impact of lantana camara invasion on a cattle/wildlife ... · lantana camara, is a shrub species...

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IMPACT OF Lantana camara INVASION ON A CATTLE/WILDLIFE RANCH: A

CASE OF IMIRE RANCH, WEDZA DISTRICT, ZIMBABWE

By

MARTIN ZENDE

A thesis submitted in partial fulfilment of

the requirements for the degree of

Master of Science in Tropical

Resources Ecology

Department of Biological Sciences

Faculty of Science

University of Zimbabwe

December 2014

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Abstract

The impact of Lantana camara invasion in a mixed game/cattle ranch was assessed at Imire

Ranch using remote sensing and ground truthing techniques. Vegetation communities of the

ranch were classified and mapped as a way of determining the pattern and extent of L. camara

invasion. NDVI was used as a proxy for net primary productivity to assess the impact of

invasion on primary production. LandSat images from 2001 to 2013 were used to assess the

spatial extent and trends in L. camara invasion over a twelve-year period. Differences in key

edaphic variables among invaded, managed and non-invaded sites were determined as a means

of assessing impacts of invasion on floristic composition, abundance and diversity.

Nine vegetation types were distinguished and described floristically and physiognomically.

Five woodland communities constitute the primary vegetation components of the range,

namely: Brachystegia spiciformis woodland, Brachystegia spiciformis-Julbernardia globiflora

woodland, Julbernardia globiflora-Brachystegia spiciformis mixed woodland, Acacia

sieberiana –L. camara woodland and Mixed Acacia woodland. Two grassland communities

were distinguished where Aristida sp. or wetland grasses and sedges dominated. The only

natural grassland community in the ranch is the wetland grassland as the other grassland

community represents former cultivated lands. An Acacia nilotica- Combretum molle bush

land was also distinguished as well as small Eucalyptus plantations.

The highest cover and abundance of L. camara was observed in a section of the ranch occupied

by A. sieberiana where nitrogen enriched soils present a conducive environment for the

invasive species. L. camara also tended to be confined to nutrient rich termite mounds in B.

spiciformis woodlands and in wetland grasslands with higher water and nutrient content. The

invasive species also occurred on open sandy soils and mesic conditions in Eucalyptus

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plantations. Though it only occupied a small section of the ranch, the invasive species appeared

to be highly adaptable, with the potential to spread into other vegetation types as observed in

several vegetation communities, though in small isolated patches.

L. camara invasion had lowered the diversity of herbaceous species. The area occupied by L.

camara indicated a persistent decline in NDVI between 2002 and 2013. The invasion also

appeared to have impacted on soil physico-chemical properties, raising soil pH and N, P, K

levels.

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Acknowledgements

This study was conducted at Imire Ranch. My appreciation also goes to Reilly, Katy and Imire

Ranch staff for their assistance during the course of the study. I am deeply indebted to Professor

Kativu and Professor Mhlanga who supervised the study. The Biological Sciences Department

provided all field equipment. I sincerely appreciate the guidance of Dr Nhiwatiwa and all staff

of the Department of Biological Sciences who helped me through my studies. Warmest thanks

go to my family and my friends. Thank you all for your support.

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Table of contents

Abstract ...................................................................................................................................... ii

Acknowledgements ................................................................................................................... iv

Table of contents ........................................................................................................................ v

List of tables……………...……………..........………….…………………………......……viii

List of figures……………...........………………………………………………………….…ix

Chapter 1: Introduction .............................................................................................................. 1

1.1 Background information ...................................................................................................... 1

1.2 Problem Statement ............................................................................................................... 3

1.3 Justification .......................................................................................................................... 4

1.4 Objectives of the study......................................................................................................... 4

1.5 Hypothesis............................................................................................................................ 5

Chapter 2: Literature Review…………………..........…………………………………….…..6

2.1 Impacts of invasive plant species ......................................................................................... 6

2.2 Biology, distribution and habitat characteristics of L. camara ............................................ 7

2.2.1 Biology of L. camara................................................................................................. 7

2.2.2 Origin and distribution of L. camara ......................................................................... 8

2.2.3 Biological characteristics of L. camara ..................................................................... 9

2.3 Invasive attributes of L. camara ........................................................................................ 10

2.3.1 Fitness homeostasis and phenotypic plasticity ........................................................ 10

2.3.2 Interaction with animals .......................................................................................... 10

2.3.3 Geographical range .................................................................................................. 11

2.3.4 Vegetative reproduction .......................................................................................... 11

2.3.5 Fire tolerance ........................................................................................................... 12

2.3.6 Competitive ability .................................................................................................. 12

2.3.7 Allelopathy .............................................................................................................. 12

2.4 Impacts of L. camara .................................................................................................. 13

Chapter 3: Materials and Methods…………………….......…………………………………16

3.1 Study Site ........................................................................................................................... 15

3.2 Recognisance survey .......................................................................................................... 16

3.3 Spatial distribution of L. camara ....................................................................................... 17

3.4 Vegetation survey .............................................................................................................. 17

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3.5 Primary productivity .......................................................................................................... 19

3.5.1 Remote sensing data ................................................................................................ 19

3.5.2 NDVI Sampling ....................................................................................................... 19

3.6 Species assessment............................................................................................................. 20

3.6.1 Species identification ............................................................................................... 20

3.6.2 Species abundance ................................................................................................... 20

3.6.3 Species richness ....................................................................................................... 20

3.6.4 Density ..................................................................................................................... 20

3.7 Soil sampling ..................................................................................................................... 20

3.7.1 pH determination ..................................................................................................... 21

3.7.2 Nitrogen determination ............................................................................................ 21

3.7.3 Phosphorous determination ..................................................................................... 22

3.7.4 Potassium determination ................................................................................................. 22

3.7.5 Soil moisture determination..................................................................................... 22

3.8 Data analysis ...................................................................................................................... 23

3.8.1 Trend Analysis for NDVI values ............................................................................. 23

3.8.2 Compilation of the error matrix ............................................................................... 23

3.8.3 Calculation of Cohen’s Kappa ................................................................................. 24

3.8.4 Classification of Vegetation .................................................................................... 25

Chapter 4: Results

4.1 Mapping and classification of vegetation types at Imire Ranch ........................................ 26

4.2 Spatial extent and temporal trends of L. camara invasion at Imire Ranch ........................ 30

4.4 Temporal variation in net primary productivity on a L. camara invaded site ................... 40

4.5 Variations in edaphic factors among L. camara invaded, managed and non-invaded sites

.................................................................................................................................................. 43

Chapter 5: Discussion .............................................................................................................. 48

5.1 Vegetation classification .................................................................................................... 48

5.2 Spatial distribution of L. camara ....................................................................................... 51

5.3 Impact of L. camara invasion on herbaceous plant species diversity................................ 52

5.4 Impact of L. camara on rangeland production................................................................... 54

5.5 Impact of L. camara on soil physico-chemical properties ................................................. 55

5.6 Conclusion ......................................................................................................................... 56

5.7 Recommendations .............................................................................................................. 57

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References…………………………………………………………………………………....58

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List of tables

Table 3.1: Modified Braun-Blanquet Scale (source. Timberlake et al., 1993)………...........18

Table 4.1: Shannon Weiner values for the non-invaded site………………………………...30

Table 4.2: Shannon Weiner values for the invaded site……………………………………..39

Table 4.3: Shannon Weiner values for the managed site……………………………………39

Table 4.4: Summary of biodiversity measurements…………………………………………40

Table 4.5: One way ANOVA summary results of soil physico-chemical properties on L.

camara occupied, managed and unoccupied sites. Values with different superscripts differ

significantly (Tukey’s HSD; P ˂ 0.05)……………………………………………………….43

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List of figures

Figure 3.1: Map of Imire Ranch (Wedza District) showing study site where studies on L.

camara were conducted…………………………………………………………………........16

Figure 4.1: Dendrogram of Hierarchical Cluster Analysis of vegetation plots from Imire

Ranch…………………………………………………………………………………………28

Figure 4.2: Imire Ranch (Wedza District) showing sites occupied by L. camara…………..31

Figure 4.3: Landsat imagery (30 m x 30 m) of Imire Ranch showing spatial extent of L. camara

invasion in 2001………………………………………………………………………………32


invasion in 2004………………………………………………………………………………33


invasion in 2007………………………………………………………………………………34


invasion in 2010……………………………………………………………………………...35


invasion in 2013…………………………………….……………………………..................36

Figure 4.8: Mean annual NDVI trend analysis for a L. camara invaded site at Imire Ranch,

(2001 to 2013)………………………………………………………………………………..41

Figure 4.9: Box and Whisker plot illustrating comparison of mean NDVI on a L. camara

invaded site at Imire Ranch (2001-2013)……………………………………………...……..42

Figure 4.10: Mean soil pH per site. Error bars represent 1 SE……………………...............44

Figure 4.11: Mean soil total nitrogen content per site. Error bars represent 1 SE…………..45

Figure 4.12: Mean soil phosphorous content per site. Error bars represent 1 SE…………...45

Figure 4.13: Mean soil potassium content per site. Error bars represent 1 SE……………...46

Figure 4.14: Mean soil moisture content per site. Error bars represent 1 SE……………….47

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Chapter 1: Introduction

1.1 Background information

Invasive alien species (IAS) are a threat to biodiversity through their proliferation and spread,

as they displace or kill native flora and fauna, thus affecting ecosystem services. They have

since been recognised as important agents of global environmental change (Vitousek et al.,

1996) and rank second among threats to global biodiversity after direct anthropogenic habitat

destruction and landscape fragmentation (Drake et al., 1989; UNEP, 2004; Sharm et al., 2005).

Alien species invasion is one of the major ecological problems of recent years (Sharm et al.,

2005). Its economic impact is of major global concern (Pimentel et al., 2000).

Invasive alien species alter ecosystem structure and function and affect the abundance and

diversity of native species (Weiss and Noble, 1984; Gurevitch and Padilla, 2004). They may

also displace mature vegetation or limit juvenile recruitment (Yurkonis et al., 2005).

Mechanisms that limit resident species recruitment in invaded communities include

competition for such resources as light, nutrients and space (Tilman, 1987; Davis et al., 2000).

Such mechanisms also include some non-resource mediated interferences like allelopathy

(Lwando - Tembo, 2008).

Lantana camara, is a shrub species that originates from tropical America. The Invasive Species

Specialist Group (IUCN, 2001) included it among the world’s 100 worst invasive species. Like

other invasive species, it has profound effects on biodiversity, especially in semi-arid areas of

Africa, Australia, India and the Pacific Islands (Dogra et al., 2009).

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The insidious weed is thought to have been introduced in Africa in the 1930s (Verdcourt, 1992),

and has since naturalized itself in many of the continent’s semi-arid areas including some

nature reserves where it is negatively effecting ecosystem functioning. In Zimbabwe L. camara

is spreading widely with significant negative impacts. It readily invades rangelands

outcompeting native species, thus resulting in reduced forage, while its impenetrable thickets

impede access to understorey species. In addition, it drastically reduces biomass of native

species, thus reducing the abundance of forage, consequently resulting in loss of biodiversity

(Witt, 2010).

L. camara has several negative economic impacts. It lowers productivity of beef and dairy

pastures. It is also considered a serious weed of plantation timber and orchards (Swarbrick et

al., 1998). L. camara is poisonous to livestock, particularly cattle. Symptoms of L. camara

poisoning include photosensitization, loss of appetite, jaundice, liver and other organ damage.

If untreated, it can result in death (Queensland Government, 2001).

L. camara is a pest that can overrun rangeland and forest plantations (Lottyniemi, 1982). The

species has invaded parts of Imire Ranch, on the central plateau of Zimbabwe, where it poses

threat to the sustainability of the ranch. The species is considered to have been introduced into

the ranch and surrounding areas through human transportation and by birbs. It subsequently

spread through bird dispersal. Its extent and density has increased over the years, posing

challenges to pasture and fodder management in the ranch. Efforts to control L. camara have

so far been unsuccessful (Reilly, personal communication). Rangeland and pastures comprises

more than 60% of the land area of Imire Ranch. These provide a valuable resource for wildlife

and recreational activities, as well as livestock grazing. Invasion by L. camara is likely to

reduce overall forage quantity and quality.

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Carrying capacity at any given time is determined by range condition. This is assessed as a

function of grass composition, biomass and cover, and is interpreted as a stage in plant

succession (Trollope, 1990). As noted, L. camara can lower forage quality and quantity. It

also depletes soil nutrients and moisture levels, thus reducing the carrying capacity of a

rangeland. The present study sought to assess the extent of L. camara invasion and its impact

on rangeland productivity at Imire Ranch.

1.2 Problem Statement

Invasive alien species often have significant effects on native biodiversity and ecological

processes as they alter community composition and affect nutrient and hydrological cycles and

fire regimes (Yurkonis et al., 2005). Alien plant invasions have recently created an

environmental problem in Zimbabwe as they impact on natural forests, agricultural lands,

waterways and riverine ecosystems.

Rangeland weeds can have a significant impact on both humans and the environment. Their

impact on human activities can be associated with livestock production, including interfering

with grazing practices, lowering yield and quality of forage, increasing costs of livestock

management and production, slowing animal weight gain, reducing quality of meat, milk, wool,

and hides, and poisoning livestock. In addition, infestations can reduce recreational land values

(Joseph, 2000).

Noxious weeds cause more economic loss on rangeland than all other pests combined (Quimby

et al., 1991).The financial impacts of invasive plants on rangelands are substantial and have

been estimated to cost ranchers some US$5 billion annually just on control measures (Masters

and Sheley, 2001).

The primary objective of Imire Ranch is the maintenance of an economically viable mixed

livestock-wildlife ranching system. However, the invasion of the ranch by L. camara is likely

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to reduce the forage quantity and quality, thereby negatively impacting on the sustainability of

this farming system.

1.3 Justification

Rangelands provide a principal source of feed for livestock both in private farms and communal

grazing lands. Livestock and wildlife production are major enterprises in Zimbabwe (Abel and

Blaikie, 1989). However, there is increased encroachment by L. camara on many Zimbabwean

rangelands. This has the potential of reducing forage quantity and quality.

The impact of L. camara invasion is important for the management of the ranch. There is need,

therefore, to establish the species distribution pattern and its impact on native plant species as

a step towards its control.

1.4 Objectives of the study

The primary aim of the present study was to determine the pattern of invasion of L. camara at

Imire Ranch and determine its impact on production.

The specific objectives were:

1. to classify the vegetation types in Imire Ranch;

2. to determine the trend in abundance and spatial pattern of L. camara at Imire Ranch in

the past decade;

3. to assess the impact of L. camara invasion on floristic composition, abundance and

diversity;

4. to determine trends in net primary production on invaded sites;

5. to determine the key differences between invaded and non-invaded sites in soil pH,

nitrogen, phosphorous and potassium.

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1.5 Hypothesis

Ho: Invasion by L. camara has not affected primary production at Imire Ranch.

Ho: Invasion by L. camara has had no impact on floristic composition, abundance and diversity.

Ho: Invasion by L. camara has not affected soil pH, nitrogen and phosphorous of Imire Ranch.

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Chapter 2: Literature Review

2.1 Impacts of invasive plant species

Alien species invasion has become one of the most serious global environmental problems

(Sousa, 1984; Gurevitch and Padilla, 2004). The pace of invasion has accelerated during the

last century primarily due to increased modification of habitats (Schei, 1996). Species invasion

has now become the second most serious threat to global biodiversity after habitat destruction

(Schei, 1996), and number one cause of species extinction in island habitats.

The range of impacts of species invasion is wide (Perrings, 2005). These include reduced crop

and pasture production through competition for light, decreased moisture and nutrient status,

displacement of crop and pasture species through allelopathy, contamination of harvested

crops, increased secondary hosts for pests and pathogens, and interference with harvesting.

Invasive plant species may also reduce quality of livestock (through toxicity that leads to

retarded growth) and livestock products (meat, milk, fleece or hides). They may cause

reproductive failure in livestock. Where they encroach into water bodies, invasive species

affect water quality and quantity and impede its accessibility. Estimates of yield losses of not

less than 10% in less developed countries and 25% in least developed countries have been

suggested (Akobundu, 1987).

IUCN (2001) has estimated that invasive plant species result in reduction of up tob12% in crop

yields in the USA. This equates to a monetary loss of about US$ 33 billion annually. Pimentel

et al., (2001) have estimated annual crop losses of up to US$ 27.9 billion of due to exotic

weeds. In the case of India, estimates put crop losses up to 30% annually. This is worth about

US$ 90 billion (Singh, 1996). Approximately 45% of weeds in USA pastures are alien species.

These account for about US$1 billion loss in pasture production per annum (Pimentel et al.,

2001).

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The Food and Agricultural Organisation of the United Nations (2003) has noted that the

majority of the 100-200 million nomadic and trans-human pastoralists of the world reside in

Africa. These significantly contribute to the continent’s agricultural production. In the case of

the Sudan, Senegal, Niger, and Kenya, for example, pastoralism contributes some 80%, 78%,

84% and 50% of the Gross Domestic Product (GDP), respectively. This thriving industry is

threatened by invasive plant species particularly L. camara (Pimentel et al., 2001).

2.2 Biology, distribution and habitat characteristics of L. camara

2.2.1 Biology of L. camara

L. camara is a member of family Verbenaceae. It is a pan tropical weed that has invaded

pastures and native forests in more than 60 countries (Parsons and Cuthbertson, 2001). The

species is a low erect or subscandent, vigorous shrub, with stout recurved prickles, and a strong

odour. L. camara grows up to 2.4 m, and has a very strong root system. It sprouts new shoots

even after repeated cutting or burning (Frensham et al, 1994). L. camara flowers are small,

orange or of varying shades of white to red. In Australia, these are red, pink, white/pale pink

and orange (Parson and Cuthbertson, 2001). Inflorescences are produced in pairs in the axils

of opposite leaves. Yellow colouration is associated with visual cue to pollinating insects, and

pollination may stimulate colour change (Day et al., 2003). In many regions of the world, the

species flowers for much of the year if adequate moisture and light are available (Duggin and

Gentle, 1998). In cooler or drier regions, flowering may only occur in the warmer or wetter

periods of the year (Winder, 1980; Swarbrick et al., 1998).

Its fruits are small, drupaceous and shining, with two nutlets, greenish blue to black, and

dispersed by birds. The seeds germinate very easily (Day et al., 2003).

The species is now largely an artificial hybrid that has been subjected to intense horticultural

improvement in Europe since the sixteenth century. It exists in many different forms or

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varieties throughout the world and flower colour is the key character for distinguishing between

different forms (Thomas and Ellison, 2000). Through RAPD analysis, Singh (1996) has

observed great genetic diversity within L. camara.

L. camara occurs in diverse habitats and on a variety of soil types. This implies that it has the

potential to invade many parts of Zimbabwe (Palmer and Pullen, 1995). It, however, generally

performs best in open areas with minimum shade such as degraded land, pasture lands, edges

of tropical and subtropical forests, beach fronts and forests recovering from fire or logging

(Thakur et al., 1992). It also invades forest plantations and riparian areas. The diverse and

broad geographic distribution of L. camara is a reflection of its wide ecological tolerance.

2.2.2 Origin and distribution of L. camara

L. camara is a native of South America and was introduced in India in the early nineteenth

century as an ornamental plant where it now grows densely throughout the country (Thakur et

al., 1992). The species was first reported in Australia in 1841, and by 1897, it had become one

of the most troublesome weeds (Van Oosterhout et al., 2004). It now forms some impenetrable

thickets on forest edges and covers approximately 4 million hectares across Australia (Van

Oosterhout et al., 2004). L. camara has invaded millions of hectares of grazing land across the

globe (Thakur et al., 1992). It is also a problem weed in croplands of 14 major crops that

include coffee, tea, rice, cotton and sugarcane (Diatloff, 1975). L. camara grows under a wide

range of climatic conditions. Its geographic range in Australia coincides with the 750 mm

isohyet in southern Queensland and the 1250 mm isohyet in the north (Harley,1973), with

infestations restricted to creek lines in drier areas (Diatloff, 1975).

Not only is the geographic range of L. camara expanding, but its density within its geographic

range is increasing. This has been recognised as a future threat to ecosystems in Australia

(Haseler, 1966), the Solomon Islands and Vanuatu (Harley, 1973). On the contrary, there are

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several regions where L. camara is currently limited by intact forests which inhibit its growth

(Duggin and Gentle, 1998). Increased logging and habitat disturbance in many regions of the

world provide suitable habitats for the species (Gentle and Duggin, 1998). In countries where

there are still large areas of native forests, such as Papua New Guinea, L. camara is restricted

to small, isolated infestations among abandoned settlement sites (Day et al., 2003). It, however,

has the potential to spread widely following further clearing of forests for timber or agriculture.

In Zimbabwe, the species is threatening moist evergreen rain forests of the Eastern Highlands

(Timberlake and Musokonyi, 1994), and is widespread elsewhere, especially in pasturelands.

2.2.3 Biological characteristics of L. camara

L. camara does not appear to have an upper temperature or rainfall limit and is often found in

tropical areas receiving 3000 mm of rainfall per year (Cilliers, 1983). It also occupies areas

where temperatures frequently drop below 5°C (Van Oosterhout et al., 2004). The species

cannot survive under dense and intact canopies of taller native forest species, and is susceptible

to frosts and saline soils (Nathan and Muller-Landau, 2000). It also tends to rot in boggy or

hydromorphic soils, and is sensitive to aridity (Van Oosterhout et al., 2004).

The plant usually flowers in the first growing season after its establishment. Honeybees,

sunbirds (India) and humming birds (Brazil) play a minor role in pollination and fruit-set is

limited by pollinator abundance (Goulson and Derwent, 2004). Pollination results in 85% fruit-

set (Goulson and Derwent, 2004). A successfully pollinated inflorescence bears about eight

fruits (Barrows, 2008), although 25 to 28 fruits can occur (Sharma et al., 2003). L. camara

seeds germinate at any time of the year as long as there is sufficient soil moisture and light and

optimal temperatures (Parsons and Cuthbertson, 2001). Germination rates are low (4 to 45%)

due to seed dormancy, low seed viability and or meiotic instability. However, low germination

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rates can be offset by the extremely low rates of seedling mortality experienced in the field

(Sahu and Panda, 2008).

2.3 Invasive attributes of L. camara

L. camara possesses a number of attributes that characterize it as an invader (Drake et al., 1989;

Lodge, 1993). There is still uncertainty as to which attributes make some species more invasive

than others, or what makes some ecosystems more susceptible to invasion (Mack et al., 2000).

Some of the invasive characteristics typical of L. camara are described below.

2.3.1 Fitness homeostasis and phenotypic plasticity

Homeostasis is evidenced by the ability of L. camara to occupy diverse environments that vary

from rainforest margins to semiarid areas and from cool uplands to low-lying hot areas. The

species also displays immense phenotypic plasticity (Dorken and Barrett, 2004) and varies in

size and structure. Likewise, its phenology varies with the area it occurs, with some forms

being deciduous yet others being almost evergreen. Artificial defoliation of L. camara during

spring produces more stems and results in a greater allocation of biomass for reproduction than

when carried out in autumn. Thus the species compensates for defoliation, thereby exhibiting

its invasive potential (Broughton, 2003).

2.3.2 Interaction with animals

The success of L. camara can be attributed to the presence of a range of pollinators, accounting

for the high percentage of fruit-set (Dorken and Barrett, 2004). Once formed in high numbers,

the seeds of L. camara are dispersed efficiently through the participation of a variety of animal

dispersal agents that feed on the fruit. The process of invasion is further enhanced by nutrient

additions through animal droppings, canopy removal, and soil disturbance which create a good

seed bed. Physical soil disturbance associated with cattle grazing can also increase resource

availability and removal of competitive biomass (Duggin and Gentle, 1998). L. camara benefits

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from the destructive foraging activities of vertebrates, such as pigs, cattle, goats, horses, sheep,

and deer, through enhanced vegetative propagation (Fensham et al., 1994). This might be the

reason for the spread of L. camara in rangelands, as in the case at Imire Ranch.

2.3.3 Geographical range

L. camara has a widespread distribution (from 35°N to 35°S) beyond its native range, and is

naturalized in more than 60 countries (Day et al., 2003). The distribution of L. camara varieties

reveals patterns that substantially overlap when considered in terms of the climax model (Day

et al., 2003) or Myer’s biodiversity hot spots model (Myers et al., 2000). This indicates severe

threat to ecosystems in hot spots areas under L. camara invasion. L. camara cover an altitudinal

range of up to 2000 m in the Pulnis hills in southern India (Mathews, 1972). Its distribution is

still expanding, with many countries and islands that were listed in 1974 as not having L.

camara (Galapagos Islands, Solomon Islands, Palau, Saipan, Tinian, Yap and Futuna Island)

(Thaman, 1974) having been invaded recently (Waterhouse and Norris, 1987). The density of

L. camara infestations within its native range is also increasing. This has been recognized as

an additional threat to ecosystems (Mack et al., 2000).

2.3.4 Vegetative reproduction

Once established, the rapid vegetative growth of L. camara facilitates the formation of large,

impenetrable clumps (Van Oosterhout et al., 2004). The most common means of vegetative

spread is through layering, where horizontal stems produce roots when they come in contact

with soil, suckering can also occur (Swarbrick et al., 1998). Prostrate stems can root at the

nodes if covered by moist soil, fallen leaves or other debris. In Australia, the species is

commonly well-spread by landholders who dump vegetative material in infested bush land

(Day et al., 2003).

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2.3.5 Fire tolerance

L. camara burns under hot, dry conditions, even when green (Gujral and Vasudevan, 1983).

Moderate and low intensity fires can promote the persistence and spread of L. camara thickets,

rather than reducing them (Natural Heritage Trust, 2004). Removal of competing neighbouring

plant species and increases in soil nutrients following burning raises L. camara germination

potential (Gentle and Duggin, 1997; Duggin and Gentle, 1998). Under conditions of increased

soil fertility (Duggin and Gentle, 1998), the species’ re-establishment is encouraged following

mechanical or chemical control of mature plants.

2.3.6 Competitive ability

Under conditions of high light intensity, soil moisture, and soil nutrients, the mortality rate of

mature L. camara plants is very low (Sahu and Panda, 2008). In forest communities, the species

has the potential to block succession and displace native species, resulting in reduced

biodiversity (Loyn and French, 1991; Duggin and Gentle, 1998).

L. camara infestations can be so persistent that they impede the regeneration of rainforest

species (Lamb, 1991). It is also capable of interrupting the regeneration processes of native

species through impeded germination, reduced early growth rates, and increased mortality of

competing species (Mack et al., 2000). This results in marked changes in structure and floristic

composition of natural communities. As the density of L. camara increases, species richness

tends to decrease (Fensham et al., 1994). Sites invaded by L. camara often have low species

richness. The species does not invade intact forests, but is found on its margins (Humphries

and Stanton, 1992) where there are fewer other species to compete with.

2.3.7 Allelopathy

The allelopathic effect of L. camara is well-documented (Ambika et al., 2003). It results in

severe reductions in seedling recruitment of nearly all species under its cover (Lwando-Tembo,

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2008). No growth or only growth of competing species is observable close to L. camara. This

is attributed to its allelopathic effects.

About 14 phenolic compounds have been identified in L. camara. These can reduce seed

germination and growth of young competing species (Jain et al., 1989). A number of aromatic

alkaloids and phenolics have also been extracted from various parts of L. camara (Khan et al.,

2003). These may inhibit the growth of other plants, depending on their concentration (Ambika

et al., 2003). Generally the concentration increases from root, stem to leaf (Chaudhary and

Bhansali, 2002), thus making the leaf toxic to browsers (Ambika et al., 2003).

Studies have highlighted these phytochemicals as an important component of L. camara’s

competitive strategy. Even though some authors have found that L. camara-derived chemicals

act negatively on many crops and plantation species, no thorough studies on effects of these

chemicals on native flora have been conducted (Swarbrick et al., 1995). Doubts have, however,

been expressed on L. camara allelopathic effects under field conditions (Sharma et al., 1988).

2.4 Impacts of L. camara

L. camara infestations have been observed as being so persistent such that they completely

stall the regeneration of rainforest for up to three decades (Lamb, 1991). So severe is its impact

such that the species is ranked as the most significant weed of non-agricultural areas

(Bellingham et al., 2005). Its competition may have caused the extinction of Linum cratericola

(Linaceae). L. camara is also a major threat to endangered plants in the Galapagos Archipelago

(Motooka et al., 2002). The replacement of native pastures by L. camara is threatening the

habitat of the sable antelope in Kenya (Lake and Leishman, 2004).

L. camara climbing stems can reach up to more than 20 meters, getting into the forest canopy

(Fensham et al., 1994; Hiremath and Sundaram, 2005). This causes devastating crown fires

when forests burn. High L. camara density increases mid storey fuel loads resulting in intense

14

fires which destroy the canopy (Frensham et al., 1994). This subsequently promotes spread of

L. camara since the canopy opens up and provides optimum light intensity for its growth

(Sharm et al., 2003).

Replacement of native vegetation by L. camara thickets may reduce the amount of available

forage and habitat for native animals. In the case of Zimbabwe, the relationship between L.

camara density and carrying capacity has never been investigated.

Removal of native plants creates gaps for invaders. In attempts to physically remove L. camara,

this also creates gaps that facilitate invasion and spread of secondary plant invaders (Ticktin et

al., 2006). L. camara spoils scenery and has an adverse effect on recreation (Simelane, 2002).

Lamb (1991) has documented the ability of alien invasive species to alter hydrological patterns,

soil chemistry, moisture holding capacity and erodibility. The subsequent loss in plant

biodiversity results in alterations of soil chemistry, geomorphological processes, fire regimes,

hydrology, levels of soil erosion, land transformation and disruption of ecosystem processes

and functioning (Cowling et al., 2004). Day et al., (2003) have considered loss of pasture as

the greatest single cost of L. camara invasion in grazing areas of Queensland. Reduction in

environmental quality (Heffernan, 1998) may result from replacement of native vegetation by

monospecific stands of L. camara.

15

Chapter 3: Materials and Methods

3.1 Study Site

The study was conducted at Imire Ranch in Wedza (Figure 3.1). Geographically the ranch is

located between latitudes 18o26'S and 18o31'S and longitudes 31o26'E and 31o32'E. The ranch

is 130 km east of Harare, south of Marondera. It comprises of approximately 10, 000 acres.

Chinyeka River is the primary perennial river system that traverses the ranch. Other small

streams are intermittent, flowing only during the rainy season. Imire Ranch experiences two

main seasons: a wet season which extends from November to March and a dry season which

extends from April to October. The mean annual rainfall is about 750 mm, and it falls mainly

during the wet season. The range previously combined tobacco farming with livestock and

wildlife rearing. Tobacco farming has since stopped, and only livestock and game ranching

are presently practised. There are approximately 900 head of cattle which graze in the same

area with elephant (Loxondonta africana), buffalo (Synceras caffer), giraffe (Giraffa

camelopardis), hippopotamus (Hippopotamus amphibious), zebra (Equus burchelli), eland

(Orynx gazelle), sable (Hippotragus niger), blesbok (Damaliscus dorcus phillipsi), impala

(Aepyceros melampus) and nyala (Tragelaphus angasii).

The ranch has been extensively dammed in order to provide water supplies to livestock and

wildlife.

16

Figure 3.1: Map of Imire Ranch (Wedza District) showing study site where studies on L.

camara were conducted

3.2 Recognisance survey

A recognisance survey was carried out in order to familiarize with current vegetation

distribution in the ranch. Core sites of L. camara invasion were identified and their

geographical position recorded using a Geographical Positioning System (GPS) receiver

(GARMIN, GPSmap 76 S, Am). Non-invaded sites were also identified and geographically

referenced.

Three core sites were identified, namely (i) Acacia sieberiana woodland with > 70% invasion

L. camara invasion, (ii) adjacent site to A. sieberiana woodland where L. camara was

17

previously removed with < 10% invasion and (iii) a grassland with isolated trees of A.

sieberiana with no L. camara.

3.3 Spatial distribution of L. camara

Polygons of L. camara invaded sites were established by taking GPS recordings of the area. A

random starting point was selected at each site. All woody and grass species occurring in the

area were recorded and the area was expanded until no new species could be encountered. Care

was taken not to transgress any obvious environmental boundaries like from a grass land to a

woodland. Data were digitalized and geo-referenced to a location and visualized in the form of

a map and then analyzed to reveal relationships, patterns and trends. This allowed for a rapid

establishment of spatial relationships among layers.

3.4 Vegetation survey

Woody species were assessed for cover-abundance using a modified Braun-Blanquet Scale

(Table 3.1).

18

Table 3.1: Modified Braun-Blanquet Scale (source. Timberlake et al., 1993)

Species frequency and estimate of aerial cover Braun-Blanquet

symbol

Few with small cover

Numerous but less than 5% cover or scattered with up to 5% cover

Any number with 5-25% cover



Any number with more than 75% cover

+

1

2

3

4

5

The Braun-Blanquet approach is based on three principal assumptions:

i. Classification and interpretation of communities can be based on floristic

composition

ii. Some species in a community give a more sensitive expression of relationships than

others; and

iii. These indicator species will be used to organize communities into a hierarchical

classification (Whittaker, 1975).

The production of maps of Imire Ranch was done using GIS after transferring Landsat

images onto base maps.

19

3.5 Primary productivity

3.5.1 Remote sensing data

A 1:50000 map was acquired from the Surveyor General’s Office in Harare. The map was

scanned and saved in a TIF format at the GIS library of the University of Zimbabwe’s

Department of Geography and Environmental Science. The maps were then digitised using a

GIS software (ArcGIS 10).

This part of the study was based on LANDSAT TM 4-5 imagery. This LANDSAT imagery

was acquired at a spatial resolution of 30 m by 30 m and a temporal resolution of 16 days. Only

images taken in March were used in order to avoid the effect of leaf fall. Cloud free images

were selected for the calculation of NDVI. The images were downloaded and pre-processed

using GIS software. NDVI graphs were produced for years: 2002 to 2013.

The map projection for the images was from the Universal Transverse Mercator (UTM), Zone

36 South, WGS 1984 reference Spheroid. Near Infra-Red (NIR) (band 4), Red (band 3), Green

(band 2) and Blue (band 1) of LANDSAT images were used for the purpose of visualisation as

a colour composite and for estimating of primary production.

3.5.2 NDVI Sampling

The study adopted a stratified sampling procedure. The whole ranch was categorised into two

land classes, Woodland and Grassland, using the supervised classification process. Thus land

cover type was used as the stratum. GIS sampling points for each of the strata were randomly

generated using ArcGIS Software, and the area of each stratum was used to determine the

number of sampling points per stratum. The sampling points were then overlaid on NDVI maps

for the purpose of extracting statistical data for each year. The NDVI values were calculated

using the ArcGIS Software, and were then used as a proxy for primary production.

20

3.6 Species assessment

Species assessments were carried out in three site categories namely, invaded, non-invaded and

managed. Three parameters were assessed from each of the sites: species richness, species

abundance and density.

3.6.1 Species identification

Species were identified in situ. Plants that could not be identified were pressed in a plant press

and later identified at the National Herbarium. In situ species identification was aided by field

guides.

3.6.2 Species richness

Ten quadrats of 4 m x 4 m were randomly laid in each of the three sites: invaded, managed

and non-invaded areas. All herbaceous species in each of the quadrats were recorded.

3.6.3 Species abundance

The total number of individual for each the herbaceous species within each quadrat was

recorded. This provided a measure of abundance for each of the species.

3.6.4 Density

Density was considered as the number of plants per given area. In this study density was

estimated per hectare using the average number of plants in all the quadrats from each treatment

and simple proportion.

3.7 Soil sampling

Soil samples were collected from each of the three sites using an auger. A systematic sampling

procedure was followed where soil samples were collected from the 4 corners of the quadrat

and at the center at a depth of 20 cm. Samples from each quadrat were bulked into a composite

sample in plastic bags. The plastic bags were then placed into paper bags, which were then

21

sealed and taken to the University of Zimbabwe, Department of Biological Sciences Laboratory

for assessment of pH, Nitrogen, Phosphorous, Potassium and soil moisture.

3.7.1 pH determination

Some 20 g of soil was weighed into a numbered conical flask and 50 ml of distilled water was

added to the soil. The suspension was thoroughly mixed and left to settle for 10 minutes. The

pH was measured using a digital pH meter (Systronics 335) in the supernatant suspension of

1:2.5 soil to liquid ratio.

3.7.2 Nitrogen determination

The nitrogen content of the soil samples was determined by the Kjeldalhl method. The method

involves the heating the soil with sulfuric acid, which decomposes the organic substance by

oxidation to liberate the reduced nitrogen as ammonium sulfate. In this step, potassium sulfate

was added to raise the boiling point of the medium (from 337 °C to 373 °C). Chemical

decomposition of the sample was completed when the initially very dark-colored medium has

become clear and colourless.

The solution was then distilled with a small quantity of sodium hydroxide, which converted

the ammonium salt to ammonia. The amount of ammonia present, and thus the amount of

nitrogen present in the sample, was determined by back titration. The end of the condenser was

dipped into a solution of boric acid. Ammonia reacts with the acid and the remainder of the

acid was then titrated with a sodium carbonate solution by way of a methyl orange pH indicator.

Degradation: Sample + H2SO4 → (NH4)2SO4 (aq) + CO2 (g) + SO2 (g) + H2O (g)

Liberation of ammonia: (NH4)2SO4 (aq) + 2NaOH → Na2SO4 (aq) + 2H2O (l) + 2NH3 (g)

Capture of ammonia: B (OH) 3 + H2O + NH3 → NH4+ + B (OH) 4

–

Back-titration: B (OH) 3 + H2O + Na2CO3 → NaHCO3 (aq) + NaB (OH) 4(aq) + CO2 (g) + H2O

22

3.7.3 Phosphorous determination

The Olsen sodium bicarbonate method was used to determine phosphorous content in the soil.

A 1 gram scoop of air-dried soil and 20 milliliters of 0.5 molar sodium bicarbonate (NaHCO3)

solution were shaken for 30 minutes. The blue colour in the filtered extract was developed with

molybdate- ascorbic acid reagent and measured with the Brinkman PC 900 probe colorimeter

at 880 nm. Results were reported as mg/kg phosphorus (P) in the soil. As with the Bray P-1

test, potentially available organic P was not measured by the test.

3.7.4 Potassium determination

The flame atomic absorption spectrophotometry method was used to determine soil potassium

content. The more readily soluble potassium was removed by a single extraction with 1M

HNO3, at a wide acid: soil ratio. This fraction was discarded. Three successive extractions with

1 M HNO3, at a narrower acid: soil ratio were then made and the K concentration in these

extracts was determined using flame atomic absorption spectrophotometry. The results of the

extractions, were averaged to give the potassium (K) value. The method was adapted from that

described by Blakemore et al., (1987).

3.7.5 Soil moisture determination

The Oven Drying Method as described by Hausenbuiller (1975) was used to assess soil

moisture content. A labelled dry crucible was measured (W1) and 10 g (+/- 0.01) of soil was

added into the crucible (W2). The soil was then dried at 105 0C for 2 hours and was allowed to

cool and weighed (W3). Soil moisture percentage was calculated as:

[(W2 – W3) /W2] * 100

23

3.8 Data analysis

Data were analysed in SPSS 16.0 (2002) and PCORD programme packages. Descriptive

statistics were used to summarise data and graphical representations were made in Microsoft

Office Excel 2007 to show variation in measured variables.

In addition, non-woody species diversity indices were calculated using Shannon-Wiener

diversity index (H’):

H'= -pi*ln pi

Where pi is the importance value contributed by the ith species.

Species richness was also measured for all the sites sample

Normality tests were performed on all measured variables to determine whether they satisfied

ANOVA assumptions (Quinn and Keough, 2002) using Q-Q.

3.8.1 Trend Analysis for NDVI values

The mean NDVI for each year and the long-term or overall mean for all the years were

calculated. NDVI change for each year was calculated by subtracting the long-term mean

NDVI from the average mean annual NDVI for all the years. A trend analysis was conducted

using R Statistical Software to determine the slope and the significance of the trend in primary

production as estimated by the NDVI (Millard et al., 2002).

3.8.2 Compilation of the error matrix

The error matrix (similar names include confusion matrix, correlation matrix, or covariance

matrix) summarizes the relationship between two datasets, often a classification map or model

and reference test information or alternative model. A point shape file of 30 random points was

generated in a GIS environment using the random number generator. The point shape file was

converted to a Key Hole Mark-up Language (KML) and exported to the digital browser,

24

Google Earth for visualisation and matching of the newly created field IDs. The newly matched

attribute values of the random point shape file were used to extract values to points from the

classified raster of the study area using the Spatial Analyst of ArcGIS 10. An error matrix was

derived from the matching of the classified Landsat image with the corresponding class values

as calculated in the GIS environment. The rest of the calculations were based on a match

between the numbers of points recognized in the Google Earth Image that matched with the

classified values.

3.8.3 Calculation of Cohen’s Kappa

Kappa gives an insight into the overall classification scheme and whether or not the

classification achieved the results better than what could have been achieved strictly by chance.

The formula for kappa is:

------------------------------------ (1)

Observed is overall accuracy. Expected is calculated from the rows and column totals. Hence,

what would be expected based on chance is given as follows:

……………………………………… (2)

From the calculation, the product matrix was 598 and the cumulative sum was 1600 and this

gave an expected frequency of 37%. As such,

= 68 %

This means that the classification of the Landsat image (30 m x 30 m resolution) is 68 % better

than what could have occurred strictly by chance.

Expected

ExpectedObservedKappa

1

sumCumulative

xoductmatriPr

37.01

37.080.0

K

25

3.8.4 Classification of Vegetation

Hierarchical Cluster Analysis was used to produce a classification of the vegetation using

species presence-absence data. The method divides the samples into a hierarchy of dissimilar

entities and the results are presented as a dendrogram. Cluster analysis was carried out using

the Bray-Curtis index with the program PAST. The Bray-Curtis index was chosen because of

its ability to deal with matrices with a high component of zero data entries i.e. it will not be

influenced by joint absences.

The description of the vegetation types were based on structure and key species components

(Pratt et al., 1966). This is interpreted as physiognomic classification where vegetation types

are described in terms of structure, characteristic species, important associate species and

topographic features (Timberlake et al., 1993).

26

Chapter 4: Results

4.1 Classification of vegetation types at Imire Ranch

The Hierarchical Cluster Analysis produced nine clusters representing nine vegetation types.

These comprised of five woodlands, two grasslands, one bush land and one Eucalyptus

plantation (Figure 4.1). The vegetation types are defined by one or more co-dominant species,

and were derived from Landsat imagery and field observations. The vegetation types are

described below.

A. Aristida sp. grassland

This grassland community is dominated by Aristida sp., with a cover abundance greater than

75%. Due to grazing pressure or edaphic factors dominance sometimes shifts to either

Hyparrhenia filipendula or Sporobolus pyramidalis. Other grass species include

Craspidorachis africana, Digitaria milanjiana, Pogonarthria squarrosa, Loudetia simplex and

Hypathelia dissoluta with cover abundance of less than 5%. This community occurs on

formerly cultivated land (old fields). It includes some few scattered Acacia sieberiana and

Lantana camara.

B. Wetland grassland

This community occurs on seasonally water logged sites, and is dominated by wetland sedges

(Carex sp., and Cypres sp.,) with cover abundance ranging between 50-75%. Hyparrhenia

filipendula occursionaly occurs. Other grass species include Setaria sphacelata, Leersia

hexandra, Sporobolus pyramidalis, Aristida junciformis, Cynodon dactylon and Loudetia

simplex. Pennisetum purpureum occurs along stream banks.

27

C. Brachystegia spiciformis-Julbernardia globiflora woodland

This vegetation community has Brachystegia spiciformis and Julbernardia globiflora as co-

dominants, both with a cover abundance ranging between 50-75%. Scattered within this

woodland community are Combretum molle, Pseudolachnostylis maprouneifolia and Carissa

edulis. Grass species include Hyparrhenia filipendula, Aristida sp. and Craspidorachis

africana.

D. Brachystegia spiciformis woodland

This vegetation community is dominated by Brachystegia spiciformis, with a cover abundance

of between 50-75%. Brachystegia spiciformis co-dominates with the naturalised Dodonaea

viscosa. Other woody associates include Acacia sieberiana, Dichrostachys cinerea, Ziziphus

mucronata, Combretum molle, Carissa edulis, Peltophorum africanum, Acacia nilotica,

Maytenus senegalensis and Burkea africana. Carissa edulis and Lantana camara are confined

to termite mounds. The grass component includes Hyparrhenia filipendula, Aristida sp.,

Sporobolus pyramidalis, Digitaria milanjiana and Pogonarthria squarrosa.

E. Acacia sieberiana –Lantana camara woodland

This woodland community comprises of Acacia siberiana trees at 6 m or more height, with

clumps of the invasive Lantana camara. The latter forms a thick understory, with cover

abundance ranging between 5-25%. Scattered within the community are Julbernardia

globiflora, Combretum molle, Ziziphus mucronata and Dichrostachys cinerea.

28

A- Aristida sp. grassland (range 59-5)

B-Wetland grassland (range 19-57)

C- Brachystegia spiciformis-Julbernardia globiflora woodland (range 56-4)

D-Brachystegia spiciformis woodland (range 28-37)

E- Acacia siberiana –Lantana camara woodland (range 3)

F- Acacia nilotica- Combretum molle bush land (range 24-10)

G-Julbernardia globiflora-Brachystegia spiciformis mixed woodland (range 9-8)

H-Eucalyptus plantantion (range 62-12)

I-Mixed Acacia woodland (range 48-35)

Figure 4.1: Dendrogram of Hierarchical Cluster Analysis of vegetation plots from Imire Ranch

29

F. Acacia nilotica- Combretum molle bush land

This is a low tree or bush community, with Acacia nilotica and Combretum molle as co-

dominants. The dominant grass species is Hyparrhenia filipendula, with cover abundance

greater than 75%. Hyparrhenia filipendula sometimes co-dominates with Sporobolus

pyramidalis with cover abundance of 25-50%. This community is under heavy grazing and

browsing pressure, with most of the tree species reduced to bushes. Other species include

Carissa edulis, Diospyros lycioides, Ziziphus mucronata and Peltophorum africanum.

Combretum molle, Carissa edulis and Lantana camara are confined to termite mounds.

G. Julbernardia globiflora-Brachystegia spiciformis mixed woodland

This mixed miombo woodland has Julbernardia globiflora as the dominant species, with an

abundance of 25-50%. Brachystegia spiciformis is common, with cover abundance of 5%.

Terminalia sericea, Monotes glaber, Baikea africana, Peltophrum africanum, Maytenus

senegalensis, Uapaca kirkiana, Combretum molle, Ozoroa insiginis, Carissa edulis and

Ziziphus mucronata also frequently occur. This community has been invaded by Lantana

camara whose cover abundance, however, is still low. Common grass species include Themeda

triandra, Sporobolus pyramidalis, and Aristida sp. and Pogonarthria squarrosa. Less frequent

grass species include Hyparrhenia filipendula and Digitaria milanjiana.

H. Eucalyptus grandis plantantion

Some sections of Imire Ranch are under Eucalyptus plantation. Lantana camara and

Dichrostachys cinerea are frequently found scattered within this community, with

Brachystegia spiciformis and Burkea africana occasionally occurring. Grass species include

Digitaria milanjiana, Hyparrhenia filipendula and Pogonarthria squarrosa.

30

I. Mixed Acacia woodland

This woodland community comprise of Acacia siberiana (50-75%), Acacia nilotica (5-25%)

and another Acacia species. Other less frequent species include Ziziphus mucronata, Baikea

africana, Combretum molle and Lantana camara. The grass component includes Digitaria

milanjiana, Sporobolus pyramidalis and Pogonarthria squarrosa.

4.2 Spatial extent and temporal trends of L. camara invasion at Imire Ranch

The sites established after ground truthing as invaded by L. camara are marked with red stars

on Figure 4.2. The spatial extent of L. camara invasion in relationship to the area of the ranch

is shown in Figure 4.2. L. camara was found to be widely distributed. It has primarily invaded

the A. siberiana woodland, but also occurring in the B. spiciformis woodland and B.

spiciformis-J. globiflora woodland communities. It has also established an understory in

Eucalyptus plantations. L. camara occurred to a lesser extent in wooded grasslands and open

grasslands. Heavy infestations of the species mainly occurred in areas dominated by A.

siberiana and B. spiciformis. The trend in spatial extent of L. camara invasion at Imire Ranch

between the years 2001 to 2013 is as illustrated in Figures 4.3 to 4.7. The maps show that L.

camara invasion increased over time. The area occupied by L. camara in 2001 is much smaller

than that occupied by the species in 2013.

31

Figure 4.2: Imire Ranch (Wedza District) showing sites occupied by L. camara

32


invasion in 2001

33


invasion in 2004

34


invasion in 2007

35


invasion in 2010

36


invasion in 2013

37

4.3 Impact of L. camara invasion on herbaceous species composition, abundance and

diversity

Approximately 418 individual herbaceous plants from 10 species were recorded in quadrats

sampled from a site that was not invaded by L. camara (Table 4.1). The Shannon Weiner index

(H’) for this non-invaded site was 1.89. Some 79 herbaceous plants from four different species

were recorded in quadrats sampled from a L. camara invaded site (Table 4.2). The Shannon

Weiner index (H’) for this invaded site was 0.98. Some 83 individual herbaceous plants

belonging in five different species were recorded from quadrats sampled from a managed site.

The Shannon Weiner index (H’) for this site was 1.10 (Table 4.3).

Plant abundance was highest in the non-invaded site, followed by the managed site. The

invaded site had the least plant abundance (Table 4.4). The non-invaded site had the highest

species richness of 10, followed by the managed site with 5. The invaded site had the lowest

species richness of 4. However, there was no significant difference between the managed site

and the invaded site. Species diversity and richness varied among the three sites as revealed by

the Shannon-Winner Index (H’). The non-invaded site had the highest species diversity (H’=

1.89) followed by the managed site (H’= 1.10). The invaded site had the lowest species

diversity (H’= 0.98) (Table 4.4).

38

Table 4.1: Shannon Weiner values for the non-invaded site

Species Number found ΣPiln(Pi)

Hibiscus meensei 141 -0.36657

Desmodium uncinatum 28 -0.18109

Dicerocaryum zangueburum 22 -0.15497

Leucas martinicensis 57 -0.27169

Unidentified sp. 14 -0.11375

Bidens pilosa 74 -0.30652

Conyza sumatrensis 26 -0.17275

Aloe sp. 6 -0.06090


Boophane disticha 1 -0.01443

Total 418 -1.89395

39

Table 4.2: Shannon Weiner values for the invaded site

Species Number found ΣPi ln(Pi)



Dicerocaryum zangueburum 18 -0.33701

Boophane disticha 3 -0.12420

Total 79 -0.98263

Table 4.3: Shannon Weiner values for the managed site

Species Numberfound ΣPi ln(Pi)



Leucas martinicensis 2 -0.08976


Bidens pilosa 44 -0.33644

Total 83 -1.09549

40

Table 4.4: Summary of biodiversity measurements

Site Abundance Species Richness Shannon Weiner

Invaded 79 4 0.98

Managed 83 5 1.10

Non-invaded 418 10 1.89

4.4 Temporal variation in net primary productivity on a L. camara invaded site

Temporal variations in mean NDVI on a site occupied by L. camara during years 2001 to 2013

is illustrated in Figure 4.8. Mean NDVI significantly (p=0.0000023) varied between year 2001

and year 2013. There was an overall decline in mean annual NDVI (slope = -0.04) over the 12-

year period. Mean NDVI value for year 2001 was significantly higher (0.73) than the mean

(0.34) of the other years. Mean annual NDVI declined significantly from the time L. camara

was first recorded in 2001 until 2013 (Figure 4.8).

41

Figure 4.8: Mean annual NDVI trend analysis for a L. camara invaded site at Imire Ranch,

(2001 to 2013)

42

Figure 4.9: Box and Whisker plot illustrating comparison of mean NDVI on a L. camara

invaded site at Imire Ranch (2001-2013)

The three year NDVI values for 2001, 2004, 2007 and April 2013 satellite images showed a

decline in primary productivity from 2001 to 2013 (Figure 4.9). NDVI values for 2001 indicate

that the vegetation was in a healthy state. The April 2004 image shows a significant (p=0.0043)

decline of vegetation condition from year 2001. The April 2013 NDVI indicates poorer

vegetation condition than for April 2001. These NDVI values indicate an overall decline in

primary productivity on the L. camara invaded site.

43

4.5 Variations in edaphic factors among L. camara invaded, managed and non-invaded

sites

Table 4.5: One way ANOVA summary results of soil physico-chemical properties on L.

camara occupied, managed and unoccupied sites. Values with different superscripts differ

significantly (Tukey’s HSD; P ˂ 0.05)

Site pH Nitrogen % Phosphorous Potassium Moisture %

Invaded 6.382b 0.3350c 39.33c 259.2c 13.00a

Managed 6.089ab 0.1600b 17.51b 224.0b 15.40a

Non-invaded 5.842c 0.0830a 9.43a 178.3a 11.00a

P-value 0.004 <.001 <.001 <.001 0.391

LSD 0.2739 0.04982 5.30 14.14 6.58

The site invaded by L. camara had a mean pH value of 6.3. The non-invaded site had a mean

pH of 5.8, while the site where L. camara was under control recorded a mean pH value of 6.1

(Figure 4.5). Table 4.5 and Figure 4.10 illustrate that mean pH among all the sites was

significantly different.

44

Figure 4.10: Mean soil pH per site. Error bars represent 1 SE

The highest mean nitrogen level was recorded on the site invaded by L. camara followed by

the managed site and lastly the non-occupied site. The non-invaded site had the least mean soil

nitrogen content (Figure 4.11). There were significant differences (P˂0.05) in mean soil

nitrogen levels between the three sites (Table 4.5) (Figure 10.11). Tukey’s HSD showed that

the sites differed significantly in their nitrogen content (Table 4.1).

5.40

5.60

5.80

6.00

6.20

6.40

6.60

Invaded Managed Non-invaded

pH

site

Soil pH

45

Figure 4.11: Mean soil total Nitrogen content per site. Error bars represent 1 SE

The L. camara invaded site recorded the highest mean phosphorous content, followed by the

managed site, and lastly the non-invaded site (Figure 4.12). Significant differences (P˂0.05) in

phosphorous content were observed amongst the three sites (Table 4.5). Tukey’s HSD showed

that all sites differed significantly in their phosphorous levels (Table 4.5).

Figure 4.12: Mean soil phosphorous content per site. Error bars represent 1 SE

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40


Nit

roge

n c

on

ten

t %

Site

Soil total Nitrogen content

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00


Ph

op

ho

rou

s co

nte

nt

mg/

kg

Site

Soil Phosphorous content mg/kg

46

Highest mean soil potassium content was recorded on the L. camara invaded site. The non-

invaded site had the least mean soil potassium content, followed by the managed site (Figure

4.13). There were significant differences (P˂0.05) in mean soil potassium levels amongst the

three sites. Tukey’s HSD showed that all the groups differed significantly (Table 4.5).

Figure 4.13: Mean soil Potassium content per site. Error bars represent 1 SE

The invaded site had the highest mean soil moisture content, followed by the managed site and

lastly the non-invaded site (Figure 4.14). There were no significant differences in moisture

content amongst the three sites (Table 4.5). Tukey’s HSD showed that the three sites had

similar moisture content (Table 4.5).

0.00

50.00

100.00

150.00

200.00

250.00

300.00


Po

tass

ium

co

nte

nt

mg/

kg

Site

Soil Potassium content

47

Figure 4.14: Mean soil moisture content per site. Error bars represent 1 SE

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00


Mo

istu

re c

on

ten

t %

Site

Soil Moisture Content

48

Chapter 5: Discussion

5.1 Vegetation classification

The vegetation of Imire Ranch is part of an upland miombo that typifies the central plateau of

Zimbabwe. It consists of deciduous miombo woodland with B. spiciformis and J. globiflora as

co-dominants (Wild and Barbosa 1968; White, 1983). The identified nine different

communities reflect variations in edaphic factors (nutrient and moisture status), anthropogenic

disturbance and grazing/browsing pressure. Campbell (1995) noted that miombo exhibits

pronounced heterogeneity which influences small scale variations in structure and species

composition. This variation, as stated above is linked to fire, herbivory, land use and soil

characteristics (Mapaure, 2001).

A. sieberiana-L. camara woodland formed a localized vegetation type within the northern

section of the ranch. It can be assumed that this woodland community represents a former A.

sieberiana-dominated woodland that has been transformed by L. camara. L. camara forms

almost impenetrable thickets within the understorey. This is the only site that is heavily

invaded by L. camara. Acacia species, like most other legumes, have a symbiotic relationship

with bacteria (Rhizobium sp.) which enables them to convert atmospheric nitrogen into a form

that plants can absorb (Timberlake et al., 1999). It is possible that L. camara is benefiting

through nitrogen provision from this association that is why it is mostly concentrated in the

with Acacia sieberiana.

A mixed Acacia woodland community was found on the southern part of the ranch. This

community is co-dominated by three Acacia species, namely A. sieberiana, A. nilotica and A.

boliviana. The first two species are indigenous, but A. boliviana is a native of South America

(Argentina, Bolivia) that was introduced as a fodder species, and has since spread from former

49

cultivated areas. Sections of the ranch with Eucalyptus represent plantation sites. L. camara is

the only other woody species occurring within the Eucalyptus plantations. Its presence is

associated with its invasive capacity, being tolerant to nutrient poor conditions associated with

Eucalyptus trees. A single bushy species frequently occurred in the open grasslands, namely A.

nilotica. The A. nilotica-C. molle bush land is a remnant community associated with heavy

browsing pressure.

Pope (1995) identified and mapped four woodland communities at Imire Ranch. These were

described as Brachystegia dominated (miombo) woodland, Julbernardia dominated woodland,

Mixed woodland and Riverine woodland. The Brachystegia dominated (miombo) woodland

occurred throughout the ranch. This was identified as the dominant community in the present

study. Pope (1995) noted that the Julbernardia dominated woodland only occurred in patches

in a band across the middle of the ranch. This community was also distinguished in the present

study.

The present study, however, employed a much finer resolution technique that managed to

reveal the heterogeneity of the miombo woodland, thus separating the woodlands into different

communities, namely B. spiciformis woodland, B. spiciformis-J. globiflora woodland and J.

globiflora-B. spiciformis mixed woodland. These minor communities reflect edaphic

heterogeneity (soil nutrient and moisture status). Within the northern section of the ranch, Pope

(1995) described a mixed woodland that comprised of Diospyros lycioides, Parinari

curatellifolia, Maytenus senegalensis, B. spiciformis, C. molle and P. africanum. This

community was not identified in the present study. Rather, the dominant vegetation

50

communities within this section were the B. spiciformis woodland and Mixed Acacia

woodland. Within a very fine scale, B. spiciformis, J. globiflora and T. sericeae appear to co-

dominate, thus forming what Pope (1995) considered as a mixed woodland. Significant

variation in tree dominance at micro-scale was observed in the northern section of the ranch,

with the frequency of such species as M. senegalensis and M. glaber varying within short

distances. Mapaure (2001) noted that elephant modified Brachystegia dominated woodlands to

T. sericea dominated woodland thickets. The impact of elephant browsing is quite noticeable

on several sites at Imire Ranch. Details of such impact, and its possible role in transforming

vegetation communities, needs to be investigated further.

One vegetation community described by Pope (1995), but not identified in the present study,

is Riverine Woodland, which according to him, occurred in small patches in the central part of

the ranch and along main drainage lines. This community was described as predominantly

consisting of tall trees and dense cover of Ziziphus mucronata and Rhus tenuinervis. Streams

and all major drainage channels were sampled in the course of the present study. Communities

in the drainage channels were a continuum of those on adjacent upper slope sites, and did not

show any association whatsoever with typical riparian vegetation.

Pope (1995) described a bush land community (Acacia karoo dominated community) from the

northern section of the ranch that occurred on red soils. This type was not confirmed in the

present study. Instead, a mixed Acacia woodland community comprising of A. siberiana, A.

nilotica and A. boliviana, intermixed with Z. mucronata, C. molle and B. africana was

identified.

51

Two grassland communities were also distinguished where Aristida sp. or wetland grasses

dominated. Pope (1995) described one grassland community which he referred to as Open

Grassland that was dominated by Cenchrus ciliaris, Enneapogon cenchroides and

Pogonarthria squarrosa. He also mapped a large expanse of cultivated land within the ranch.

Imire Game Ranch was then run as a mixed cattle, maize and tobacco estate. Game was only

introduced in the 1970s. Crop cultivation has since stopped, and the ranch converted to an

integrated cattle and wildlife ranch. The croplands have since been transformed to open

grasslands. Natural grassland communities are only confined to seasonally water-logged sites

which are frequently subjected to firing. The present vegetation classification provides a basis

for veld management and rotational browsing or grazing for the ranch.

5.2 Spatial distribution of L. camara

L. camara had highest cover and abundance in a section of the ranch colonized by A. siberiana.

The nitrogen enriched soils within the A. siberiana woodland appear to present an optimal

environment for L. camara invasion. In B. spiciformis woodlands, L. camara tended to be

confined to termite mounds, although it also occurred on open sandy soils. Termite mounds, as

already noted, have nutrient rich soils, hence the high frequency of L. camara on such sites. L.

camara clumps were also recorded in wetland grasslands. Wetlands are rich in resources,

especially soil moisture. One likely explanation for wetlands colonization by L. camara is soil

moisture availability. L. camara also occurred in mesic conditions under Eucalyptus

woodlands. This demonstrates the species’ high plasticity in environments that it occupies. L.

camara presently occupies a relatively limited area of the ranch. There seems to be some

potential for its spread into other vegetation type as the species already appears in several

vegetation communities, though in small isolated patches, except in the B. spiciformis

woodland where relatively large clumps presently occur. Since the B. spiciformis woodland is

widespread in the ranch, the likelihood of L. camara extending its range into other sections of

52

the ranch is great. Thus, despite the heterogeneity of habitats in the ranch, L. camara is likely

to extend beyond the area it currently occupies.

Elsewhere, L. camara has been reported as decreasing in abundance and spatial extent when

tree density increases (Day et al., 2003). This is attributed to decreasing light intensity, as L.

camara does not tolerate shade. Further work that assesses the interactive effects of soil factors,

shading and fire on L. camara performance is needed in order to fully understand the factors

that determine its distribution in the ranch.

5.3 Impact of L. camara invasion on herbaceous plant species diversity

L. camara had the highest impact on the invaded site in terms of both species diversity and

composition, where both were low. It is likely that the vigorous growth of the species, which

has higher canopy cover than the resident herbaceous species is responsible for this negative

impact (Pysek et al., 2007). The extensive and dense root system of L. camara plays a role in

the competitive exclusion of other species from invaded communities (Day et al., 2003). The

ability to form homogenous stands, which is typical of L. camara, seems to be another effective

means of suppressing native vegetation.

Species abundance decreased with increasing invasion of L. camara from invaded, managed

to non-invaded site in that order. The invaded site had the least Shannon Weiner (H’) value,

while the highest Shannon Weiner (H’) value was recorded in the non-invaded site. These

results support Moyo’s (2000) findings in Victoria Falls National Park, Zimbabwe. Moyo

(2000) observed a general decrease in species diversity with increasing L. camara density.

Low biodiversity recorded in the invaded site could be attributed to the fact that L. camara

produces and releases phenolic acids, flavonoids, terpenes and terpenoids, some of which are

known to inhibit the growth of other plants (Sharma et al., 1988). Achhireddy et al., (1985)

also recorded stunted or impeded growth among plants growing close to L. camara stands. The

53

reduced species diversity could also be linked to the fact that L. camara reduces recruitment

and establishment of other species (Pysek et al., 2007).

Glyphosate herbicide was used to control L. camara on the managed site. Glyphosate is a

broad-spectrum, non-selective, systemic herbicide that can control most annual and perennial

plants (Tu et al., 2001). If a non-target spraying method is used, indigenous plants and crops

may also be destroyed. This non-target spraying method could have led to the defoliation of

some herbaceous species, thus contributing to the lower diversity recorded on the managed

site. This managed site still has some L. camara, but at lower density than the invaded site. The

results demonstrate a negative association between L. camara abundance and species richness

of the study sites, validating widespread fact that L. camara invasion in natural ecosystems

lowers species diversity (Gooden et al., 2009).

Despite attempts of choosing a site with little disturbances (fire and livestock and wildlife

grazing), it is most likely that the strong association between L. camara and species richness is

coincidental. Thus, in this study, interpretations of patterns of species decline in response to L.

camara must be taken with caution.

The time the study was conducted might also have impacted the findings. The study was

conducted during winter, a period at which some of the herbaceous plants are in senescence

and difficult to observe or identify. It is recommended that any future similar studies be

conducted in the growing season when all plants are at peak growth stage. Assessing the impact

of invasive species on resident communities by comparing invaded and non-invaded sites

(Levine et al., 2003) makes it possible to collect large data sets which take into account

variations in response to invasion over a wide range of environmental attributes. However,

this approach brings about some uncertainty over the character of invaded sites prior to

invasion, i.e. the extent to which the invaded site compares with non-invaded control sites, as

54

the sites may differ in several other factors other than invasion. In the present study, the non-

invaded site was located close to the invaded site, with habitat conditions closely matching

those of the invaded site.

5.4 Impact of L. camara on rangeland production

Net primary production on a site occupied by L. camara as assessed through NDVI declined

between 2001 and 2013. It is apparent that 87% of variation in mean NDVI is associated with

invasion by L. camara since 2001. Generally, healthy vegetation absorbs much of the visible

light and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation

reflects the greater part of the visible light and less near infra-red light. Bare soils, on other

hand, reflect moderately in both the red and infrared portion of the electromagnetic spectrum

(Holme et al., 1987). As such, low NDVI values mean there is low vegetation cover, and in

turn low productivity. NDVI readings are also affected by cloud contamination, atmospheric

perturbations, and variable illumination and viewing geometry, all of which tend to reduce

mean NDVI (Los et al., 1994). These factors need also to be considered in the interpretation of

the data.

Perry et al., (2009) attributed changes in vegetation communities to micro-factors like soil

moisture, nutrient levels and pollution status. Disturbed environments have reduced primary

productivity. Thus the decrease in NDVI may not be solely attributed solely to L. camara

invasion. A possible reason for the reduced NDVI of the invaded site might be overgrazing.

Several wildlife species and livestock are kept in the ranch. This may lead to overgrazing on

some sites. In a study by Bryan et al., (2009), disturbances, such as timber harvesting and

livestock grazing, were shown to influence NDVI values by facilitating the establishment of

forest floor plants. Variation in rainfall can also contribute to a decline in NDVI. When an area

55

is receiving below normal rainfall, this may lead to reduced moisture availability, hence

reduced ground cover.

5.5 Impact of L. camara on soil physico-chemical properties

L. camara affects the dynamics and composition of soil, and has some impact on ecosystem

functioning, especially soil nutrient cycling (Yelenik et al., 2007). Soil physico-chemical

properties differed significantly across sites, except for soil moisture content.

The higher soil pH levels recorded in L. camara infested soils are similar to those reported in

China (Fan et al., 2010) and India (Sharma and Raghubanshi, 2009). Although the higher pH

levels on the invaded site might be attributed to L. camara invasion, it still needs to be

ascertained whether the invasive species does not simply prefer higher pH environments. Thus,

the precise mechanism for the pH elevation in L. camara-infested soils requires further

investigation, including the contribution of its biomass and associated chemical exudates.

Nitrogen, phosphorus and potassium were highest in the invaded site, followed by the managed

site and lowest in the non-invaded site. Sites invaded by L. camara were recorded as being

richer in nutrients (Sharma and Raghubanshi 2009; Ehrenfeld, 2003). L. camara architecture

promotes accumulation of litter under the shrub, resulting in build-up of nutrients, including

nitrogen (Sharma and Raghubanshi, 2009). The high nitrogen levels on the invaded site are

attributed to nitrogen fixation by A. sieberiana and then immobilization by L. camara which

further releases it as part of litter fall. The increase in nitrogen and phosphorous levels in the

L. camara infested site is attributed to a decrease in nutrient sequestration following native

species displacement as a consequence of L. camara invasion. Nitrogen mineralization and

nitrification commonly increase in response to invasions (Ehrenfeld, 2003), which could

explain the increase in total nitrogen that was observed in the invaded site.

56

The managed site was under A. sieberiana, so was the non-invaded site which was in a scattered

A. sieberiana grassland. The two sites recorded lower nitrogen content than the invaded site.

This then could mean that the increased soil nitrogen content had something to do with L.

camara. A study by Stohlgren et al., (1998) reported some positive correlation between level

of invasion, soil nitrogen content and silt content. It was recently suggested that invasibility

was related to resource availability (Davis et al., 2000). Vranjic et al., (2000) recorded major

differences in soil phosphorous and potassium concentrations between L. camara invaded and

non-invaded sites. Levels of the soil nutrients were higher in the invaded site in this study.

There was no significant difference in mean soil moisture between the managed and non-

invaded sites. However, relatively higher mean soil moisture content was recorded in the

invaded site. These findings are consistent with observations by Ehrenfeld (2003) who noted

that soil moisture could either increase or decrease following invasion. In the present study,

higher moisture levels were recorded, but these were not significant. Sharma et al., (2003) also

observed that L. camara invasion increased moisture availability by 3 to 6%. However, results

from the present study contradict those of Hiremath and Sundaram, (2005) who observed that

L. camara negatively affected moisture availability. Results from the present study are

attributed to shading by L. camara thickets that reduces soil evapo-transpiration.

5.6 Conclusion

The study showed that Imire Ranch has been invaded by L. camara and this has reduced the

productivity of the ranch and the herbaceous species diversity. Soil pH, Ni, P and K has

increased in the invaded site while soil moisture content has slightly increased. L. camara has

potential to colonize most of the vegetation types within the ranch, which will result in the

decline in the productivity of the ranch.

57

5.7 Recommendations

There is an urgent need to come up with sustainable ways of controlling the spread of L. camara

into other vegetation types. An integrated approach that will involve mechanical and chemical

control is recommended although the use of glyphosate must be avoided since it can also

negatively affect some other native plants.

58

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impact of lantana camara invasion on a cattle/wildlife ... · lantana camara, is a shrub species...

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