Legacy Effects of Different Land-UseHistories Interact with CurrentGrazing Patterns to DetermineGrazing Lawn Soil Properties

Hugo Valls Fox,1,2,6* Olivier Bonnet,1 Joris P. G. M. Cromsigt,3,4

Herve Fritz,5 and Adrian M. Shrader1

1School of Life Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, South Africa; 2Biogeochimie et Ecologie desMilieux Continentaux, UMR 7618, Ecole Normale Superieure, 46 rue d’Ulm, 75230 Paris Cedex 05, France; 3Department of Wildlife,

Fish, and Environmental Studies, Swedish University of Agricultural Sciences, 901 83 Umea, Sweden; 4Centre for African Conser-

vation Ecology, Department of Zoology, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth, South Africa;5Laboratoire de Biometrie et Biologie Evolutive, UMR 5558, Bat. Mendel, Universite Lyon 1, 43 bd du 11 novembre 1918, 69622

Villeurbanne, France; 6Centre d’Ecologie Fonctionnelle et Evolutive, CEFE UMR 5175, CNRS - Universite de Montpellier - Universite

Paul-Valery Montpellier – EPHE, Campus du CNRS, 1919 route de Mende, 34293 Montpellier 5, France

ABSTRACT

Pastoralism and agriculture have affected range-

land ecosystems over the past millennia, including

many ecosystems that are currently protected as

reserves. However, the legacy of these land-use

practices on current ecosystem functioning remains

unclear. We studied legacy effects of former human

land use on soil physical and chemical properties in

a South African savanna. We did this by comparing

soil properties in grazing lawns (patches of short

grass maintained by the positive feedback between

grazing intensity and forage quality) with the sur-

rounding less grazed bunch grasslands within three

different human land-use history contexts: (i)

Abandoned bomas: permanent stone enclosures

where livestock were kept overnight, and dung and

urine accumulated for several years or decades. (ii)

Old fields: areas where vegetation was cleared, soil

tilled, and cultivated, but received little or no fer-

tilization. (iii) Natural grasslands: not cultivated but

grazed by livestock before the establishment of the

reserve and wildlife thereafter. Former human land

use rather than soil texture was the main deter-

minant of grazing lawn location. Moreover, lawn

soil properties also varied among land-use histories.

In all grazing lawns, soil nutrient concentrations

were higher than in adjacent grasslands but aban-

doned bomas contained three times more phos-

phorus, and twice as much nitrogen and carbon

than old fields and natural grassland lawns. In ad-

dition to past land use, soil texture influenced lawn

soil nutrients: Concentrations of phosphorus,

potassium, calcium, magnesium, total nitrogen,

and carbon in lawns were higher on clayey soils

than sandy soils, whereas phosphorus, C:N ratio,

and pH did not change with soil texture. Our study

confirms previous findings on the effect of human

land use on savanna heterogeneity, but also high-

lights how legacy effects may vary among different

historic land-use practices.

Key words: savanna; boma; nutrient cycling;

Ithala Game Reserve; herbivory; resource hotspots.

Received 11 June 2014; accepted 27 January 2015

Author contributions O. Bonnet, J. P. G. M. Cromsigt, and H. Fritz

came up with the initial idea; H. Valls Fox, A. M. Shrader, and O. Bonnet

designed the study; H. Valls Fox and O. Bonnet collected and analyzed

the data and all authors contributed to the manuscript.

*Corresponding author; e-mail: hugo.valls@cefe.cnrs.fr

EcosystemsDOI: 10.1007/s10021-015-9857-x

� 2015 Springer Science+Business Media New York

INTRODUCTION

Pastoralism and cultivation have substantially

changed vegetation patterns and nutrient distri-

butions across African savannas over the past mil-

lennia (Prins 2000). Moreover, many areas

currently dedicated to wildlife conservation have

historically been affected by these practices (Ellis

and Ramankutty 2008). Several studies have

shown that former human land-use practices may

influence foraging patterns of wild savanna grazers

after agricultural activities have stopped, with po-

tentially large cascading effects on other parts of the

ecosystem (Blackmore and others 1990; Young and

others 1995; Donihue and others 2013). The legacy

of former pastoralism and cultivation may persist

for decades (McLauchlan 2007) or even millennia

(Cobo and others 2010). However, in this study, we

show that the legacy effects of livestock husbandry

and cultivation may differ considerably.

A typical practice of pastoralism in African sa-

vannas is the overnight containment of animals in

enclosed bomas (or corrals) to protect livestock

against predation. This practice leads to prolonged

accumulation of dung and urine in localized areas

from months to decades resulting in elevated soil

nutrient concentrations and organic matter content

for decades after boma abandonment (Augustine

2003). Bush clearing is also a central practice in the

creation of boma sites, creating open glades with a

specific herbaceous and woody vegetation com-

munity in an often denser savanna matrix

(Porensky and Veblen 2012). In comparison, the

legacy of cultivation differs substantially from that

of bomas. Native vegetation is removed during the

cultivation of fields. In contrast to bomas, cultiva-

tion often leads to prolonged nutrient and soil

organic matter loss caused by the replacement

of native vegetation by crops, biomass removal,

and especially tilling (Roberts and others 2003;

McLauchlan 2007).

After abandonment, wild grazers may select for

and maintain former boma sites as open, nutrient-

rich resource hotspots, or grazing lawns (Young

and others 1995). Grazing lawns are conspicuous

patches of short, mat-forming, grazing-tolerant

grass species resulting from repeated cropping by

herbivores (Figure 1; McNaughton 1984; Cromsigt

and Kuijper 2011). Continuous grazing in these

lawns keeps grass short and sustains high-quality

forage, generating a positive feedback between

herbivore consumption and the nutritional value of

the grass (McNaughton 1979, 1984; Bonnet and

others 2010). Localized dung and urine deposition

by herbivores strengthens this feedback by accel-

erating the rate of nutrient cycling in the top soil of

grazing lawns and facilitating sustained plant

growth (Ruess and McNaughton 1987), likely me-

diated by increasing microbial activity (Bardgett

and Wardle 2003; Sankaran and Augustine 2004;

de Vries and others 2012). The intense grazing also

shifts the grass community toward more grazing-

tolerant and palatable species with higher litter

quality which may further accelerate soil nutrient

cycling (Bardgett and Wardle 2003; Cromsigt and

Kuijper 2011). Although grazing lawns were ini-

tially described in the Serengeti plains in Tanzania

(McNaughton 1984) and not originally associated

with abandoned bomas, more recent studies sug-

gest that the short grass hotspots in former boma

sites are functionally similar to the lawns, as

originally formalized by McNaughton (Cromsigt

and Kuijper 2011).

In contrast, we know much less about the effects

of abandoned fields on the foraging patterns of wild

savanna grazers and to what extent these grazers

may maintain grazing lawns in old fields. The cre-

ation of open habitat through bush clearing and the

removal of tall grass in old fields create the neces-

sary starting conditions to initiate the development

of grazing lawns (Cromsigt and Olff 2008). Similar

to former boma sites, the open habitats in old fields

may attract herbivores by reducing predation risk

(Ford and others 2014), whereas the removal of tall

caespitose grasses facilitates colonization by higher

quality lawn grass species (Huisman and Olff 1998;

Cromsigt and Olff 2008; Novellie and Gaylard

2013). Once grazed upon, herbivores would be

Figure 1. An example of a typical grazing lawn from the

study area. During the growing season, the mat-forming

Cynodon dactylon grasses are cropped daily by resident

grazers. Note the sharp edge with the surrounding bunch

grass matrix (Sporobolus sp.).

H. Valls Fox and others

more likely to return to these sites comprising

younger, more nutritious forage (Bonnet and oth-

ers 2010), thus initiating a positive feedback known

as grazing optimization (de Mazancourt and others

1998). This process may then initiate a similar de-

velopment of grazing lawns on old fields as on

former boma sites. The contrasting effects of culti-

vation and bomas on nutrient and organic matter

build-up should, however, lead to grazing lawns

with quite contrasting post-agro-pastoral biogeo-

chemical signatures. Whereas lawn soils on former

boma sites should be enriched with organic matter

and less volatile nutrients such as phosphorous, we

predict that lawn soils on old fields do not show this

enrichment. In addition, we currently lack under-

standing of the spatial extent at which land use

affects soil parameters and resulting grazing lawns.

This is particularly relevant, because this scale may

differ between land-use types, with old fields

being significantly larger than boma sites (Lewu

and Assefa 2009).

Soil parent material likely modifies the legacy

effects of different land uses (McLauchlan 2007).

Theoretical (de Mazancourt and others 1998) and

empirical evidence (Ruess and McNaughton 1987)

suggests that the positive feedback between herbi-

vores, plants and soil biota characterizing grazing

lawns should only come into being in nutrient-rich

contexts (Bardgett and Wardle 2003). Several

studies indeed confirmed that nutrient-rich soil

patches favored the formation of grazing lawns

(Cromsigt and Olff 2008) and were associated with

their distribution and persistence (Scholes and

Walker 1993). A recent study discussed another

important aspect of parent material, namely, soil

texture as a potential driver of soil compaction

(Veldhuis and others 2014). This builds upon pre-

vious studies that showed that grazing tolerance in

grass species may in fact reflect adaptations to low

water availability (Coughenour 1985). Veldhuis

and others (2014) discuss how herbivore trampling

drives soil compaction of lawn soils, hereby

reducing water infiltration rates and favoring

the more drought-tolerant lawn grass species,

specifically on fine textured soils. Indeed, grazing

lawns were first described on fertile, fine textured

volcanic soils of the Serengeti plains (McNaughton

1984). However, other studies have shown that

resource hotspots derived from old bomas have

been described across a wide range of soil types,

including fine textured clayey soils (Veblen 2012),

loamy soils (Young and others 1995; Augustine

2003), and coarser sandy soils (Cech and others

2010; van der Waal and others 2011). We currently

lack studies that describe grazing lawn soil prop-

erties for different former land uses and with var-

iation in soil texture.

Our aim was to quantify the relative importance

of human land-use legacy and soil parent material

(especially texture) on the soil nutrient and carbon

status of grazing lawns, and to compare legacy ef-

fects among different land uses. We conducted our

study in Ithala Game Reserve, South Africa, which

provided us with a unique setting that combined

different past land-use practices with contrasting

soil geological origins within a relatively small area.

Moreover, as we were unsure about the spatial

extent of past land-use legacies and expected this

extent may differ between the larger old fields and

smaller bomas, we compared soil from within in-

tensively used grazing lawns with close as well as

far away tall grassland controls (10 m and 200 m

away from the lawn, respectively). To assess the

legacy effects of different land-use types, we com-

pared lawns found in abandoned bomas, old fields,

and natural grasslands. Natural grasslands are areas

comprising indigenous grasses and forbs that are

grazed by wildlife and livestock (Allen and others

2011). Based on the different impacts to soil nu-

trients and texture, we predicted that the properties

of grazing lawns would differ with past land use.

We expected large stocks of soil nutrients and car-

bon in former bomas to persist long after their

abandonment and attract herbivores to these

lawns. Grazing lawns in old fields would have

originated from the rapid onset of grazing opti-

mization after herbivores aggregated in the large

disturbed grasslands that emerged and not from

large stocks of soil nutrients and carbon. These

differences may then be further mediated by var-

iation in soil texture, where lawns should be pre-

dominantly found on fertile fine textured soils if

nutrient limitation drives grazing patterns.

MATERIALS AND METHODS

Study Area

We conducted the study in Ithala Game Reserve

(hereafter Ithala), KwaZulu-Natal, South Africa

(27�30¢S; 31�20¢E). The 30,000 ha reserve com-

prises a series of deep valleys running from Ngotshe

Mountain (alt. 1446 m) to the Pongola River (alt.

320 m). Soils are generally shallow and undiffer-

entiated ranging from coarse textured (<15% clay)

dystrophic regosols of granitic origin in the east to

more fine-grained (15–35% clay) and eutrophic

leptosols soils in the center and west (Figure 2A; van

Rooyen and van Rooyen 2008; Dewitte and others

2013). Climate is sub-tropical. From 1973 to 2010,

Legacy of Land Use on Grazing Lawn Soil Properties

annual rainfall ranged from 394 to 1164 mm

(mean = 763 mm, CV = 27%) and 82% of rainfall

occurred during the rainy season from October to

March. Mean daily temperatures range from 14 to

27�C in January during the rainy season to 4–21�Cin July during the cold dry season.

Ithala contains a wide variety of habitat types.

Open areas comprise 37% of the reserve. These

include the following: Natural grasslands which

comprise 19% of the reserve and are dominated by

the grass species Trachypogon spicatus, Tristachya

leucothrix, Hyparrhenia hirta, Senecio microglossus, and

Bewsia biflora; Old field grasslands that comprise 9%

of the reserve and are dominated by the grass

species Hyparrhenia hirta and Sporobolus africanus,

and 9% riparian vegetation dominated by the tree

Syzygium cordatum and sedge Cyperus sexangularis.

The most dominant habitats are the dense bush-

veld/thickets comprising 52% of the reserve, and

containing the tree species Acacia nilotica, Combre-

tum apiculatum, Euclea schimperi, Greyia sutherlandii,

and grasses such as Tristachya leucothrix, Melinis

nerviglumis, Eragrostis curvula, and Themeda triandra.

Woodlands comprise 11% of the reserve and con-

taining both trees (Nuxia oppositifolia, Dovyalis zey-

heri, Salix mucronata, Dalbergia armata, and

Combretum hereroense) and the grasses (Phragmites

australis, Imperata cylindrica, Paspalum urvillei, and

Paspalum dilatatum) (van Rooyen and van Rooyen

2008). Main grazers in the reserve include white

rhinoceros (Ceratotherium simum c. 41 ind.), plains

zebra (Equus quagga c. 1600 ind.), wildebeest

(Connochaetes taurinus c. 1044 ind.), warthog (Pha-

cochoerus aethiopicus c. 2000 ind.) and impala

(Aepyceros melampus c. 2200 ind.).

Until the establishment of the game reserve in

1972, Ithala was a patchwork of fields (now termed

old field grasslands) and natural grassland areas

interspersed by homesteads each containing en-

closed stone wall corrals (or bomas) where live-

stock were kept at night. Van Rooyen and Van

Rooyen (2008) identified 111 old field grassland

patches across Ithala, ranging from 0.2 to 250 ha

with a mean of 22 ha and median of 7.1 ha. These

old field grassland patches would consist of multi-

ple associated fields and the average size of indi-

vidual old fields in Ithala was therefore likely

around 1–2 ha and similar to current small-scale

farming in KwaZulu-Natal, South Africa (Lewu and

Assefa 2009). Hence, old fields were much larger

than bomas that were typically between 10 and

30 m radius. Although we do not have data on the

lifetime of such bomas for Ithala, we know from

similar historic and current practices elsewhere in

Africa that they are likely used for several decades

if not longer (Blackmore and others 1990; Young

and others 1995). Moreover, the fact that in Ithala

bomas were encircled with stonewalls and not

thorny branches suggests long-term use. Native

grazers were absent during this period, but subse-

quently reintroduced as the farms were abandoned

and the fenced reserve expanded (van Rooyen and

van Rooyen 2008). Ultimately, this varied land-use

history provided us with a unique opportunity to

compare grazing lawns found in areas with differ-

ent land-use histories within a single reserve.

Figure 2. A A three-class representation of soil texture in lawns and control sites. Soil texture was coarser (<15% clay)

for the dystric regosols of granitic origin in the east and finer (15–35% clay) for eutric leptosols in the West, FAO/USDA

standard abbreviations are used: Cl clay, Si silt, Sa sand, and Lo loam. The soil texture R package v.1.2.13 was used for this

plot (Moeys and Shangguan 2014). B Mean percent sand ± SD for all sites according to geology (west: light bars and East:

dark bars) and land use (old bomas, natural grasslands and old fields). Tukey Contrasts: groups sharing the same letter were

not significantly different (P > 0.05).

H. Valls Fox and others

Grazing Lawn Selection According toLand-Use History

During the wet season, during which we sampled,

grazing lawns contrasted sharply with surrounding

taller bunch grassland (Figure 1) and mostly con-

sisted of short monospecific patches of Cynodon

dactylon (mean height = 4 cm). We know from

previous work in a similar savanna system near

Ithala that during the wet season grazing lawns

need to be intensely grazed, nearly daily, to remain

in such a short state (Cromsigt and Olff 2008;

Bonnet and others 2010). Moreover, from other

work by some of the co-authors, we know that in

Ithala white rhinoceros spend nearly all of their

foraging time in grazing lawns (Bonnet and Shra-

der unpublished data).

We first located grazing lawns using high-

resolution aerial photographs (1/30,000) from 2003

taken by the South African Department of Land

Affairs. Grazing lawn presence was then confirmed

on foot in 2009 and 2010. This mapping exercise

showed that within Ithala grazing lawns were

generally smaller than 0.5 ha and only covered

0.14% of the reserve. Grazing lawns were pre-

dominantly found in open old field grasslands

where herbivore densities were highest (pers. ob-

servation) and to a lesser extent natural grasslands

(Table 1). Former bomas were distinguished from

other grazing lawns in the field based on evidence

of human occupation. We limited our study to

boma lawns that were found within the remains of

rectangular stone walls. To differentiate between

old fields and natural grasslands sites, we used the

vegetation map generated by van Rooyen and van

Rooyen (2008). To generate their map, van Rooyen

and van Rooyen (2008) used both physical features

of the landscape (for example, terraced rows on

predominantly north-facing gentle slopes) and

present grass species composition to determine the

location of the old field grasslands across the re-

serve. Because most of Ithala has been subjected to

human land use, grazing lawns in natural grass-

lands were scarce; however, we often found they

were located close to natural nutrient hotspots such

as termite mounds or white rhinoceros dung mid-

dens.

To further confirm that our grazing lawn classi-

fications did indeed reflect different previous land-

use histories, we used (1/30,000) aerial pho-

tographs from 1969 to 1976 in order to confirm

that the lawns used in our study matched land use

at the time of the proclamation. We then confirmed

the classification was coherent with the oldest

series of photographs we were able to exploit dat-

ing from 1948; indicating these land uses had per-

sisted for at least two or three decades. Nonetheless,

it is possible that some of the grazing lawns in the

old field grasslands had been boma sites prior to

1948. Although bomas were also found in old field

grasslands, we restricted our choice of boma lawns

to those that clearly fell outside of old fields to

avoid possible confounding effects between these

two land-use histories. As a result, by selecting

these boma sites we reduced the possibility of a link

between old field lawns and boma lawns.

In total, we sampled 32 grazing lawns (13 boma

sites, 8 old field sites, and 11 natural grassland

sites). We only sampled grazing lawns larger than

50 m2 to limit edge effects. Each grazing lawn was

paired with two control grassland sites, one close

(10 m from the chosen lawn) and one far from the

lawn (200 m from any grazing lawn). We took a

close and far away control because we were unsure

of the spatial extent of the legacy of agricultural

land use, and because we expected that this extent

might differ between the larger old fields and the

smaller bomas. To limit confounding effects, both

controls were sampled on the same day as the

grazing lawn and were located in random direc-

tions from the grazing lawn, while remaining in the

same landscape unit (for example, past land use,

vegetation type, and hillside).

Table 1. Main Habitat Types in Ithala Game Reserve

Habitat type Surface area (ha) Surface area (%) Lawn surface area (ha) Grazing lawn cover (%)

Bushveld/woodland 18,581 63 12 0.06

Old field grassland 2535 9 18 0.71

Natural grasslands 5658 19 10 0.18

Riparian areas 2509 9 1 0.06

Total 292,831 100 41

The proportion of each habitat covered by lawns is expressed in m2 ha-1.1Built-up areas and cliffs inaccessible to grazers were excluded.

Legacy of Land Use on Grazing Lawn Soil Properties

Soil Collection and Analysis

We collected soil samples from February to March

2011. For each site, we collected five top soil sub-

samples (depth: 0–5 cm) and three deep soil sub-

samples (depth: 25–30 cm). We collected all

samples from a 10 m 9 5 m rectangular area using

a steel cylinder (height = 5 cm, diameter =

7.5 cm). Subsamples were pooled in the field and

stored in open plastic bags to air dry at room tem-

perature (�25�C) for about 3 days. Prior to analy-

sis, we removed any gravel from the samples.

Pooled samples were analyzed using standard

methods by the Department of Agriculture Soil

Fertility Laboratory at Cedara, KwaZulu-Natal,

South Africa (Manson and Roberts 2011). pH, cal-

cium, magnesium, (Ambic-2) phosphorus, and

potassium were analyzed through KCl extraction

following Hunter (1975). Total nitrogen and carbon

were measured by automated dry dumas combus-

tion using a LECO TruSpec CN analyzer (Leco

Corporation, Michigan, USA; Matejovic 1997).Soil

texture (3 classes) was determined hydrometrically

(Day 1965).

Statistical Procedure

To account for spatial correlation between lawns,

close and far sites, we analyzed variation in soil

properties using mixed linear models (Zuur and

others 2009) computed with the nlme package in R

(Pinheiro and others 2011). We used percent sand

as a proxy for soil texture because percent sand was

negatively correlated with the other soil fractions

(cor = -0.89 clay and cor = -0.80 silt). We applied

a logit transformation to percent sand to maintain

homogeneity of variance. Prior to analysis, we

natural log-transformed soil nutrient concentra-

tions to improve the homogeneity of variance. We

did not transform pH and C:N ratio, as ho-

moscedasticity was met. Soil samples from each

lawn and their paired controls were defined as

triplets for the random effect to account for spatial

correlation and timing of soil collection. Likelihood

ratios confirmed that adding this random effect

increased model fit (P < 0.0001, for all variables).

Fixed effects included the covariate soil texture

(percent sand), and three factors: distance (lawn,

close control, far control), land use (boma, old field,

natural grassland), and depth (top soil, deep soil) as

well as the interactions between these variables.

We compared fixed effects using Akaike’s Infor-

mation Criterion (AIC). When the best models had

similar AICs (DAIC < 2), the model with least in-

teractions was preferred. The final model was re-

calculated using restricted maximum likelihood

(REML) as this method provides more accurate

estimators. The significance of each independent

variable was finally evaluated using Wald F tests.

The contribution of different factor levels was es-

timated using Tukey contrasts with the multcomp

package (Hothorn and others 2008). When inter-

actions rendered these comparisons meaningless,

the model was recalculated for each land use (bo-

ma, old field and natural) or depth class (top soil

and deep soil) by taking subsets of the data corre-

sponding to these classes.

RESULTS

Legacy Effects of Different Land-UseHistories

Overall, boma lawn soils had higher concentrations

of P, N, and C than old field and natural grassland

lawn soils (Table 2). P concentration in the topsoil

of boma lawns (122 ± 16 mg kg-1) was about

Table 2. Main Fixed Effects of Mixed Linear Models for Soil Nutrient and C Concentrations, and pH, F, andP Values are Reported for Wald F tests

Soil Land use Distance Depth Soil texture

df F P df F P df F P df F P

K ns ns ns 2.61 31.3 <0.0001 1.88 164 <0.0001 1.88 18.2 <0.0001

Ca ns ns ns 2.62 20.4 <0.0001 1.90 66.3 <0.0001 1.90 23.4 <0.0001

Mg ns ns ns ns ns ns 1.90 7.14 0.0090 1.90 60.4 <0.0001

P 2.29 4.61 0.018 2.58 49.9 <0.0001 1.90 96.8 <0.0001 ns ns ns

N 2.29 4.57 0.019 2.62 6.13 <0.0001 1.89 191 <0.0001 1.89 57.1 <0.0001

C 2.29 3.54 0.042 2.58 1.38 0.26 1.89 196 <0.0001 1.89 50.9 <0.0001

C:N 2.29 0.72 0.49 2.58 33.4 <0.0001 1.87 15.4 0.0002 1.87 1.93 0.17

pH ns ns ns 2.62 39.7 <0.0001 1.89 11.1 0.0013 ns ns ns

Non-significant variables (ns) were removed from the final model.

H. Valls Fox and others

three times higher than in natural grassland lawns

(45 ± 14 mg kg-1) and old field lawns (39 ± 21

mg kg-1) (Figure 3D). Similarly, top soil N con-

centrations in boma lawns (4.7 ± 0.7 g kg-1) were

about twice the level of N concentrations in natural

grassland (2.4 ± 0.4 g kg-1) and old field lawns

(2.1 ± 0.5 g kg-1) (Figure 3E). This pattern also

held for total carbon (bomas: 48 ± 7 g kg-1, nat-

ural grasslands: 27 ± 4 g kg-1, old fields: 25 ± 6

g kg-1) (Figure 3F). In contrast, the C:N ratio was

lower in boma lawns (10.4 ± 0.3) compared to

lawns in natural grasslands (11.2 ± 0.4) and old

fields (11.9 ± 0.4) (Figure 3H; Table 2). We found

no effect of past land use on lawn soil properties in

terms of K, Ca, Mg, and pH (Tables 2, 3; Figure 3).

Grazing lawn soils had higher concentrations of

P, N, K, Ca, higher pH, and lower C:N ratio com-

pared to the close (10 m) and far (200 m) control

soils (Figure 3; Table 2). The only nutrient that did

not vary among lawn and control soils was Mg

(Tables 1, 2; Figure 3C). However, the spatial ex-

tent of the elevated soil nutrient concentrations

varied among nutrient type and across the different

land-use histories (Table 3). With regard to P, boma

lawns had concentrations that were ten times

higher than controls, whereas for lawns in old

fields and natural grasslands P concentrations were

only four times higher (Figure 3D). Concentrations

of C and N were significantly higher in boma lawns

compared to close controls (C dist: F2,24 = 4.0,

P = 0.031; N dist: F2,24 = 7.4, P = 0.003), but not

for lawns in old fields or natural grasslands: C (dist:

F2,36 = 2.5, P = 0.095) and N (dist: F2,36 = 3.2,

P = 0.054). This means that only boma lawns rep-

resented nutrient hotspots for N, P, and C com-

bined, whereas old field and natural grassland

lawns were only hotspots for P (albeit to a lesser

degree than boma lawns). Moreover, for some

nutrients, the patterns did not differ between the

different land-use histories. For example, top soil

(z = -0.21 and P = 1) and deep soil (z = 0.15 and

P = 0.42) concentrations of K did not differ be-

tween lawns and close sites, but these were more

than twice the K concentrations recorded at the far

sites across all land-use histories (Figure 3A). In the

case of Ca, concentrations in lawns of the different

land-use histories were 1.5 higher than close con-

trols (z = 3.3 and P = 0.003), which in turn were

1.2 times higher than far controls (z = -3.1 and

P = 0.006, Figure 3B).

Finally, with regard to soil depth, differences in

P, K, and C:N between top and deep soils were

more pronounced in the far control soils than in

the lawn soils of the different land-use histories

(Figure 3). In contrast, for Ca and Mg, patterns

varied between lawns and controls (see dis-

tance 9 depth in Table 2; Figure 3). Lastly, we

found that deep soil pH in the grazing lawns of the

different land-use histories was generally a full pH

unit higher than in the close controls (z = 8.03,

P < 0.001) (Table 2; Figure 3H), whereas the top

soil pH of the lawns was similar to close sites

(z = 1.64, P = 0.56). However, top and deep soils

pH did not differ for close (z = 0.79, P = 0.97) or far

away control sites (z = 1.85, P = 0.42) (Table 2;

Figure 3H).

The Influence of Soil Texture on GrazingLawn Soil Properties

Grazing lawns covered a wide range of soil textures

that were not associated with specific land-use

histories, but rather with the two distinct geological

areas in Ithala with coarse granitic soils in the East

and finer soils of volcanic origin in the West

(F1,28 = 73.7, P < 0.0001; Figure 2A). Soil texture

did not differ between grazing lawns and either

controls (F2,62 = 1.15, P = 0.32), nor did it differ

according to land use (Figure 2B). However, most

soil nutrient concentrations varied with soil tex-

ture. Clayey soils had higher concentrations of K,

Ca, Mg, total N, and C than sandy soils for lawns as

well as control sites (Tables 2, 3; Figure 4A–F). In

contrast, P, C:N, and pH were not influenced by soil

texture (Tables 2, 3; Figure 4D, G, H).

DISCUSSION

Former land use was the key factor explaining

grazing lawn location in Ithala. More than 40 years

after the reserve was established, three quarters of

grazing lawns were found on locations that were

substantially altered by former livestock husbandry

(bomas) or agriculture (old fields) (Table 1). Fur-

thermore, soil nutrient content in the grazing lawns

was influenced by both former land use and soil

texture. Specifically, P concentrations in the grazing

lawns found in abandoned bomas were three times

higher than in lawns found in old fields or natural

grasslands. In addition, N and C concentrations were

twice as high in boma lawns compared to the lawns

in the other locations (Figure 3). On clayey soils,

grazing lawns were hotspots for all measured soil

nutrients, whereas on sandy soils they only differed

from controls for P, C:N, and pH (Figure 4). By

collecting samples within and away from grazing

lawns, we found that soils from boma lawns con-

trasted more strongly with close controls than old

fields and natural grassland lawn soils, particularly

for N, P, and C.

Legacy of Land Use on Grazing Lawn Soil Properties

Figure 3. Mean soil

potassium (K), calcium

(Ca), magnesium (Mg),

phosphorus (P), carbon

(C), nitrogen (N), C:N

ratio, and pH. Bars are

grouped from left to right

according to land use: old

bomas, natural

grasslands, and old fields.

Colors indicate distance:

grazing lawn (dark gray),

close (light gray

d = 10 m), and far (white

d = 200 m). Soil depth is

indicated by the shading of

the bars: top soil (0–5 cm;

full bars) versus deep soil

(15–30 cm; shaded bars).

Error bars represent 1

standard deviation.

H. Valls Fox and others

Legacy Effects Differ Between Types ofFormer Human Activities

The sharp boundary that separates abandoned bo-

ma grazing lawns from control sites is consistent

with previous work from South Africa (Blackmore

and others 1990) and East Africa (Young and oth-

ers 1995; Augustine 2003; Cech and others 2010).

Even after 40 years of abandonment, the boma

sites in our study area were characterized by higher

soil organic content associated with a unique, thick

top soil horizon created by the accumulation of

manure. In contrast, grazing lawns found in nat-

ural grasslands and old fields exhibited remarkably

lower P, Ca, total N, and C than lawns found in

former bomas. All lawns had similarly elevated le-

vels of pH compared with control soils. These re-

sults thus suggest that the type of land use

influences grazing lawn characteristics.

Several studies point out that nutrient concen-

trations in bomas gradually decrease due to a ne-

gative nutrient balance (Young and others 1995;

Augustine 2003; Muchiru and others 2009). Ac-

cordingly, we cannot rule out that grazing lawns

found in old fields or natural grasslands do not

originate from even older bomas pre-dating our

1948 aerial pictures. However, as explained in the

methods, we avoided selecting old boma sites in, or

close to, old fields (for example, avoiding sites with

remains of stone walls or old grinding stones). The

fact that the practice in Ithala had been to encircle

bomas with stone walls, and not thorny bushes as

elsewhere in Africa (Young and others 1995) made

it possible for us to avoid even very old boma sites.

Thus, we reduced the possibility of an overlap be-

tween old field lawns and very old abandoned

bomas. In addition, because the fields we selected

had been cultivated for at least 2–3 decades, we

believe that even if some fields would have been

very old boma sites, legacy effects of these bomas

would have been strongly diluted through decades

of quite intense cultivation (including the creation

of terraces and annual plowing).

One could also argue that bomas and old fields

were originally restricted to different parts of the

landscape. For example, if bomas had been located

in good grazing areas for cattle, which might still be

very attractive for wild grazers, this may explain

the difference in legacy effect with old fields.

However, we did not find a correlation between

land-use history and soil texture (Figure 2), nor

with the other soil properties we measured (see far

controls Figure 3). Moreover, bomas in Ithala were

not necessarily directly linked to the best grazing

areas but instead found in the vicinity of home-

steads. These homesteads were on slopes and crests,

possibly to avoid risk of diseases prevalent in the

valleys where the good grazing lands are situated

(Feely 1980). Livestock were then herded to good

grazing areas during the day and brought back to

the boma during the night. Hence, we do not think

differences in the positioning of boma and old field

sites explain their difference in legacy effects.

Soil nutrient and carbon hotspots may contain a

substantial proportion of the total stocks of a given

ecosystem (Ostle and others 2009). This might be

the case for former bomas and grazing lawns de-

rived from them. Unfortunately, we lacked soil

bulk density measures to come to reliable estimates

of ecosystem-scale contributions of lawns to nu-

trient and carbon stocks. However, doing a rough

calculation, grazing lawns cover less than 0.2% of

Ithala (Table 1) and P concentrations in lawns were

on average eight times higher than controls. Based

on these estimates, the legacy of human land use

Table 3. Interactions Between Main Fixed Effects of Mixed Linear Models for Soil Nutrient and C Con-centrations, and pH, F, and P Values are Reported for Wald F tests

Soil Distance 9 land use Distance 9 depth Texture 9 depth

df F P df F P df F P

K ns ns ns 2.88 8.71 0.0004 ns ns ns

Ca ns ns ns ns ns ns ns ns ns

Mg ns ns ns ns ns ns ns ns ns

P 4.58 4.40 0.0036 2.89 6.86 0.0017 ns ns ns

N ns ns ns ns ns ns 1.89 11.5 0.0010

C 4.58 2.57 0.047 ns ns ns 1.89 15.5 0.0002

C:N 4.58 3.37 0.015 2.87 4.98 0.0090 1.87 13.7 0.0004

pH ns ns ns 2.89 30.5 <0.0001 ns ns ns

Non-significant variables (ns) were removed from the final model.

Legacy of Land Use on Grazing Lawn Soil Properties

Figure 4. Mean soil

potassium (K), calcium

(Ca), magnesium (Mg),

phosphorus (P), carbon

(C), nitrogen (N), C:N

ratio, and pH according to

5 USDA soil texture

classes. Darker bars

correspond to finer soils

and lighter bars to coarser

soils. Bars are grouped

according to distance:

grazing lawn, close

(d = 10 m), and far

(d = 200 m). Shading

distinguishes soil depth:

top soil (0–5 cm; full bars)

versus deep soil (15–

30 cm; shaded bars). Error

bars represent 1 standard

deviation.

H. Valls Fox and others

accounts at most for 2% of the total nutrient and

carbon stocks in Ithala. Total estimates of soil nu-

trient stocks in all horizons are needed to assess the

human legacy by comparing former bomas and

grazing lawns to other nutrient hotspots such as

dung middens and termite mounds. Although the

contribution to total stocks may be small, lawns

likely are crucial nutrient hotspots for mammalian

grazers because they present concentrations of key

limiting nutrients, such as P, above the nutritional

requirement thresholds, in contrast to surrounding

bunch grassland (McNaughton and others 1988).

Grazing lawns can also be attractive for wild her-

bivores because these open habitats reduce preda-

tion risk (Ford and others 2014). Indeed, previous

studies demonstrated that lawns of a scale similar to

the ones in our study were preferred by a wide

range of grazer species (Cromsigt and Olff 2006).

Grazing Lawns as Hotspots: A Matter ofScale

We found that soil P, C, N, Ca, and pH declined and

C:N increased within a few meters of former boma

grazing lawns (Figure 3). Similar edge effects have

also been reported by other studies (Blackmore and

others 1990; Porensky 2011; Porensky and others

2013). Potassium was the only nutrient with si-

milar concentrations for lawn and close control

soils, although concentrations were still higher

than in the far control (also see Muchiru and others

2008; Veblen and Young 2010). In contrast, in old

field and natural grassland grazing lawns, only P

and Ca concentrations were clearly elevated in

comparison with control soil, although lawns did

not stand out for other nutrients. These results

indicate that abandoned bomas create more con-

trasting nutrient hotspots than the other land-use

types. It also shows that, although the spatial ex-

tent of old field impact is much larger than that of

bomas, the legacy of bomas is in fact stronger,

probably because the intensity of land use in bomas

is stronger and more concentrated than in old

fields. In contrast to our findings, recent studies in

Hluhluwe-iMfolozi Park, South Africa (Stock and

others 2010), Kruger National Park, South Africa,

and Serengeti National Park, Tanzania, (Arnold and

others 2014) found that soil nutrients did not differ

between lawn and bunch grass plots despite dif-

ferences in foliar nutrient concentrations. Howev-

er, it remains unclear whether the lawns described

in these studies are associated with former human

land use or originate from localized grazing by

native wild grazers (Waldram and others 2008).

Furthermore, in their study, Stock and others

(2010) sampled soil from under lawn and bunch

grass patches co-occurring within the same

40 9 40 m site. Hence, their controls could have

been situated within a lawn site. In contrast, both

our controls were clearly away from the lawns out

in the caespitose grassland. In addition, we in-

cluded a far control site at least 200 m away from

any mapped lawn to confirm our findings. In the

case of Arnold and others (2014), it seems they did

not necessarily compare grazing lawns with control

sites composed by a clearly distinctive plant com-

munity. At a fine scale, Arnold and others (2014)

selected grazing lawns based on grass height, not

grass species, and grass species composition did not

significantly differ between their grazing lawns and

control sites. As a result, they might have compared

intensely grazed with less intensely grazed areas

within lawn sites. In contrast, by selecting grazing

lawns dominated by the lawn grass Cynodon dactylon

and control grassland sites with clearly different

grass species compositions (that is, tall, caespitose

species), we compared clearly different grassland

communities and not grazing lawns in different

states of intensity of grazing.

Our results are consistent with the existence of

perennial, compositional, or stoloniferous, grazing

lawns (see Cromsigt and Olff 2008) that are main-

tained by grazers and develop a distinct enriched

soil and grazing-tolerant community (Blackmore

and others 1990; Young and others 1995; Augustine

2003; Muchiru and others 2008; Veblen and Young

2010). Browsers may also contribute to lawn

maintenance by depositing nutrient-rich feces,

while using the lawns as resting sites with relatively

low predation risk (Augustine 2004; Ford and oth-

ers 2014). These perennial lawns differ from grazed

patches that are temporarily grazed short (structural

or caespitose ‘lawns,’ see Cromsigt and Olff 2008) that

may occur when a disturbance such as fire, mow-

ing, trampling, or localized intense grazing keeps

the tall caespitose grass short (Archibald 2008;

Cromsigt and Olff 2008). Younger grass phenology

may attract grazers to these temporary ‘lawns’ by

providing high-quality forage. However, at this time

scale, soil properties and species composition would

not have changed and this may provide an alter-

native explanation to the apparently conflicting

results found by different studies.

Soil Texture: A Modifier of Land-Use andGrazing Effects

In Ithala, soil texture does not appear to govern

grazing lawn location (Figure 2). This confirms

previous findings that grazing lawns can be found

Legacy of Land Use on Grazing Lawn Soil Properties

on a range of soil textures ranging from clayey to

sandy soils whether they originate from bomas

(Young and others 1995; Augustine 2003; Cech

and others 2010; van der Waal and others 2011;

Veblen 2012) or from free-ranging herbivore

grazing (Cromsigt and Beest 2014). This suggests

that in our study soil texture varied at a larger

spatial scale than patterns remaining from past land

use or emerging from current grazing intensity.

This is further confirmed by the fact that soil tex-

ture did not vary among the land-use types in our

study (Figure 2). In Ithala, bomas were built and

fields were plowed on very diverse soil types where

grazing lawns were established locally by herbi-

vores. However, whereas the human legacy set the

template for today’s landscape heterogeneity, this

study does not undermine the importance of

edaphic factors such as soil texture as shown by

many other studies (Anderson and others 2007;

Veldhuis and others 2014). In our study, soil tex-

ture influenced which soil nutrients were elevated

in lawn soils (Figure 4). Whereas most of the soil

nutrient concentrations were higher in lawn soils

on clay, on sandy soils only P, C:N ratio, and pH

differed between lawns and controls. This may

indicate the importance of P:C:N stoichiometry and

pH in processes that govern grazing lawn persis-

tence.

N:P Stoichiometry, Potential Role of SoilBiota, and Grazing Lawn Persistence

N and P are major nutrients that shape many

grazed ecosystems (Cech and others 2010).

Whereas P may persist for decades (Augustine

2003) or even centuries (Blackmore and others

1990), N is much more volatile and its availability is

driven by short-term flows (Scholes and Walker

1993; de Mazancourt and others 1998; Coetsee and

others 2010). High concentrations of P may be

necessary for grazing optimization (Chapin and

McNaughton 1989) and P hotspots may be vital for

herbivores in dystrophic contexts (Augustine 2003,

2004; Verweij and others 2006; Treydte and others

2011). We found that P was the only nutrient that

was clearly elevated in grazing lawns for all land-

use histories and across a wide range of soil tex-

tures. This may indicate that P concentration is a

key process that governs grazing lawn persistence

(Cech and others 2010). The persistence of nutri-

ents in lawns is likely further mediated by soil biota

and the role of such biota perhaps varies among the

different land uses. Increased N inputs and higher

pH negatively impacts fungi (de Vries and others

2006), and lower C:N and higher pH in former

boma lawn soils thus likely resulted in a high mi-

crobial versus fungal biomass ratio (de Vries and

others 2006). Such higher microbial biomass may

in turn enhance nutrient cycling rates and facilitate

plant nutrient uptake (Bardgett and Wardle 2003).

Previous studies have stressed the importance of

such microbe-induced high rates of N cycling in

grazing lawns (McNaughton and others 1997;

Frank and others 2000) even in the absence of N

stocks (Coetsee and others 2010). Future studies

exploring soil biota composition across different

former land uses may reveal to what extent dif-

ferent microorganisms may alter the legacy of past

human land-use practices and drive grazing lawn

persistence.

CONCLUSION

In many protected areas, former human activity

has created nutrient hotspots across the landscape

that influences the grazing patterns of wild herbi-

vores. Grazing lawns illustrate this legacy due to

the strong interactions between herbivores, plants,

and the underlying soil. The legacy of bomas dif-

fered substantially from other land uses due the

persistence of large stocks of P, N, and C within

restricted areas. P limitation in surrounding grass-

lands may favor the maintenance of these grazing

lawns over decades or even centuries, whereas soil

organic matter may alter the role of soil biota in

nutrient cycling. Conversely, grazing lawns in old

field and natural grasslands were also soil nutrient

hotspots but did not have the typical higher soil

organic matter content found in boma top soil and

had less pronounced P stocks. Ultimately, our study

revealed how the legacy of different land-use his-

tories modifies the balance between soil properties

and grazers by shaping C:N:P stoichiometry in

grazing lawns. As such, we confirm that human

activities have a long-lasting legacy that shapes

landscapes and ecosystems over decades and

probably longer, including within so called pristine

protected areas.

ACKNOWLEDGMENTS

We would like to thank KZN wildlife for giving us

the opportunity to conduct our research in Ithala

Game reserve. We also thank our colleagues from

UKZN particularly Vincent Chaplot (IRD) for advice

on soil sample collection and analysis. We are

grateful to Jacques Gignoux and Simon Chamaille-

Jammes for their comments on earlier versions of

the manuscript. Funding for the project was pro-

vided to A.S. (UKZN) by the National Research

H. Valls Fox and others

Foundation (NRF) and to H.F. (LBBE) by the CNRS

INEE ‘‘Zone Atelier’’ grant, and the CNRS GDRI

Biodiversity Dynamics in Southern Africa. H.V.F. was

supported by the Ecole Normale Superieure as an

eleve fonctionnaire stagiaire. J.P.G.M.C. was sup-

ported by a Marie Curie Career Integration Grant

(PCIG10-GA-2011-304128) and by the Swedish

Thematic research program WILDLIFE & FORESTRY.

We thank two anonymous reviewers for their com-

ments which greatly improved a previous version of

this manuscript.

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