nursery fertilization and tree shelters affect long-term field response of acacia salicina lindl....

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Forest Ecology and Management 215 (2005) 339–351

Nursery fertilization and tree shelters affect long-term

field response of Acacia salicina Lindl. planted in

Mediterranean semiarid conditions

Juan A. Oliet a,*, Rosa Planelles b, Francisco Artero b, Douglass F. Jacobs c

a E.T.S. Ingenieros Agronomos y de Montes de la Universidad de Cordoba, Avda. Menendez Pidal s/n, 14071 Cordoba, Spainb Departamento de Medio Ambiente, Instituto Nacional de Investigacion Agraria y Alimentaria,

Carretera de La Coruna, km 7,5, 28040 Madrid, Spainc Department of Forestry and Natural Resources, Hardwood Tree Improvement and Regeneration Center,

Purdue University, West Lafayette, IN 47907-2061, USA

Received 16 December 2004; received in revised form 11 April 2005; accepted 10 May 2005

Abstract

Transplant stress limits establishment of newly planted seedlings in semiarid Mediterranean regions, which are characterized

by very low precipitation and poor fertility soils. Nursery cultural regimes which influence stock quality, as well as silvicultural

treatments applied at outplanting may affect the capacity of seedlings to establish successfully. We examined the influence of

nursery mineral nutrition and application of individual tree shelters on 9-year seedling performance of the leguminous species,

Acacia salicina Lindl., planted on a degraded site in southeastern Spain. Survival was significantly greater throughout the

duration of the study for seedlings fertilized at high rates, while initial benefits to field growth associated with nursery

fertilization diminished after 4 years. A significant relationship was established between P supplied in the nursery and both

seedling survival and root dry weight after the first growing season (R2 = 0.68 and 0.77, respectively), though no relationship was

detected for N. The capacity of this species to fix N through root nodulation apparently dictates that P fertility is relatively more

important to initial establishment on low fertility sites characteristic of this region. Survival of protected seedlings became

significantly greater than that of non-protected seedlings following an extended drought after the sixth year. Stem diameter was

significantly greater for non-protected seedlings as of the fourth year but height was greater for protected seedlings throughout

the study, reflecting differential carbon allocation within the sheltered environment. Our results suggest that mineral nutrient

status of nursery stock (especially high P content) and tree shelters may positively affect long-term plantation establishment of

A. salicina seedlings in semiarid Mediterranean climates.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Acacia salicina Lindl.; Forest seedling nutrition; Ecological restoration; Legumes; Phosphorus; Tree shelters

* Corresponding author. Tel.: +34 957 218655; fax: +34 957 218563.

E-mail address: [email protected] (J.A. Oliet).

0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2005.05.024

J.A. Oliet et al. / Forest Ecology and Management 215 (2005) 339–351340

1. Introduction

Transplant shock is described as an interruption in

the normal physiology of a seedling after outplanting

caused mainly by water stress provoked by temporary

impairment of seedling root function or poor root–soil

contact (Folk et al., 1996; Kavanagh and Zaerr, 1997;

Grossnickle, 2000). Transplant shock is one of the

most frequent causes of reforestation failure, and can

be very intense in semiarid conditions with limited soil

water availability. Morphological and physiological

attributes of nursery stock largely impact the capacity

of seedlings to resist post-transplant water stress.

Aspects such as biomass distribution between shoot

and roots (which affects the balance between water

uptake and evaporative demand), osmotic adjustment

and other water stress tolerance components, resis-

tance to cold, root growth capacity and carbohydrate

status each affect capacity of seedlings to establish on

the site (Puttonen, 1997).

Fertilization in the nursery is one of the most

important cultural practices for plant quality in

reforestation, especially for seedlings produced in

containers in which the limited volume seriously

hinders growth (Landis, 1989). Fertilization affects

shoot and root growth of plants, improves post-

transplant rooting and growth capacity, and increases

resistance to water stress, low temperature and disease

(van den Driessche, 1980, 1991a, 1992; Haase and

Rose, 1997; Shaw et al., 1998; Malik and Timmer,

1998; Grossnickle, 2000; Floistad and Kohmann,

2004). These properties are of vital importance for

successful early establishment under unfavorable

conditions (Puttonen, 1997; Birchler et al., 1998),

and can be influenced substantially by alternative

fertilization regimes. Moreover, remobilization of

internal nutrient reserves enables outplanted seedlings

to be partly independent of external nutrient availability

(Cherbuy et al., 2001). Thus, mineral nutrient reserves

can play an important role after planting, when nutrient

uptake is limited by poor root–soil contact (Timmer and

Aidelbaum, 1996; Malik and Timmer, 1998), and a

decrease in tissue mineral nutrient concentrations

occurs (Close and Beadle, 2004). Nutrient loading by

applying increasing doses of fertilizer can be effective

in building up internal reserves that will be used after

planting (Quoreshi and Timmer, 2000; Salifu and

Timmer, 2003). Many studies have confirmed the

influence of mineral nutrition on seedling quality for

reforestation, though most of these focus on conifers

from wet, temperate forests, with emphasis on N

additions (van den Driessche, 1988; Larsen et al., 1988;

Green and Mitchell, 1992; Green et al., 1994; Folk et al.,

1996; Timmer and Aidelbaum, 1996; Tan and Hogan,

1997; Irwin et al., 1998; Quoreshi and Timmer, 2000;

Jose et al., 2003). Relatively little is known, therefore,

regarding the relationship between nursery fertilization

with N or additional macronutrients on capacity of

seedlings of species from other ecoregions to resist

transplanting stress.

Another tool to help minimize transplant shock is

the use of individual tree shelters to protect outplanted

seedlings. Although shelters help to prevent damage

resulting from animal browse (Potter, 1991), tree

shelters also act as a small greenhouse providing a

modified microclimate that may affect both survival

and growth after planting (Potter, 1991; Bergez and

Dupraz, 1997, 2000; Dupraz and Bergez, 1999; Jacobs

and Steinbeck, 2001). Although many studies with

tree shelters have been conducted in temperate

regions, few experiments have been reported in dry

regions characterized by low transpiration rates and

higher temperatures. In these regions, plant response

to tree shelters appears to be species-specific, with

many species exhibiting improved survival and growth

when protected with shelters (Marques et al., 2001;

Oliet et al., 2003).

Many studies regarding seedling outplanting

response focus only on results incurred during the

first field season. However, some authors emphasize

the importance of tracking development for longer

timescales (Racey and Gerum, 1983; Burdett, 1990;

McDonald, 1991; Rose and Atkinson, 1992; Simpson

et al., 1994; Cain and Barnett, 1996; Jacobs et al.,

2004). This may sometimes alter the conclusions of a

single season study, due to interactions between

experimental treatments and time. In particular, the

effects of tree shelters or the combination of nursery

mineral nutrition and tree shelters may be prolonged

for several seasons after planting (Jacobs, 2004; Oliet

et al., 2000, respectively). However, few studies

consider more than first year response, especially

when examining effects associated with nursery

fertilization treatments (Puertolas et al., 2003).

Acacia salicina Lindl. is a N-fixing leguminous

shrub or tree which is native to the arid zone of South

J.A. Oliet et al. / Forest Ecology and Management 215 (2005) 339–351 341

Table 1

N, P and K amounts per plant supplied by fertilizer treatments (rates

of each formulation per liter of substrate)

Formulation 9-13-18 16-8-9

Rate (g l�1) 1.5 3.25 5.0 3.25 5.0 7.0

N (mg/seedling) 38.6 83.7 128.7 148.7 228.8 320.3

P (mg/seedling) 24.3 52.7 81.0 32.3 49.8 69.8

K (mg/seedling) 64.1 138.9 213.7 69.5 106.7 149.6

Australia, but has been introduced to other regions as a

multipurpose species (Le Houerou, 1986; Rehman

et al., 1999). It successfully establishes on degraded

areas (Grigg and Mulligan, 1999). In Spain, A. salicina

has been introduced in some Mediterranean areas to

examine its capacity to serve as a source of fodder for

livestock (Correal et al., 1988), as well as for use as an

ornamental and to rehabilitate disturbed areas

(Tilstone et al., 1998). The objective of this study

was to evaluate the individual and combined effects of

both nursery fertilization and tree shelter protection at

planting on mid to long timescale response of A.

salicina in a degraded land of a semiarid Mediterra-

nean region of Spain.

2. Materials and methods

A. salicina seedlings were produced from May

1992 to planting time in the Boticario Centre (28240W,

368520N, elevation 60 m), Almeria, Spain. Plants were

grown in 230 ml individual cell containers filled with a

4:1 (v/v) sphagnum peat moss—vermiculite growing

medium in which fertilizer treatments were mixed.

Fertilizer treatments consisted of three rates of two

different controlled-release Osmocote1 (Scotts Co.,

Marysville, OH, USA) formulations plus a non-

fertilized treatment for comparison. The formulations

used were:

1. 9
-13-18: 9N (6.1% NH4-N and 2.9% NO3-N)-5.7P-
14.9K. Rates: 1.5, 3.25 and 5.0 g l�1 substrate.

2. 1

Table 2

Annual rainfall recorded on the planting site during the study period

Year Precipitation (mm)

1993 205

1994 187

1995 122

1996 193

1997 204

1998 42

1999 176

2000 229

2001 99

6-8-9 + 3Mg: 16N (6.6% NH4-N and 9.4% NO3-

N)-3.5P-7.5K. Rates: 3.25, 5.0 and 7.0 g l�1

substrate.

Each formulation had an equivalent stated nutrient

release period: 12–14 months at 21 8C. Micromax1

(Scotts Co.), a solid mixture of microelements, was

added at 0.15 g l�1 for all treatments. Fertilizer

treatments were designed to supply an increasing

amount of N per plant, from 38.6 mg (1.5 g l�1 9-13-

18) to 320.3 mg (7 g l�1 16-8-9) while creating

different N-P-K-balances (Table 1). Treatments in the

nursery were arranged as a completely randomized

design. Height and basal stem diameter were measured

on 30 randomly selected 9-month old seedlings per

treatment sampled directly from the nursery containers.

On 3 March 1993, A. salicina seedlings were planted

on a degraded plain (28200W, 368510N, elevation 30 m)

of Almeria, Spain. According to FAO taxonomy, the

soil belongs to cambic arenosol group formed on

calcareous parent material (Perez, 1989), with a first

sandy horizon (95% sand) 30 cm depth upon a sandy–

loamy horizon (66% sand, 29% loam). Carbon (0.5%)

and fertility of the profile are very low (0.05% total N,

0.75 ppm POlsen and 0.13 mg g�1 K) and pH is high

(pHH2O 8:5) (Perez, 1989). Annual rainfall and mean

temperature of the area are 200.2 mm and 18.5 8C,

respectively, with frequent strong southwest winds,

according to Spanish National Institute of Meteorology

reports (data averaged from 1969 to 2001). Precipita-

tion during the study period was collected and measured

by a pluviograph installed on the planting site (Table 2).

Annual rainfall from 1993 to 2001 averaged 160.5 mm,

with an average of 30 mm from May to September (data

not shown). The experimental site was fenced to restrict

access to rodents and herbivores. Cross ploughing to a

depth of 80 cm was accomplished prior to planting.

Seedlings were planted in manually opened pits

(0.3 m � 0.3 m � 0.3 m) at 1.5 m � 1.5 m spacing.

The 7 (nursery fertilization) � 2 (with or without tree

shelters at planting) factorial treatments were arranged

as a randomized complete block design with four

replications. Tree shelters (standard unventilated,


translucent, circular, twin-walled polypropylene tubes

0.6 m tall � 0.11 m wide, Tubex Co., South Wales,

UK) were installed at planting. The experimental unit

consisted of a row of 16 seedlings, each block

containing 14 rows. Manual weeding was conducted

annually. Seedling height and groundline stem diameter

(GSD) from all living plants were measured in June and

October 1993, 1994, November 1995, January 1997,

February 1999, December 2000 and January 2002.

Height and diameter data were transformed into

slenderness quotient (height:GSD) and stem volume

index (SV), using the following formula: stem

volume = (1/3) � p � (1/4)(GSD)2 � height. Volume

index (VI) was calculated on a per hectare basis by

combining survival and mean SV per experimental unit.

Slenderness quotient is an important morphological

indicator of seedling quality (Thompson, 1985) which

was used to help evaluate shoot development, in

particular among protected and non-protected plants.

Stem volume index provided a more accurate indicator

of shoot biomass than height or GSD independently

(van den Driessche, 1988; South and Mitchell, 1999).

Volume index, computed by correcting stem volume

per treatment with survival on a per hectare basis,

emphasizes the variation among treatments in capacity

to occupy the site. In September 1993, a sample of three

plants per treatment replication among non-protected

seedlings was randomly selected, GSD and height were

measured, and seedlings were destructively harvested

(12 plants per fertilization level, 84 plants in total).

After cautious excavation, root systems were extracted,

taking care to retain roots >1 mm diameter. Shoots

were separated from roots and dry mass of each fraction

was determined by oven drying at 65 8C for 24 h and

weighing. Following this sampling, a significant shoot

dry weight over SV power regression model was fitted

(R2 = 0.894, P < 0.001, n = 84).

Data from the planting experiment were analyzed

using analysis of variance (ANOVA) for a randomized

Table 3

Height and basal stem diameter of containerized Acacia salicina seedlings a

of each formulation per liter of substrate)

Formulation 9-13-18

Rate (g l�1) 0 1.5 3.25

Height (cm) 19.3c 34.3b 40.2ab

Diameter (mm) 2.3d 3.1c 3.6bc

Within a row, means with different letters (a, b and c) indicate significan

complete block design with four blocks. Analysis of

survival and growth was made by two-way ANOVA

(main factors consisting of fertilization in the nursery

and tree shelter at planting), with the treatment mean

for each block comprising the experimental unit (each

row of 16 plants). Any significant formulation � rate

interaction was noted in the text. For plantation

survival percentages, comparison data were arcsine

transformed (Steel and Torrie, 1989), though data are

reported as original means with standard errors. Data

from the destructive sampling of planted seedlings

were subjected to one-way ANOVA (with fertilization

in the nursery as the main factor) with the three plants

excavated in each block for each treatment comprising

the experimental unit. Seedling morphology before

planting was assessed using one-way ANOVA for a

completely randomized design. For each analysis,

when ANOVA was significant, statistically significant

differences between means were identified using

Fisher’s protected least significant differences

(L.S.D.) test (Steel and Torrie, 1989). To assess the

relationships between certain variables, Pearson

correlation coefficients were calculated, and linear

regression models were fitted to quantify relationships

among certain variables. Effects were considered

significant when P < 0.05. SPSS Version 11.00 (2001)

was used for all statistical tests.

3. Results

3.1. Seedling morphology

Height ranged from 19.3 cm (non-fertilized) to

49.7 cm (fertilized with 7 g l�1 of 16-8-9) (Table 3),

although plant height response to rate within a

formulation was not significant. Among fertilized

treatments, all but 1.5 g l�1 of 9-13-18 were in the

same statistical height group. Basal stem diameter

fter 9 months as affected by fertilizer treatments in the nursery (rates

16-8-9

5 3.25 5 7

42.2ab 44.5ab 45.7ab 49.7a

3.9ab 4.1ab 4.1ab 4.4a

t differences (n = 30).


Fig. 2. Linear regression models fitted for the relationships between

Acacia salicina root dry biomass (Root DW) and survival after the

first field growing season (September and October 1993, respec-

tively) and P supplied in the nursery.

ranged from 2.3 mm (non-fertilized) to 4.4 mm

(fertilized with 7 g l�1 of 16-8-9). Within the 9-13-

18 formulation a significant shift appeared when rate

increased from 1.5 to 5 g l�1 (Table 3), but no

significant basal stem diameter response occurred

from 3.25 to 7 g l�1 within 16-8-9. Linear correlations

between both height and basal stem diameter and N

and P supplied were positive and significant

(P = 0.015 for N, P = 0.031 for P and P = 0.009 for

N, P = 0.033 for P, respectively, n = 7).

3.2. Outplanted biomass after first summer

Whole plant, shoot and root biomass after the first

summer for non-protected seedlings were significantly

affected by fertilization in the nursery (ANOVA

P = 0.011, 0.016 and 0.006, respectively). Plants

fertilized with 9-13-18 experienced a significant

increase in root dry weight with rate (Fig. 1), while

this was not observed among 16-8-9 fertilized A.

salicina seedlings. Maximum root biomass was

attained with 5 g l�1 of 9-13-18. Within a formulation,

shoot dry weight was not significantly affected by rate,

although a positive trend was observed (Fig. 1).

Maximum shoot biomass was reached with 7 g l�1 of

16-8-9. A significant positive linear regression model

was fitted for P supplied in the nursery and mean root

biomass after planting (Fig. 2), while no significant

relationship was found for N supplied. Mean shoot

biomass was positively correlated to both N and P

Fig. 1. Mean (+S.E.) biomass (dry weight, DW) of Acacia salicina

after the first field growing season as affected by nursery fertilizer

treatments (rates of each formulation, 9-13-18 and 16-8-9, per liter

of substrate). For each fraction (shoot or root), columns marked with

different letters indicate significant differences (n = 12).

supplied (R2 = 0.780, P = 0.008 and R2 = 0.814,

P = 0.005, respectively, n = 7).

3.3. Planting response: survival

A significant interaction between both factors

(nursery fertilization and tree shelter) was present in

June 1993 (P = 0.027), but was not detected for the

remainder of the study period. After the first summer

(October 1993), A. salicina seedling survival decreased

in all fertilization treatments, and differences were

significant through January 2002 (P < 0.001 for all

dates). Survival in October 1993 of non-fertilized plants

and plants fertilized with 1.5 g l�1 of 9-13-18 decreased

40.6 and 24.1%, respectively, while the reduction in

survival for the remainder of treatments was lower

(Table 4). In 1994 and 1995 the decrease in survival was

similar among treatments, ranging from 11.2% in non-

fertilized plants to 2% in plants fertilized with 5 g l�1 of

16-8-9. However, in 1996–1998 mortality was more

severe and several shifts in significance of differences

between treatments appeared: while A. salicina

fertilized in the nursery with 3.25 g l�1 of 16-8-9

experienced a survival decrease of 19.7%, 5 g l�1 of 9-

13-18 provoked a 6% reduction in survival (Table 4).

During 1999–2001 no significant changes in survival

occurred. Following the first summer, 5 g l�1 of 9-13-

18 showed the best performance, followed by plants

fertilized with 5 g l�1 of 16-8-9. At the end of the period

considered (January 2002), survival among fertilized

treatments ranged from 49.9% (1.5 g l�1 of 9-13-18) to


Table 4

Survival (%) of Acacia salicina (�S.E., n = 8) during 9 years as affected by nursery fertilizer treatments (rates of each formulation per liter of

substrate)


Rate (g l�1) 0 1.5 3.25 5 3.25 5 7

June 1993# 83.6 � 3.1 95.3 � 2.6 96.9 � 1.7 98.4 � 1.0 100.0 � 0.0 94.5 � 2.5 93.8 � 2.0

October1993 43.0 � 5.9c 71.2 � 5.5b 86.4 � 4.5a 92.1 � 2.6a 90.7 � 1.7a 86.5 � 4.0a 86.8 � 2.6a

October 1994 31.8 � 5.5c 62.4 � 6.7b 79.3 � 4.4a 87.4 � 3.5a 84.7 � 2.0a 84.6 � 4.8a 81.6 � 2.3a

November 1995 31.8 � 5.5c 62.4 � 6.7b 78.3 � 4.4a 87.4 � 3.5a 83.9 � 2.0a 84.6 � 4.8a 81.6 � 2.3a

January 1997 27.1 � 4.6d 59.1 � 6.6c 68.6 � 4.7c 86.7 � 3.6a 72.7 � 4.0bc 81.7 � 4.1ab 74.3 � 4.1bc

February 1999 19.0 � 4.6d 49.9 � 8.0c 65.3 � 5.6bc 81.4 � 4.0a 64.2 � 5.1bc 70.2 � 2.3ab 66.2 � 6.8b

December 2000 18.2 � 4.5d 49.9 � 8.0c 64.4 � 6.3bc 80.5 � 4.7a 63.4 � 5.2bc 68.3 � 2.3ab 65.4 � 6.5bc

January 2002 18.2 � 4.5d 49.9 � 8.0c 64.4 � 6.3bc 80.5 � 4.7a 63.4 � 5.2bc 67.3 � 2.4ab 64.6 � 6.8bc

Within a row, means with different letters (a, b and c) indicate significant differences.# A nursery fertilization � tree shelter at planting interaction occurred precluding statistical analysis of main effects.

80.5% (5 g l�1 of 16-8-9), with survival of remaining

treatments ranging from only 63.4 to 67.3% (Table 4).

Tree shelters did not significantly affect post-

planting survival of A. salicina until 1999. However,

following this year, survival of non-protected plants

was reduced to 54.9%, while survival of protected

seedlings exhibited a less pronounced reduction,

reaching 64.0% in February 1999 (Fig. 3). As

mentioned previously, no substantial changes in

Fig. 3. Mean (�S.E., n = 28) survival, height, groundline stem diameter a

duration as affected by tree shelters at planting. (*) and (***) indicate P

survival occurred during 1999, 2000 and 2001 and

the significant difference in survival among protected

and non-protected plants was maintained.

3.4. Planting response: growth

Similar to the response for survival, a significant

interaction between both factors (nursery fertilization

and tree shelter at planting) appeared in June 1993

nd slenderness quotient of Acacia salicina during the 9-year study

< 0.05 and P < 0.001, respectively.


Table 5

Height (cm) of Acacia salicina (�S.E., n = 8) during the 9-year study period as affected by nursery fertilizer treatments (rates of each

formulation per liter of substrate)


Rate (g l�1) 0 1.5 3.25 5 3.25 5 7

June 1993# 13.9 � 2.0 33.0 � 4.3 39.6 � 4.2 44.7 � 5.5 44.7 � 3.7 44.7 � 3.5 48.2 � 3.3

October 1993 23.4 � 4.6c 42.0 � 8.9b 49.1 � 8.9ab 48.5 � 8.3ab 48.9 � 9.0ab 43.2 � 6.4ab 50.8 � 8.7a

October 1994 38.0 � 6.6b 56.0 � 9.9a 62.8 � 7.8a 63.8 � 7.5a 61.6 � 9.9a 54.7 � 7.7a 62.3 � 10.1a

November 1995 43.2 � 6.7b 57.6 � 9.3a 63.1 � 7.1a 65.1 � 6.5a 63.1 � 9.2a 60.8 � 7.5 a 63.2 � 9.4 a

January 1997 61.6 � 10.4 64.7 � 11.6 77.7 � 9.1 75.8 � 9.4 74.9 � 10.5 72.7 � 9.6 75.4 � 9.7

February 1999 70.9 � 10.4 69.9 � 11.2 84.5 � 8.4 84.4 � 10.3 83.0 � 9.0 83.3 � 9.6 85.1 � 10.3

December 2000 79.0 � 10.9 73.7 � 9.4 91.1 � 7.8 90.7 � 10.7 95.8 � 6.7 95.3 � 11.7 94.2 � 10.5

January 2002 88.3 � 12.9 84.8 � 9.1 103.9 � 8.3 99.7 � 11.5 95.3 � 7.4 110.4 � 12.5 106.1 � 11.1

Within a row, means with different letters (a, b and c) indicate significant differences.# A nursery fertilization � tree shelter at planting interaction occurred precluding statistical analysis of main effects. Note: when a treatment

effect was not significant the multiple comparison test was not conducted.

(P = 0.001) for height, though it disappeared for the

remainder of the study period. From October 1993 to

the end of 1995, height response was significantly

affected by nursery fertilization (P < 0.01), with plants

fertilized with maximum rates of both formulations

achieving highest values (Table 5). However, from 1996

to the end of the study period, no further significant

effect on height occurred. Maximum differences in

mean height among fertilizer treatments ranged from

34.3 cm in June 1993 to 22.1 cm in January 2002

(Table 5). Groundline stem diameter (GSD) was

significantly affected by fertilization in the nursery

during the entire study period, though P-values

decreased over time (P < 0.001 from June 1993 to

January 1997, P < 0.05 for February 1999 to January

2002). Correspondingly, the number of statistically

different groups according to multiple comparison tests

decreased from 5 to 2 between 1993 and 2002 (Table 6).

Nine years following planting, significant differences

were only present between non-fertilized or low

fertilized (1.5 g l�1 substrate of 9-13-18) A. salicina

seedlings and the remainder of nursery fertilization

treatments, with maximum differences of 10.9 mm

between the smallest (non-fertilized, GSD = 20.8 mm)

and largest (fertilized with 5 g l�1 of 16-8-9,

GSD = 31.7 mm) plants. Slenderness quotient

decreased in all treatments with time after planting

(Table 6), though differences among treatments still

persisted in 2002 (P < 0.05). Interactions between

nursery fertilization and tree shelter protection on

slenderness quotient occurred at the 1993, 1995 and

1997 measurements (data not shown). Stem volume

index differences as affected by nursery fertilization

were significant during the first 3 years after planting

(P < 0.01 in 1993 and P < 0.05 in 1994 and 1995) but

differences were no longer significant in January 1997,

4 years after planting. High variability was present

within each treatment at the end of the study period

(Table 6). Volume index on a per hectare basis 9 years

after planting was significantly affected by nursery

fertilization (P < 0.05), with maximum difference of

2.31 m3 ha�1 between non-fertilized plants and those

fertilized with 7 g l�1 of 16-8-9, followed by a

2.17 m3 ha�1 difference between the former treatment

and the lowest rate of 9-13-18 (1.5 g l�1, Table 6).

Height was significantly affected by tree shelter

application from June 1993 (three months after

planting) to the end of the study period (P < 0.001

for all dates). In October 1993, a decrease in mean

height was observed in non-protected plants (Fig. 3),

while height of protected plants increased. From this

time to the end of the study period, differences in

height between protected and non-protected plants

remained relatively consistent, ranging from 45.3 cm

in January 1997 to 35.0 cm in January 2002 (Fig. 3). In

contrast, GSD was not affected by tree shelter

protection until the third year after planting (Novem-

ber, 1995), when GSD was significantly greater for

non-protected seedlings (Fig. 3). Statistically signifi-

cant differences continued (P < 0.05 for all dates)

through January 2002. Maximum slenderness quotient

difference among protected and non-protected plants

was 7.1 cm mm�1 after the first summer (October

1993), though it was progressively reduced to


Table 6

Groundline stem diameter, slenderness quotient, stem volume index before (June 1993) and after (October 1993) the first growing season and 9

years after planting (January 2002) and volume index 9 years after planting (January 2002) of Acacia salicina (�S.E., n = 8) as affected by

nursery fertilizer treatments (rates of each formulation per liter of substrate)


Rate (g l�1) 0 1.5 3.25 5 3.25 5 7

Stem diameter (mm)

June 1993 2.2 � 0.1e 3.6 � 0.1d 4.3 � 0.1c 5.2 � 0.2b 5.1 � 0.1b 5.0 � 0.1b 5.5 � 0.1a

October 1993 3.1 � 0.3e 5.1 � 0.4d 6.3 � 0.4bc 7.2 � 0.4ab 6.8 � 0.8abc 5.9 � 0.2cd 7.6 � 0.7a

January 2002 20.8 � 2.6b 21.1 � 1.5b 29.1 � 1.8a 28.5 � 3.4a 27.0 � 2.2ab 31.7 � 3.0a 29.6 � 4.0a

Slenderness (cm: mm)

June 1993# 6.4 � 0.9 9.1 � 1.1 9.3 � 1.0 8.5 � 0.9 8.7 � 0.7 8.9 � 0.7 8.7 � 0.6

October 1993# 7.8 � 1.5 8.3 � 1.7 7.9 � 1.4 7.0 � 1.2 7.4 � 1.3 7.3 � 1.1 7.4 � 1.3

January 2002 4.6 � 0.7c 4.5 � 0.6bc 3.9 � 0.5a 4.0 � 0.5ab 3.9 � 0.4a 4.1 � 0.5ab 4.2 � 0.6abc

Stem volume (cm3)

June 1993 0.2 � 0.0e 1.2 � 0.2d 2.2 � 0.2c 3.6 � 0.6b 3.5 � 0.4b 3.3 � 0.3b 4.4 � 0.3a

October 1993 0.8 � 0.3d 4.7 � 1.6cd 7.6 � 1.9abc 10.2 � 2.0ab 9.2 � 3.5abc 5.1 � 0.8bcd 10.8 � 2.9a

January 2002 227.5 � 108.7 136.8 � 32.5 470.9 � 79.2 564.4 � 187.8 320.5 � 73.2 728.1 � 222.4 858.0 � 371.0

Volume index (m3 ha�1)

January 2002 0.22 � 0.10b 0.35 � 0.11b 1.42 � 0.32ab 2.04 � 0.66a 0.96 � 0.26ab 2.10 � 0.60a 2.52 � 1.22a

Within a row, means with different letters (a, b and c) indicate significant differences.# A nursery fertilization � tree shelter at planting interaction occurred precluding statistical analysis of main effects. Note: when a treatment

effect was not significant the multiple comparison test was not conducted.

2.8 cm mm�1 coinciding with the final measurement

(Fig. 3). As mentioned above, an interaction between

nursery fertilization and tree shelter protection was

detected for slenderness quotient in 1993, 1995 and

1997; when no interaction appeared, ANOVA was

significant for the tree shelter main factor (P < 0.001).

Stem volume of plants in tree shelters was only

statistically different (P < 0.01) during the first year

following planting, with protected seedlings having

greater volumes (data not shown). Likewise, volume

index on a per hectare basis 9 years after planting was

not affected by shelter treatments (data not shown).

4. Discussion

4.1. Nursery fertilization and survival at planting

Overall survival of A. salicina after 9 years was

relatively high, considering the rainfall shortage

during the whole period, which included years with

precipitation as low as 42 mm (1998, Table 2). In a

planting trial undertaken near our study site from

1988 to 1991 to compare response of different multi-

purpose tree species, A. salicina exhibited the best

performance among species in terms of survival and

growth (Tilstone et al., 1998).

The maximum rate (5 g l�1 substrate) of 9-13-18

(the richest P fertilizer) promoted the highest survival

following the first year after planting. When examin-

ing the relationships between survival or root dry

weight and P supplied in the nursery, significant

regression models were fitted (Fig. 2), while no

significant models were found for N. The role of P in

the enhancement of root growth after planting may

help explain this finding. Many authors have

suggested a positive effect of P on root development

(Timmer and Armstrong, 1987; Salisbury and Ross,

1994), and several studies have confirmed this

relationship. For instance, while dry weight of

Pseudotsuga menziesii (Mirb.) Franco nursery seed-

lings was affected by both N and P supplied in a

nursery fertilization experiment, root growth capacity

after planting was mostly influenced by P (Bigg and

Schalau, 1990). Likewise, Dominguez et al. (2000)

reported that Pinus pinea L. seedlings grew more roots

in the nursery and attained higher post-transplant root

growth capacity values when fertilized at the highest

rate of P. In a study with Picea glauca � Picea

engelmanii, new root biomass from seedlings grown in


the presence of P after 12 weeks was almost two times

greater than that of seedlings grown without P (Folk

and Grossnickle, 2000). Enhanced root system

morphology associated with P additions might help

improve survival on harsh sites. For instance, Planelles

(2004) found a significant improvement in survival of

outplanted Ceratonia siliqua L. in response to P

supplied in the nursery under low fertility Mediterra-

nean semiarid field conditions. The response we

observed may be accentuated by the scarcity of P in

the soil (see Section 2), though a study of Pinus

halepensis Mill. seedlings in the same location and

with the same nursery fertilization treatments showed

a significant and positive response of survival to N

supplied, but not to P (Oliet et al., 1997). This suggests

a species-specific response of outplanted seedlings to

mineral nutrition in the nursery. Thus, capacity of

leguminous species, like A. salicina, to fix atmo-

spheric N may reduce dependence on internal N to

help support establishment after planting. Nodulation

of another leguminous shrub, Retama sphaerocarpa L.

was enhanced when growing in low fertility condi-

tions and this promoted N uptake efficiency, while P

uptake was not affected by nodulation (Valladares

et al., 2002). Therefore, as N dependence decreases, P

dependence increases, particularly in low fertility soils

and for leguminous species, since legume nodules

responsible for N fixation have a high P requirement

(Vance, 2001). Likewise, outplanting performance of

some other conifer species is largely affected by P

reserves in needles produced in the nursery (van den

Driessche, 1991b; Folk and Grossnickle, 2000).

Nine years following planting, differences in first

year survival among fertilization treatments in the

nursery still persisted, and from the fourth year on

there were no relevant shifts in survival and

composition of statistical groups, indicating that

treatment responses had consolidated. Survival of

non-fertilized or low fertilized (1.5 g l�1 9-13-18)

plants was significantly lower for the entire study

period, reflecting superior performance of nutrient

loaded seedlings during both the period of initial

establishment and throughout early plantation devel-

opment. A positive field response from nutrient

loading may persist over time as a result of the initial

advantage of rapid root growth which may help to

enhance subsequent uptake of nutrients (McAlister

and Timmer, 1998).

4.2. Nursery fertilization and growth after

planting

In contrast to survival response, the effect of

fertilization in the nursery on post-planting height

persisted only 3–4 years. Moreover, during these

years, plant height was strongly related to initial

seedling height in the nursery (rPearson = 0.992, 0.853,

0.764, 0.843 and 0.797 in June 1993, October 1993,

1994, November 1995 and January 1997, respectively,

P < 0.05 for all dates). Post-planting height is largely

associated with initial seedling size in the nursery

(Roth and Newton, 1996; Villar-Salvador et al., 2000;

Puertolas et al., 2003). However, after 4 years, the field

height of initially smaller plants from non-fertilized or

low fertilized treatments did not differ significantly

from the height of the taller fertilized seedlings. Small

plants tend to have greater height growth rates

irrespective of nutrient status or other treatments

applied in the nursery, which acts to minimize initial

size differences of taller plants, particularly on harsh

planting sites (Tuttle et al., 1988). However, this effect

often does not become apparent until several years

after planting (Rose and Ketchum, 2003). For

instance, Oliet et al. (2000) reported no significant

height differences after three to four years for

outplanted P. halepensis seedlings among fertilizer

treatments (excepting non-fertilized plants) exposed

to the same nursery fertilizer treatments as in this

study. Furthermore, some authors have reported that a

nutrient concentration effect promoting field height

growth (irrespective of initial size), only persisted for

one year (Irwin et al., 1998; Puertolas et al., 2003). In

contrast, GSD and slenderness quotient differences

among nursery fertilization treatments in our study

still persisted 9 years after planting. No significant

differences in SV were present in 2002, indicating that

shoot biomass per plant was not affected by nursery

fertilizer treatments at a mid to long timescale.

However, when considering survival combined with

stem volume (i.e., volume index) in the analysis, the

highest fertilized rates of both formulations promoted

significantly higher per hectare volume.

4.3. Tree shelters and planting response

Tree shelters did not affect survival until the fifth to

sixth year, when survival of non-protected plants was


reduced compared to that of protected seedlings. This

phenomenon was first noted at the February 1999

measurement, after the intense drought of 1998. By

this time, shoot height exceeded that of the shelter

(mean height of protected trees >100 cm, Fig. 3) so

shelter microclimate should not have had as strong an

effect on plant growth conditions compared to earlier

in the study, particularly since the majority of foliar

canopy had emerged from the shelter (data not

shown). However, shelters may have provided some

continuous benefit even at this point by minimizing

transpirational demand of protected foliage associated

with drying winds (Bergez and Dupraz, 1997). Several

other studies reported significant differences in first

year seedling survival as affected by tree shelters,

some suggesting a species-specific response to shelter

microclimatic conditions under Mediterranean envir-

onments (Costello et al., 1996; Marques et al., 2001;

Oliet et al., 2003). However, corresponding with our

results, some studies have also found changes in

survival response with time. For instance, survival of

Quercus rubra L. and Fraxinus pennsylvanica Marsh.

was not affected during the first year after planting, but

significant differences appeared in years 3 and 8,

respectively, with improved seedling survival for each

species when protected with shelters (Ponder, 2003).

Height of protected plants increased significantly

compared to non-protected plants during the first year

after planting, when mean seedling height was below

the length of the shelter (60 cm). This response of

protected plants is associated with reduction in light

availability within the tubes, which stimulates height

growth (Potter, 1991; Jacobs, 2004). Furthermore,

reduced height development in non-protected seed-

lings during the first year may be associated with

transplanting stress caused by desiccation in the very

windy conditions of the experimental field. For the

continued duration of the study period, height

differences remained relatively constant. Various

results have been reported in previous studies,

suggesting species and/or site specificity. Gillespie

et al. (1996) found that protected Q. rubra trees were

still significantly taller 5 years following planting than

non-protected trees. Dupraz (1997) reported that

Juglans regia L. trees emerged from the top of the

shelter during the first growing season, but the height

advantage of protected trees diminished after 10 years.

Similarly, Ponder (2003) reported significant differ-

ences in first year height of Q. rubra, J. nigra L. and F.

pennsylvanica, though in year 10 the differences were

significant only for Q. rubra. Further, non-protected P.

halepensis plants outperformed protected plants 5

years following planting (Oliet et al., 2000). Stimula-

tion of height within the tree shelter environment may

subside once the plant reaches the top of the shelter.

Wind-induced stem and leaf movement of non-

protected trees promotes greater diameter growth

(Kjelgren and Rupp, 1997; Bergez and Dupraz, 2000).

Since shelters were not removed in our study,

continuous dynamic pressure on the basal stem

provoked by wind enhanced sturdiness at the expense

of height growth by allocating more carbon to

diameter growth and less to height growth (Gillespie

et al., 1996). This helps to explain how differences in

stem diameter among protected and non-protected

trees increased with time and differences in slender-

ness quotient were high compared to other long-term

studies (Oliet et al., 2000; Johansson, 2004). In spite of

differences in height and slenderness quotient, stem

volume was not affected by sheltering after 9 years.

5. Conclusions

Seedling quality attributes of nursery-stock and

application of tree shelters at planting each affected

outplanting response of A. salicina throughout several

years. In particular, while the effect of mineral

nutrition in the nursery on height growth diminished

after 3–4 years, the differences in survival were

maintained for the 9-year study duration. The effect of

tree shelters on survival became apparent after several

years, while differences in growth were established

after the first year and remained consistent throughout

the study for many morphological traits. This response

suggests that more than one or two seasons provides a

more useful assessment of the effects of nursery

practices and stock quality variables, as well as

silvicultural treatments applied in the field, on

response following planting and should therefore be

emphasized in afforestation and reforestation experi-

mental trials.

In arid and windy areas, A. salicina establishment is

enhanced by protecting seedlings after planting with

tree shelters and by nursery cultural treatments which

promote high seedling P content. Mineral nutrition of


seedlings for plantation establishment must be

optimized to match both species necessities and site

ecological conditions.

Acknowledgments

We gratefully acknowledge the financial support

of the National Institute for Agriculture and Food

Technology and Research (Spanish Department of

Science and Technology). The comments of two

anonymous reviewers substantially improved the

manuscript.

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