Effect of pruning on Prosopis juliflora: considerations for tropical dryland agroforestry

Journal of Arid Environments (2003) 53: 441–455 doi:10.1006/jare.2002.1069

Effect of pruning on Prosopis juliflora: considerations for tropical dryland agroforestry

Mohamed A. Elfadl* & Olavi Luukkanen Tropical Silviculture Unit, University of Helsinki, PO Box 28, Helsinki 00014, Finland (Received 8 June 2001, accepted 11 July 2002) Effects of pruning on biomass growth in Prosopis juliflora were examined under dryland conditions in the Sudan. Growth parameters were followed for a total period of 32 months and water content, and gas exchange measurements were carried out. Heavily pruned trees yielded more than six times larger usable wood volume and produced 60% more leaf biomass than the control. The water status in pruned trees was improved which also had a more efficient CO2 assimilation rate, associated with higher stomatal conductance. The results and their implications for the management of sparsely spaced P. juliflora are discussed in relation to agroforestry. # 2002 Elsevier Science Ltd. Keywords: Prosopis juliflora; shoot pruning; ecophysiology; silviculture; agroforestry; drylands; Sudan

Introduction Prosopis juliflora (Swartz) DC. is a medium-sized drought-tolerant leguminous multipurpose tree belonging to the family Mimosaceae. It has an extensive natural range in the south-western United States and northern Mexico, and it is widely found as introduced species in tropical arid and semi-arid regions (Burkart, 1976). Prosopis species have been highly valued as a multi-purpose resource that can contribute to socio-economic development in rural communities (Felker, 1979; Fagg & Stewart, 1994; Tewari et al., 1998). Prosopis juliflora was introduced to the Sudan in 1917 (Broun & Massey, 1929), and to date its evaluation has mostly concentrated on biomass yield and coppicing ability (Ballal, 1986; Elfadl, 1997). The species has become naturalized and has been found to be one of the most promising suitable species for afforestation and reforestation programmes in the arid and semi-arid areas of the country (Luukkanen et al., 1983). Despite its merits in the Sudan, the species also has been regarded as a noxious weed on fertile irrigated agricultural lands in the country (Ahmed ElHouri, 1986), whilst no problems of weedy invasion have been reported over the sandy soil areas of western Sudan. Prosopis juliflora is known to become established and survive on drylands through drought avoidance and/or tolerance mechanisms (Nilsen et al., 1981; Soyza et al., 1996). In general, the productivity of plants growing in arid regions could be improved through direct application of water or by silvicultural practices that reduce *Corresponding author. Fax: +358 9 191 58646. E-mail: [email protected] 0140-1963/03/040441 + 15 $30.00/0

# 2002 Elsevier Science Ltd.

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water loss. One of these practices is shoot pruning, which involves removal of branches to a predetermined height, thus reducing the total leaf area of the plants. Under dryland conditions, the successful establishment of trees is dependent on fast root growth. However, this characteristic is not desirable for trees prescribed for integration with agricultural crops. Different options exist to reduce the lateral extension of the tree root system so as to reduce competition. Shoot pruning has become a management practice in agroforestry for reducing both above- and belowground competition with associated crops (Fownes & Anderson, 1991; Sinclair et al., 1998), supplying organic materials to the soil (Mafongoya et al., 1998) and providing mulch during the cropping season (Kadiata et al., 1998). However, as the functional balance of the tree is altered through pruning, it reacts both morphologically and physiologically in response to the changes and consequently, the growth and development of shoots and foliage may be altered (Singh & Thompson, 1995). Traditionally, Prosopis species have been considered of low value for sawnwood production because of their small trunks. However, pruning of P. juliflora has shown a potential not only for improving lumber quality (Meyer & Felker, 1990), but also for stimulating diameter growth (Patch & Felker, 1997). In Prosopis-based agroforestry situations, Elfadl (1997) used shoot pruning for controlling the size of the tree canopy, thus allowing for better light regime and affecting dry mass accumulation and partitioning. Traditional agrosilvicultural systems in the Indian sub-continent, where P. cineraria forms an important natural-resource base at densities between 20 and 40 trees ha
1 scattered in the fields, were found to have an increased crop production due to improvement in microclimate and soil conditions (Puri et al., 1994). In a field study in north-eastern Nigeria, P. juliflora shoot pruning resulted in the reduction of tree root length and increased sorghum yield and root density ( Jones et al., 1998). Different promising agroforestry settings involving P. juliflora have been noted (Pasiecznik et al., 2001), but the potential of this tree species for dryland agroforestry has largely been neglected due to a lack of precise management guidelines. In intercropping systems, trees and associated crops are known to use different resources (Cannell, 1989). Shoot pruning can be an effective means of controlling the water balance of the agroforestry system ( Jackson et al., 2000). The response of trees to pruning has not generally been considered as a main criterion in tree selection for agroforestry. However, there are situations in which the response to pruning has been included in the tree selection process (Palm, 1995; Peter & Lehmann, 2000). Introducing silvicultural management practices for trees under unfavourable environmental conditions can cause shifts in carbon allocation that favour the organ most affected by the particular stress, so as to maintain a functional equilibrium (Brouwer, 1983). Many tree species exhibit changes in partitioning depending on the treatment (defoliation, shoot pruning, pollarding), and widely different responses, or even conflicting results, have been obtained (Haddad et al., 1995; Tschaplinski & Blake, 1995; Patch & Felker, 1997; Jones et al., 1998). It has been argued that the interpretation of changes in allocation patterns in response to resource supply may be misleading, if differences in plant size are not taken into account (Gebauer et al., 1996). Another view is that all species have similar patterns, and the relative magnitude of response and the differences are mainly due to environmental variations (Vanderklein & Reich, 1999). For example, nutrient deficiency and water stress have been shown to increase the partitioning of photosynthates to root growth, while competition for light favours greater carbon allocation to shoot growth (Ovaska et al., 1993). There have been few studies on pruning effects on the growth and development of trees with different crown architecture and environmental and site conditions (Bandara et al., 1999). The aim of the present study was to determine the effects of shoot pruning on growth and to examine the physiological basis for the observed growth responses in

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P. juliflora under dryland agroforestry conditions in the Sudan. Ultimately, the aim was to benefit rural farmers, land users, and development agencies involved in the promotion of agroforestry in the drylands.

Materials and methods Field experiments on P. juliflora were conducted between November 1990 and July 1993 in a plantation established near Teliebat village (latitudes 121 450 and 131 150 N and longitudes 311 450 and 321 150 E) located near Tendelti, central Sudan (Fig. 1). The trees had been planted in July 1989 using seedling stock. The stocking density was 625 trees ha
1 with original spacing of 4 m  4 m, and the area of the plantation was 27 ha. The soil was typical sandy soil with a clay content lower than 5% in the top 30-cm stratum (Table 1), indicating poor water-holding capacity and low nutrient content. The long-term average rainfall was 2927100 mm with the rains usually occurring from June to October. The experimental design was a randomized complete block with four treatments and three replications. Each replication contained 24 trees, but only the innermost eight trees, that were identical to the plot mean, were used for detailed measurements. The four treatments consisted of a control (unpruned, 0%) and three pruning treatments: removal of 25% (light), 50% (moderate), or of 75% of the branches (heavy). The initial measurements of basal diameter (BD) total height and foliage biomass were done before pruning. The basal diameter measured at 15 cm above the ground using a caliper and the total height was determined by using a telescopic measuring pole. All pruning was done from below. Pruning was accomplished by using pruning scissors in the first week of November 1990.

Figure 1. Map of the Sudan showing the location of the study area.

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Table 1. Main soil characteristics of the experimental area, Tendelti, Sudan

Soil depth (cm) 00–05 05–15 15–30 30–60 60–90 90–120 120–150

Particle size (g kg
1)

Average nutrients

Sand

Silt

Clay Carbon (g kg
1)

950 937 935 914 856 866 877

18 18 17 18 20 16 20

32 45 48 68 124 118 103

0?9 0?8 0?7 0?6 0?5 0?5 0?5

pHWater retention (v/v)% Field
1?5 MPa capacity

Nitrogen P (mg kg
1)(mg g
1) 256 224 210 201 182 172 161

8?7 6?7 4?7 4?7 3?3 3?0 3?0

7?6 7?4 7?2 7?3 7?3 7?2 6?9

14?5 15?7 17?2 18?5 14?8 15?5 14?8

6?0 6?2 6?8 7?3 6?3 7?0 6?3

At the start of the experiment, the branches removed from each pruning treatment were defoliated and their fresh foliage was weighed to determine the degree of defoliation per treatment. Leaf biomass production was measured at approximately 4month intervals (in November, March, and July) till the end of the experiment. In early November 1991 (1 year after pruning), diurnal measurements of net CO2 assimilation rates (A) and stomatal conductance for water vapour ( gs) were taken with a LI-6200 portable photosynthesis system (Li-Cor Inc. Lincoln, NE) equipped with 250-ml cuvette, for three leaves per tree and two trees per treatment per each of the three replicates (n = 72). The same trees were felled at ground level and defoliated for determination of moisture content of foliage and wood. A total of 40 discs (5-cm thickness) were excised from the main stem and branches of eight trees (two trees per treatment). The excised discs and samples of leaves were immediately wrapped in polythene bags, taken to the laboratory and weighed with a sensitive balance of 0.01-g accuracy, and their fresh weights were recorded. The wood and leaf samples were oven-dried at 801C until a constant weight was obtained. The moisture content per cent was calculated on fresh weight basis as per cent moisture content on fresh-weight (wt.) basis = (fresh wt.
dry wt./fresh wt.)  100. The data were subjected to the analysis of variance using SPSS. Student’s t-test and Tukey’s test for multiple comparisons at po0?05 were used to determine differences between treatments (means separation test).

Results The results of growth indicators for the pruning intensity treatments are shown in Table 2. After 1 year of pruning, no negative effects of pruning on the growth attributes were observed. The growth increased linearly with increasing pruning intensity. Light pruning increased the basal diameter (BD) by 41% and the height by 26% and doubled the basal area (BA) and volume as compared to the control values. Following moderate pruning, the BD per tree was twice as large as the control and the trees were also 1?7 times taller. These differences resulted in a 3?5 times larger BA and a 4?4 times larger volume than in the control trees. Differences in BD, BA, height (H), and volume between light and moderate pruning intensities were highly significant ( p = 0?003; 0?005; 0?000, 0?001 respectively). At the age of 3 years, the BD of the heavily pruned trees (6?4 cm) was more than twice as large as the control (2?9 cm) and about 66% larger than that of the light pruning treatment (4?1 cm). Mean height increment was not affected by light pruning,

EFFECT OF PRUNING ON PROSOPIS JULIFLORA IN SUDAN

445

Table 2. Effects of pruning intensity on tree characteristics at age of 3 years. First measurements were carried out simultaneously with the pruning treatment 1 year after planting

Basal diameter (cm)

Basal area (m2 ha
1)

Height (m)

Volume (m3 ha
1)

Light (pruning) Light Light

1?6 2?5 4?1a

0?147 0?462 0?959a

1?4 1?7 2?4a

0?062 0?220 0?755a

1 2 3

Mod. (pruning) Moderate Moderate

1?8 3?1 6?1b

0?178 0?522 1?967b

2?0 1?7 3?2b

0?118 0?372 2?026b

1 2 3

Heavy (pruning) Heavy Heavy

1?8 4?4 6?8b

0?193 1?143 2?426b

1?5 1?9 3?5b

0?114 0?577 2?642b

1 2 3

Control Control Control

1?7 2?0 2?9c

0?173 0?220 0?422a

1?4 1?6 1?9c

0?111 0?155 0?330c

Age (years)

Pruning intensity

1 2 3

Mean values followed by the same letter in the same column do not differ statistically as per Tukey’s test for multiple comparisons, at po0?05. Light, moderate and heavy represent intensity of pruning. Stocking density = 625 trees ha
1?

but moderate and heavy pruning treatments increased the height growth by 68% and 80% compared to the control, respectively (Table 2). Differences in BD or BA between heavily pruned trees and either of the light pruned or the control were highly significant (F3,11 = 30?027*** and F3,11 = 25?771***, respectively). The volume of heavily pruned trees was more than 4?8 times greater than that produced by trees subjected to light pruning and more than six times higher than that produced by the control trees. Tukey’s HSD-test for plot means did not detect significant differences between moderate and heavy pruning intensities in BD, BA, H or volume, however, heavy pruning treatment was more effective than any other treatments. The average fresh foliage mass for the 24 selected trees at the start of the experiment ranged between 0?89 and 0?91 kg tree
1. The 0%, 25%-, 50%- and 75%-pruning treatments resulted in removal of 0%, 43%, 58% and 68% of foliage biomass, respectively. By the end of the first year, total leaf mass had increased by 33%, 57%, 70%, and 78% over the leaf mass at the beginning of the experiment in unpruned, lightly, moderately and heavily pruned trees, respectively. Moderate and heavy pruning produced a significantly greater leaf mass (medium p = 0?058; high p = 0?034), and there was a cumulative effect for 2 years following the treatment, but, no response was observed in the third year (Fig. 2). Within 4 months after treatment the removed leaf biomass was compensated for, and the ratio (per cent increase compared to the removed) of compensation increased with pruning intensity (62%, 131%, and 169% for light, moderate and heavy pruning, respectively). In contrast, unpruned trees shed 23% of their leaves naturally. Figure 3 shows the tendency of increasing moisture content of leaves and wood as a response to increase in pruning intensity, with heavily pruned trees having a significantly higher ( po0?001) moisture content (59% and 48% for leaves and wood, respectively). Photosynthetic rates were significantly higher in pruned trees ( po0?004; Fig. 4) within 1 year after pruning, and maximum values (46?7 mmol CO2 m
2 s
1; Table 3) were obtained on leaves of heavily pruned trees. Values for leaf conductance showed trends similar to those of photosynthesis, where peak conductance (0?61 mol

M. A. ELFADL & O. LUUKKANEN

0.94

1.5

0.93

1.4

0.92

1.3

0.91

1.2

0.90 0.89

1.4

1.0 0.9

0.87

0.8

0.86

0.7 25%

1.5

1.1

0.88

0.85

1.6

July

March

November

446

1.2 1.1 1.0

0.6

50% 75% Control Pruning intensity

1.7

1.3

25%

50% 75% Control Pruning intensity

25%

50% 75% Control Pruning intensity

2.0

2.0

1.9

1.6

1.8 1.7

1.5

July

1.6

March

November

1.5 1.4 1.3

1.5 1.4

1.0

1.3 1.2

1.2

1.1

1.1

1.0

0.5 25%

50% 75% Control Pruning intensity

25%

2.0

50% 75% Control Pruning intensity

25%

50% 75% Control Pruning intensity

3.0

2.5

1.9 2.5

July

2.0

1.7

March

November

1.8

1.6 1.5

2.0

1.5 1.5

1.4 1.3 1.2

1.0

1.0 25%

50% 75% Control Pruning intensity

25%

50% 75% Control Pruning intensity

25%

50% 75% Control Pruning intensity

Figure 2. Effects of pruning regime on the development of leaf biomass of Prosopis juliflora during a period of 32 months. Values are means of foliage biomass (fresh weight) for two trees per treatment.

H2O m
2 s
1) occurred in leaves of heavily pruned trees that also had significantly higher photosynthetic rates ( po0?004). Leaf conductance increased with increasing pruning intensity but was not significantly different when lightly pruned and control trees were compared ( p = 0?41; 0?24 and 0?18 mol H2O m
2 s
1, respectively). Leaves of pruned trees had a lower stomatal resistance than leaves of control plants. The intercellular CO2 concentration (Ci) did not decrease in all treatments, and the difference between ambient CO2 concentration and Ci (Dp) was 65, 116, 109 and 136 p.p.m. for the control, light, moderate and heavy pruning treatments, respectively.

Discussion Prosopis juliflora apparently responded to pruning by faster production of leaves and enhanced photosynthesis to recover some of the lost leaf area. Shoot pruning reduced the total leaf biomass; however, within 4 months after pruning, trees showed increased

EFFECT OF PRUNING ON PROSOPIS JULIFLORA IN SUDAN 50

65

Moisture content (%)

Moisture content (%)

Foliage

60

55

50

45

Wood

45

40

35 0.00

447

25% 50% 75% Pruning intensity

0.00

25% 50% 75% Pruning intensity

Figure 3. Effect of 0%, 25%, 50% and 75% shoot pruning on mean moisture content of fresh foliage and wood (n = 64) of Prosopis juliflora. Vertical lines are standard error of the mean (S.E.).

Conductance (mol m-2 s-1 )

0.7

(a)

0.6 0.5 0.4 0.3 High Medium Low Control

0.2 0.1 0.0 0

CO2 assimilation (
mol m-2 s-1 )

50

500 1000 1500 PAR (
mol m-2 s-1 )

2000

(b)

40 30 20 High Medium Low Control

10 0 0

500 1000 1500 PAR (
mol m-2 s-1 )

2000

Figure 4. Effect of pruning treatments (0%, 25%, 50% and 75%) on (a) leaf water vapour conductance and (b) leaf photosynthesis of Prosopis juliflora measured at 12 months after pruning treatment.

448

M. A. ELFADL & O. LUUKKANEN

Table 3. Mean 7 standard deviation of prevailing environmental conditions and gas exchange parameters (A, net CO2 assimilation; gs, leaf stomatal conductance, Ci, intercellular CO2 concentration; WUE, is water-use efficiency) during measurements on Prosopis juliflora in November 1990, Tendelti, Sudan

Parameters

Average values during measurement (n = 72)

Environment Q (mmol m
2 s
1) T (1C) RH (%) VPD (kPa) Ambient CO2 (p.p.m)

18807582 40?572?51 33?876?42 5?270?26 346?673?0 Control

Gas exchange Amax (mmol m
2s
1) Amean (mmol m
2s
1) gs mean (mol m
2 s
1) Ci mean (mmol m
2s
1) Emean(mmol m
2s
1) WUE (mmol mol
1)

13?45 3?42 0?180 282?6 5?044 0?68

Treatments Low Medium pruning pruning

High pruning

41?34 11?99 0?236 237?9 4?612 2?60

46?69 26?29 0?358 208?9 4?976 5?28

40?25 17?26 0?321 230?5 4?834 3?57

above-ground growth as indicated by significant increase in stem diameter, tree height, and mass of leaves. This increase was directly proportional to the intensity of pruning. Highest photosynthetic rates were recorded in heavily pruned trees, suggesting the phenomenon known as compensatory photosynthesis. In common with other studies several mechanisms for compensation for the lost photosynthetic area, including increased photosynthetic rates (von Caemmerer & Farquhar, 1984), changes in resource allocation (Bandara et al., 1999) and utilization of reserved carbohydrates (Tschaplinski & Blake, 1994) increased with increasing severity of pruning. The present study could not discriminate between different causes of the increased biomass growth, as resource allocation and reserve carbohydrate utilization were not directly measured. Increased compensatory growth might have resulted not only from the direct effect of photosynthesis. Complete removal of the lower three-quarters of the branches (heavy pruning) resulted in a reduced sink for dry matter allocation but also reduced the maintenance respiration. Similarly, Kadiata et al. (1997) observed higher biomass production in repeatedly pruned Albizia lebbeck, Gliricidia sepium and Leucaena leucocephala trees, while a single pruning in their studies failed to induce a stimulatory effect. During the first 4 months after pruning in the present study, trees exhibited rapid refoliation and were able to produce more leaf mass than what had been removed by pruning. A treatment that removes both the source (leaves) and the sink (growing branches), may have a different physiological effects than defoliation alone. In both situations, to refoliate, the plant must either use its reserve carbohydrate resource or replace the lost resource with new assimilation (Lovett & Tobiessen, 1993). In our study, pruning increased the photosynthetic rate and hence, biomass growth, linearly with increasing pruning intensity. Responses of plants to defoliation either by pruning or lopping vary greatly (see Belsky, 1986; Singh & Thompson, 1995; Kadiata et al., 1997; Patch et al., 1998; Verdaguer et al., 2000). Increase in total biomass has been observed in most cases, e.g. in Populus  euramericana cv. ‘Negrito de Granada’

EFFECT OF PRUNING ON PROSOPIS JULIFLORA IN SUDAN

449

(Bassman & Dickmann, 1982), Oenothera biennis (Morrison & Reekie, 1995), and Eucalyptus nitens (Pinkard & Beadle, 1998). Other studies show unaffected biomass production, e.g. in L. leucocephala (Kadiata et al., 1998), Robinia pseudoacacia L. and Acer rubrum L. (Seastedt et al., 1983), or even reduced biomass production as found in Pinus resinosa (Krause & Raffa, 1996) and G. sepium (Kadiata et al., 1998). Irrespective of pruning intensity, the higher water content found in leaves and the wood of pruned trees in the present study could be due to a reduced transpiring surface area of the leaves at the beginning of the experiment. The improved water status of the treated trees during the first year suggests a deeper rooting depth and/or greater ability of pruned trees to take up water to compensate for transpiration losses. Analyses of growth responses revealed that the crowns of the pruned trees were able to produce more photosynthates for growth and maintenance. A reduction of the leaf area, and therefore transpiration, appeared to increase the leaf water potential and leaf turgor (as shown by levels of leaf and wood water content), thus maintaining stomatal opening and an increased photosynthetic rate of the pruned trees. Blake & Tschaplinski (1986) also reported enhanced leaf conductance and increased photosynthesis in defoliated Populus species. Hence, there may be a shift in the allocation pattern towards refoliation that leads to higher carbon gain and faster growth. The trend of increased leaf growth with increasing pruning intensity indicates enhanced partitioning of the current assimilate for rapid recovery of leaf area. Stimulated leaf growth has also been reported in response to defoliation in Diospyros melanoxylon Roxb. (Kotwal, 1981). Appearance of few sprouts on the main stems of P. juliflora after pruning was also observed, which may add another explanation for the increased stem growth: suppression of secondary shoots allows the growth of the main stem. Low stimulation of lateral buds has also been observed on three Quercus species after pruning (Verdaguer et al., 2000). In P. glandulosa var. glandulosa suppression of sprouts has been found to result in clear wood and a higher quality and more valuable lumber (Patch & Felker, 1997). The potential of various Prosopis species for high-value mechanical wood products has recently been also highlighted (Felker, 2000). A reduced sink in the above-ground parts of the pruned trees may allow for more transport of photosynthates to roots, which could lead to better water uptake and growth of the tress. However, the allocation of carbon to roots is affected by the water and nutrient status of the soil (Kramer, 1983; Fabiao et al., 1995; Arndt et al., 2000). Stomatal conductance is governed by environmental factors through various physiological mechanisms (Farquhar & von Caemmerer, 1982). The response of plants to water deficit in the form of reduced photosynthesis is often thought to be caused by stomatal conductance (Downton et al., 1988; Sharkey & Seemann, 1989; Vassey et al., 1991). The only possible mechanism with which stomatal closure can reduce the photosynthetic rate is through decrease in Ci. Our results showed that Ci was not reduced (283 p.p.m.) in the leaves of the control trees; in fact, it was higher as compared to the pruned treatments, suggesting that the stomata did not reduce the CO2 uptake to an extent that could cause the observed reduction in photosynthesis. Instead, there was a decrease in Dp (Ca
Ci) in the control treatment, indicating a decline in the biochemical activity and reflecting the fact that photosynthesis and growth can cease during severe water stress while respiration proceeds (Amthor, 1994). These results from unpruned trees are in agreement with those of Lauer and Boyer (1992) who found that Glycine max and Phaseolus vulgaris had higher values of Ci when subjected to water stress as reflected as low leaf water potential. With the large vapour pressure deficit (3–8 kPa), high ambient temperature (32–451C) and light intensity (41700 mmol m
2 s
1), and low relative humidity (19–52%) characteristic for the present study site (Table 3), a progressive decline in the leaf water status of many species is frequently observed (Gaafar & Salih, unpublished).

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Prosopis juliflora photosynthesizes at rates below the potential maximum when water availability is limited, as reported for many other plant species (Reich et al., 1993). The observed high CO2 assimilation rates (measurements were made during a short period during the rainy season) exhibited by the pruned P. juliflora trees (26?3 mmol CO2 m
2 s
1) were higher than those reported for many xerophytic and mesophytic broad-leaved species, including Acacia auriculiformis (16 mmol CO2 m
2 s
1; Cole et al., 1994), Tamarix ramosissima (13 mmol CO2 m
2 s
1; Cleverly et al., 1997), Ziziphus rotundifolia (12 mmol CO2 m
2 s
1; Arndt et al., 2001), Quercus alba (10 mmol CO2 m
2 s
1; Dougherty & Hinckley, 1981), A. rubrum (10 mmol CO2 m
2 s
1; Bassow & Bazzaz, 1997) and, Curatella americana (7 mmol CO2 m
2 s
1; Medina & Francisco, 1994). Table 2 and Fig. 3 show that high photosynthetic rates can be satisfactorily explained and are not surprising since the pruned trees were the largest and had accumulated the largest biomass: the average annual diameter increment was 2?3 cm and these trees had a 60% greater foliage biomass as compared to the untreated trees. Thus, the greater the leaf area, the greater the relative amount of biomass that is allocated either to the stem or to the branches that support the leaf mass. The enhanced photosynthetic rate of pruned trees paralleled the observed stomatal conductance ( gs). Photosynthetic enhancement has been shown to play an important role in moderating the impact of partial defoliation on growth (Heichel & Turner, 1983; Reich et al., 1993). Increased stomatal conductance ( gs) has been shown to be responsible for the enhancement of photosynthesis in E. nitens (Pinkard et al., 1998); the magnitude of the enhancement increased with increasing severity of pruning. In the present study, stimulation of photosynthesis after shoot pruning was apparent in the newly formed leaves and might also have been present in pre-existent leaves compared to the control indicates that water stress can cause a marked decline in the photosynthetic capacity of P. juliflora. Also, the low rates of photosynthesis of the control trees indicate that most carbon gain in P. juliflora must be during the rainy season and it remains water-limited throughout the rest of the year. Arguably, if the trees of the present experiments relied solely on moisture from deep soil layers or from ground-water (which at our site occurred at a considerable depth), the average daily photosynthetic rate and the intrinsic water use efficiency would have been expected to be the same for all the treatments (Table 3). Prosopis species have been reported to possess a wide genetic variation related to water use (Pennington et al., 1999) as well as substantial variation in other physiological and morphological traits (Felker et al., 1990). The present results suggest that inefficient stomatal control over transpiration, coupled with lack of maintenance of photosynthetic capacity in Prosopis led to poor water-use efficiency. Thus, shoot pruning in P. juliflora is desirable especially at the time of agricultural cropping so as to reduce its capacity to occupy more soil resources and to reduce competition for light (Elfadl, 1997) at the onset of the rains. Prosopis juliflora is obviously a productive species with high intrinsic wateruse efficiency only during periods of unrestricted water availability. Root depth and density are both important for water uptake (Turner, 1986). It is conceivable that pruned trees of the present experiments were able to grow deeper roots and hence to tap increasingly deeper soil water resources as the upper soil layer dried. Less depletion of soil water in the crop rooting zone associated with reduction in root length density in pruned plots of P. juliflora in a semi-arid region of north-east Nigeria has been previously reported ( Jones et al., 1998). A reduction of root density following pruning has also been found in A. saligna (Peter & Lehmann, 2000), Sesbania sesban and L. leucocephala (Fownes & Andersson, 1991), and G. sepium (Schroth & Zech, 1995). Many more species maintain a balanced shoot-to-root ratio and grow a large number of superficial roots when pruned, including Cassia siamea and Calliandra calothyrsus Meissn. (van Noordwijk & Purnomosidhi, 1995),

EFFECT OF PRUNING ON PROSOPIS JULIFLORA IN SUDAN

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Peltophorum dasyrachis Kurz (van Noordwijk et al., 1991). Generally, high root length density seems to be a trait found in highly competitive species. Adjustments in plant architecture can sometimes improve photosynthesis and growth rates of stressed trees (Brunig, 1976). Shoot pruning enhances photosynthesis and causes greater partitioning of photosynthate to shoots, for a quick recovery of leaf area in order to maintain the shoot:root ratio. In this case, the root system will be maintained and no reduction in carbohydrate reserve will take place. On the other hand, reduction of root dry matter occurs as a consequence of utilization of root carbohydrates for shoot growth as observed by Singh & Thomson (1995) in moderately and heavily lopped Alnus glutinosa. Reduction in soluble carbohydrates has been observed at partial defoliation in Platanus acerifolia Wild (Haddad et al., 1995), after quick refoliation of Q. rubra following the removal of the summer foliage (Parker, 1979), and after repeated defoliation of Eucalyptus species (Bamber & Humphreys, 1965). Hsiao & Acevedo (1974) pointed out that water stress enhances root growth not only relative to shoot growth but absolutely. Although the root system was not examined in the present study, our results suggest that no reduction in root growth occurred. Instead, there was either an increase in root growth or, at least, the root system was not reduced as a result of partial removal of branches and leaves.

Conclusions Worldwide concern of environmental degradation has increased the awareness of the importance of agroforestry in environmental rehabilitation and in provision of support to rural people in the form of food security and fuelwood production. The use of P. juliflora in agroforestry systems in the Sudan has shown important results in the past. Pruning improves the growth and yield of P. juliflora, and the effect is most pronounced when pruning is performed up to three-fourths of the total tree height. Under the conditions of the study area, where rainfall is low and variable and usually below 300 mm year
1, results indicated that pruning improves the water status of the trees. Pruned trees show greater leaf CO2 conductance and leaf biomass. In agroforestry, the competition for water can be mitigated by reducing the foliage area of trees by pruning. Shoot pruning at the onset of the rains is recommended since more water will, in this way, be available for associated agricultural crops. There is a necessity for wider spacing of P. juliflora so as to produce an agroforestry system resembling Acacia parkland landscape, so as to better integrate conservation measures with agricultural development. Pruning of Prosopis produces single-stemmed trees of larger dimensions and possibly improves the root mass. Management practices, such as wider spacing and pruning early before the rainy season, can improve the water use, light, and nutrient cycling processes in agroforestry systems based on Prosopis and possibly in other tree species as well. Agroforestry systems should aim at large-sized single-stemmed trees that improved the microclimate favourably for agricultural crops and also provide fuelwood and environmental benefits. The results highlight the caution required in relating gas exchange measurements to below-ground carbon partitioning or water uptake with depth (no root data were available in the present study). Further studies that evaluate both the above- and below-ground biomass growth with water use in P. juliflora are needed, for determining the ultimate suitability of this species for improving the productivity of drylands. This study was financed in an earlier phase by the Department for International Development Cooperation (DIDC) of the Ministry of Foreign Affairs of Finland and subsequently by the Academy of Finland. The authors thank the anonymous reviewer and Dr Peter Felker for their

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valuable comments and suggestions on the manuscript. We gratefully acknowledge this financial support. We also thank the staff of the Forests National Corporation, Tendelti, Sudan, for their assistance with field work.

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