Comparative growth, biomass production, nutrient use and soil amelioration by nitrogen-fixing tree species in semi-arid Senegal

Comparative growth, biomass production, nutrient use and soil amelioration by nitrogen-fixing tree species in semi-arid Senegal

Forest Ecology and Management 176 (2003) 253±264 Comparative growth, biomass production, nutrient use and soil amelioration by nitrogen-®xing tree sp...

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Forest Ecology and Management 176 (2003) 253±264

Comparative growth, biomass production, nutrient use and soil amelioration by nitrogen-®xing tree species in semi-arid Senegal J.D. Deansa,*,1, O. Diagnea,b, J. Nizinskia,c, D.K. Lindleya,1, M. Secka,b, K. Inglebya,1, R.C. Munroa,1 a

Centre for Ecology and Hydrology, Edinburgh, Bush Estate, Penicuik, Midlothian EH26 0QB, Scotland, UK b Institut SeÂneÂgalais de Recherches Agricoles, B.P. 2312, Dakar, Senegal c CIRAD-Amis, Laboratoire ECOTROP, Avenue Agropolis, 34398 Montpellier, Cedex 5, France Received 16 January 2001; received in revised form 26 March 2002; accepted 13 May 2002

Abstract Survival, biomass production, nutrient use and fertility of soil were examined in experimental stands of 10-year-old N-®xing indigenous Acacia nilotica (L.) Del. (eight provenances), A. senegal (L.) Willd. (one provenance), Acacia tortilis (Forsskal) Hayne (six provenances), Acacia raddiana syn A. tortilis (Forssk.) Hayne ssp. raddiana (Savi) Brenan (three provenances) and exotic N-®xing A. holosericea A. Cunn. ex G. Don (one provenance), A. cowleana Tate (one provenance), Acacia aneura F. Muell. ex Benth. (twelve provenances), Prosopis juli¯ora (Sw.) DC. (one naturalised provenance), Prosopis cineraria (L.) Druce (®ve provenances) and P. chilensis (Molina) Stuntz (two provenances) growing on a coastal site in semi-arid Senegal. Similar data were obtained for the non-N-®xing fast-growing exotic species Azadirachta indica Adr. Juss. and Eucalyptus camaldulensis Dehnh. of the same age that were growing on adjacent plots. P. juli¯ora produced more biomass than the other species and showed the greatest potential for use as a multipurpose tree. A. aneura showed the greatest potential for fodder production and despite its small stature, it ranked second for biomass among the N-®xing species. Of the indigenous species, A. nilotica was the most ef®cient user of non-renewable elements in biomass production. A. raddiana seemed pro¯igate in its consumption of both N and P in wood production. E. camaldulensis was ef®cient in its use of N in wood but its wood contained large concentrations of P and K. A. aneura, P. juli¯ora and A. nilotica had the smallest concentrations of P in wood. A. raddiana and A. indica contained large concentrations of K in their wood in contrast to A. aneura and A. nilotica which were the most ef®cient users of K in wood production. There were few signi®cant differences in fertility in soils beneath the differing tree species. Nevertheless, plots containing P. juli¯ora and P. cineraria tended to be the most fertile. Soil beneath A. indica was more fertile than soil beneath some N-®xing species. The extent of removal of plant nutrients in biomass suggests that without inputs of nutrient elements, harvest of wood and foliage is unlikely to be sustainable at the measured rate of production for more than two tree rotations of 10 years duration. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen-®xing trees; Biomass; Fodder; Plant nutrients; Soil fertility; Short-rotation forestry

*

Corresponding author. Tel.: ‡44-131-445-4343; fax: ‡44-131-445-3943. E-mail address: [email protected] (J.D. Deans). 1 The Centre for Ecology and Hydrology is a component of the Edinburgh Centre for Tropical Forests. 0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 2 9 6 - 7

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1. Introduction

2. Methods and materials

Over-exploitation of land associated with rapidly expanding human populations and the occurrence of extensive droughts have resulted in land degradation with concomitant loss of tree cover in the drylands of sub-Saharan Africa (Breman and Kessler, 1995). Whereas in peri-urban areas, farmers plant trees to produce construction timber and windbreaks, in rural areas, the principal use of wood is for fuel (ClineCole et al., 1998). Traditionally, ®rewood is harvested from natural vegetation even although awareness of the need to plant trees is increasing among populations. Diminishing crop yields have resulted in expansion of agriculture into grazing lands, woodlands and fallows (Rocheleau et al., 1988) with concomitant further loss of tree cover and increased risk of land degradation. Breman and Kessler (1995) assert that current levels of fuelwood harvesting from natural vegetation in densely populated rural areas in the southern Sahel and northern Sudanian zones are unsustainable. Consequently, and as a matter of urgency, there is a need to increase tree planting to provide the necessary timber, fuelwood and nontimber forest products which are needed if living standards are to be protected and further land degradation prevented. Although there is a long history of nitrogen-®xing tree use and culture in the western Sahel, there is relatively little information detailing comparative tree growth and production in arid and semi-arid zones. There have been many potentially useful comparative tree growth studies; however, these have rarely continued much beyond the establishment and early growth phases and thus are not necessarily of value in assessing long-term productivity. Chaturvedi and Behl (1996) reported that those species, which grew well soon after planting, did not necessarily perform well subsequently in a multispecies trial of 8 years duration in India. In that trial, Prosopis juli¯ora and Acacia nilotica grew best. The overall objective of this study in a 10-year-old trial in Senegal, was to identify fast-growing native and exotic nitrogen-®xing tree species which have good nutrient-use ef®ciency and woody biomass production. A secondary objective was to compare these performance attributes with those of fast-growing non-N-®xing species.

2.1. Study site The semi-arid study site located 15 m a.s.l. at latitude 148250 N, longitude 168580 W at Bandia in coastal Senegal (10 km from the Atlantic ocean) has a single rainy season with average annual precipitation of 350±400 mm and a dry season lasting for 9±10 months. Average monthly temperatures are in the range 26±30 8C and daily maximum dry season temperatures exceed 40 8C. Soils are vertisolic with bulk density of 1.5 g ml 1 in the top 10 cm and pH of about 6.5. They contain ferruginous concretions to 2 m depth and overlie a thick layer of sand/clay which extends to the slightly brackish water table located about 10 m beneath the soil surface. 2.2. Tree species and provenances Ten years prior to the study (1984), 49 trees of each of 10 nitrogen-®xing species that seemed to have potential for rapid, timber, fuelwood or fodder production were planted at 5 m2 spacing in 7  7 tree plots in a randomised trial containing four replicated contiguous blocks. Tree species included were A. nilotica (L.) Del. (eight provenances), Acacia senegal (L.) Willd. (one provenance), Acacia tortilis (Forssk.) Hayne (six provenances), Acacia raddiana syn A. tortilis (Forssk.) Hayne ssp. raddiana (Savi) Brenan (three provenances), Acacia holosericea Cunn. ex G. Don (one provenance), Acacia cowleana Tate (one provenance), Acacia aneura F. Muell. ex Benth. (12 provenances), P. juli¯ora (Sw.) DC (one provenance), Prosopis cineraria (L.) Druce (5 provenances) and Prosopis chilensis (Molina) Stuntz (two provenances) giving 40 provenances and 160 plots in total. Further details of provenances are presented in Appendix A. To provide some comparative data for non-nitrogen®xing trees species at 10 years of age, assessments were also made on Eucalyptus camaldulensis Dehnh. and Azadirachta indica Adr. Juss. trees. These latter species are commonly planted throughout the Sahel and were growing on the same soil type as above and at the same spacing. They had been established in two replicated trials, each containing four blocks. Plots containing these latter species were located in close proximity to

J.D. Deans et al. / Forest Ecology and Management 176 (2003) 253±264 Table 1 Details of dryland tree species studied at a coastal site in Senegala Species name A. aneura P. cineraria P. juliflora A. nilotica A. tortilis A. indica A. raddiana E. camaldulensis P. chilensis A. cowleana A. holosericea A. senegal a

by

Indigenous to Sahel

Nitrogen fixing

‡ ‡

‡ ‡ ‡ ‡ ‡

‡

‡

‡

‡ ‡ ‡ ‡

Positive responses are denoted by ‡ and negative responses .

the trial with nitrogen-®xing trees. Measurements were made on 10 trees in each plot of these latter blocks. The eucalypts had been managed in the traditional manner; harvesting occurred at 5 years of age and trees were allowed to resprout as coppice with only two shoots being permitted to grow from each coppiced stump. The A. indica trees had not been coppiced. Details for all studied tree species are presented in Table 1. 2.3. Growth, biomass and survival To ensure that plot edge effects were excluded from measurements, stem diameters 30 cm above the ground and tree heights were recorded on the nine central trees in each plot. Many trees were multi-stemmed at breast height (1.3 m above the ground, diameters at breast height (dbh)) and thus it was faster, simpler and more accurate to measure stem diameters at 30 cm height because most trees carried only between one and three stems at that height. Subsequently, regression relationships which enabled dbh to be estimated from diameter at 30 cm height were developed by measuring dbh and diameter at 30 cm height for each measured tree in each plot of block three. The resulting regression relationships were then applied to measurements at 30 cm height for trees growing in the other experimental blocks to enable estimation of dbh in these blocks. Basal area of multi-stemmed trees was assessed by summation of the individual stem basal areas for each tree. Basal diameter was subsequently re-calculated from total basal area assuming that only one stem

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was present. Where survival was less than 100% in the central zone, assessments were restricted to trees that were completely surrounded by other living stems. Where entire plots (provenances) of trees had grown poorly and had produced large numbers of thin stems (>10), they showed little potential for widescale use and in such cases, only tree height was recorded. Within species, there were no striking differences of tree architecture and hence biomass was estimated at species level, rather than at provenance level. For each species, above-ground biomass was assessed by destructive harvesting of trees. Destructive harvests were restricted to ®ve trees per species and r2 for the developed allometric relationships always exceeded 0.9. Trees selected for destructive harvesting included individuals approximating the thinnest and thickest trees in each species (diameter range determined from dbh assessments) as well as three trees approximating mean and interquartile points of the species' basal area distribution. Wood was allocated to diameter classes as follows: 1±2, 2±5 and >5 cm. Fruits, leaves and twigs (<1 cm thick) were harvested as a single entity by severing (at breast height), a single large stem/branch which carried typical amounts of leaves and fruits. Subsequently, these harvested branches were separated into woody twigs < 1 cm thick, leaves and fruits. Total amounts of leaves, twigs and fruits on each tree were assessed by proportional scaling, i.e. the quotient of total stem cross-sectional area at breast height and cross-sectional area of the sampled stem/branch at breast height was multiplied by the respective fresh weight of harvested tissue on the severed branch. For further details see Deans et al. (1999). Fresh weights of all harvested tissues were obtained in the ®eld before taking sub-samples of known fresh weight for dry weight determination (oven drying at 70 8C for 72 h) to enable calculation of dry biomass. The developed regression equations were used to calculate biomass for a tree of mean dbh in each stand. Tree survival in each plot was assessed by counting the numbers of live trees (maximum 49) during measurement of height and diameter. 2.4. Plant and soil chemistry Following dry weight determination, for the eight fastest growing species, plant tissue samples were ground to pass a 0.5 mm mesh prior to chemical analyses. Only P. cineraria and A. nilotica had produced

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suf®cient fruits to enable fruit chemical analyses to be undertaken. To check for the existence of differences in the soil ameliorating capacity of the differing species and provenances, soil samples were removed from 0 to 10, 10 to 25 and 25 to 50 cm depths adjacent to ®ve stems occupying the central region of each plot in block three of the trial. This initial soil analysis was restricted to block three because it would have been prohibitively expensive to execute such a comprehensive analysis on soils taken from all blocks. In each plot, soils from corresponding depths adjacent to each sampled tree were combined to reduce sample numbers before drying the soil and sieving the samples to pass a 2 mm screen. Soil samples were subsequently analysed for C, N, P and K. Analyses of tree growth data and soil samples from block three were used to guide further soil sampling and subsequently, soils were sampled from similar central locations in blocks one, two and four of the trial. However, again for reasons of expense and because slow growing trees are unlikely to be of interest to agroforesters, sampling was restricted to: (a) plots containing the fastest growing provenances, and (b) the 0±10 cm deep soil layer. Chemical analyses of soils and vegetation were carried out in the laboratories of L'Institut de Recherche pour le Developpement (IRD, previously known as ORSTOM) in Dakar, Senegal. For details of the individual extraction procedures and analyses, see http://www.ird.sn/act-rech/LCA/Tarifs.html. 2.5. Statistical methods Inter-speci®c (between species) and intra-speci®c (within species) growth, biomass, tissue and soil nutrient data were compared by analysis of variance. Covariance analyses were utilised to remove the effects of tree size when considering soil nutrient and carbon status. Linear regression techniques were used to derive equations which enabled estimation of tree biomass components at stated dbh. 3. Results 3.1. Growth survival and biomass Ten years after planting, there were signi®cant inter and intra-speci®c (p < 0:001, F ˆ 3:6 7:8)

differences in height, diameter and survival between the provenances of nitrogen-®xing trees (Fig. 1). There were no signi®cant differences of height, diameter or survival between replicate blocks (F ˆ 0:4 2:4). Statistics were calculated on plot means, thus the absence of standard errors for height and diameter in Fig. 1 denotes that data are mean values from single blocks because survival of the provenance/ species in other blocks was too poor to enable unbiased measurements to be made. P. juli¯ora and A. cowleana grew tallest and provenances 34 (A. raddiana from Israel), 32 (P. chilensis from Chile) and the locally derived (naturalised) provenance of P. juli¯ora had greatest basal area. Two-thirds of the provenances of the indigenous Sahelian species A. tortilis tended to be shorter than the exotic species, but overall, A. senegal from Yemen was shortest. The latter had developed a very ¯attened leaf canopy and numerous thin stems, most of which were smaller than 3 cm diameter and none exceeded 5 cm diameter. Inter- and intra-speci®c variation in survival was greater than for growth (p < 0:001, F ˆ 7:8). Trees of only three provenances (29 A. nilotica from Senegal, 13 P. juli¯ora, and 7 A. raddiana from Senegal) survived better than 90%, but more than 80% of trees of ®ve other provenances (8 P. cineraria, 14, 15 and 31 A. tortilis, and 27 A. nilotica) also survived for 10 years after planting. Overall, plants raised from the locally harvested seeds of P. juli¯ora performed best. P. juli¯ora trees were tallest, they ranked third for diameter and more than 98% of them survived. Of the indigenous species, A. tortilis tended to grow more slowly than either A. nilotica or A. raddiana. For each species, selection of the fastest growing provenances would provide increases of about 16 and 30% for height and diameter, respectively, relative to the mean values derived from all provenances of the same species. Some of the fastest growing provenances survived poorly; provenance 34, A. raddiana from Israel and 32, a provenance of P. chilensis from Chile produced the thickest stems, but survival of the latter averaged only 38% and that of the former was even poorer (18%). No living trees of A. holosericea were found in three of the four blocks and survival of provenance 40, A. aneura was so poor in every plot that unbiased comparative measurements of its height and diameter

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257

Fig. 1. Heights dbh and survival of differing tree species and provenances after 10 years of growth at a coastal site in semi-arid Senegal. Error bars are standard errors of the means.

could not be made because the surviving trees were more or less ``open grown.'' Table 2 illustrates estimated mean production of leaves and wood for the species that grew and survived well in the main trial. Data for the 10-year-old non-N®xing species E. camaldulensis and A. indica are included for comparison. Values for the former include biomass produced at an intermediate harvest, i.e. as in local usage, the trees were cut for poles at age 5, and then two stems were permitted to grow from the coppiced stumps. These two coppiced stems were harvested during this study. Estimated total biomass

production by P. juli¯ora greatly exceeded that of the other species (p < 0:01, Table 2) and estimated leaf and wood production to age 10 also differed signi®cantly between species and provenances (p < 0:01). At age 10, A. aneura and E. camaldulensis had signi®cantly (p < 0:01) greater leaf biomass than the other species and P. juli¯ora produced signi®cantly (p  0:01) more wood thicker than 2 cm and hence likely to be harvested, than all other species (Table 2). With the exception of A. raddiana, the two exotic nonN-®xing species produced signi®cantly more wood than the indigenous N-®xing species (Table 2).

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Table 2 Estimated mean above-ground biomass (S.E., kg) for native and exotic trees after 10 years of growth at a coastal site in Senegala Tree species b,c

A. aneura P. cinerariab,c P. juliflorab,c A. niloticab A. tortilisb A. indicac A. raddianab E. camaldulensisc

Leaves

Wood thinner than 2 cm

Wood thicker than 2 cm

Total above-ground biomass

14.29 10.65 8.17 8.35 2.32 8.57 2.82 15.10

34.24 22.76 26.47 35.55 18.32 11.86 27.76 11.62

58.39 35.14 106.22 37.89 27.92 76.13 53.07 59.23

106.92 67.83 140.85 81.80 48.57 96.56 83.65 85.95

(1.04) (0.38) (0.84) (0.45) (0.25) (0.62) (0.13) (0.84)

(2.40) (1.00) (2.76) (1.96) (1.98) (0.98) (1.90) (0.53)

(5.67) (1.50) (13.33) (2.10) (3.61) (6.66) (4.41) (3.20)

(9.16) (2.88) (16.93) (4.51) (5.84) (8.27) (6.44) (4.57)

a

Data presented for E. camaldulensis include biomass recorded in an intermediate harvest for poles at age 5 years. Nitrogen ®xing. c Exotic species. b

3.2. Plant nutrient concentrations and their consumption in biomass production Mean concentrations of plant nutrients contained in biomass of the fastest growing species are presented in Table 3. Nutrient concentrations in leaves and fruits were signi®cantly (p  0:001) greater than those in woody tissues and excepting N, concentrations in fruits exceeded those in leaves (p  0:01). As anticipated, nutrient concentrations in woody tissues decreased as tissue diameter increased (p  0:001). Whereas the indigenous Acacia species tortilis and A. raddiana had the largest concentrations of N in their leaves, the exotic species E. camaldulensis and A. aneura had the smallest concentrations of N in leaves (Table 3). Surprisingly, the non-N-®xing exotic species A. indica had leaf N concentrations which were comparable with those of most N-®xing species. Overall, P concentrations in leaves of exotic species tended to be smaller than P concentrations in leaves of indigenous trees. In contrast, the exotics, E. camaldulensis and A. indica had greatest concentrations of leaf K. The indigenous Sahelian species A. nilotica had the smallest concentration of K in its leaves. Nutrient concentrations in wood also differed signi®cantly between species (p  0:01) and as for leaves, E. camaldulensis had the smallest concentrations of N in wood. A. nilotica also had smaller wood N concentrations than most other species but the indigenous A. raddiana seemed pro¯igate in its consumption of both N and P in wood production. A. aneura, P. juli¯ora and A. nilotica had the smallest concentrations of P in wood and in contrast to its ef®cient use of N, wood of

E. camaldulensis contained large concentrations of P and K. A. raddiana and A. indica also contained large concentrations of K in their wood in contrast to A. aneura and A. nilotica which were the most ef®cient users of K in wood production. 3.3. Nutrients in soil beneath tree canopies Without exception, concentrations of C, N, P and K declined as depth in soil increased (p  0:001). Amounts of C, N, P and K in the top 10 cm of soil exceeded those at 10±25 cm depth by about 50, 60, 100 and 300%, respectively. There were signi®cant differences in C and nutrient concentrations in soil beneath differing tree species and provenances (Table 4). However, these differences were restricted to the top 10 cm of the soil pro®le. There were no signi®cant differences in soil nutrient or carbon content between species or provenances when soil depth exceeded 10 cm (data not presented). Consequently, soil sampling in blocks one, two and four was restricted to the top 10 cm of the soil pro®le. Soil in plots containing P. cineraria tended to be the most fertile. These plots contained signi®cantly greater concentrations of C than those containing P. chilensis and signi®cantly (p  0:05) greater concentrations of N, P and K than was found in plots containing A. tortilis. Largest amounts of N were found in soil beneath P. juli¯ora trees. Nitrogen concentrations beneath P. juli¯ora signi®cantly (p  0:05) exceeded those found in soil beneath A. tortilis. However, these latter differences were related to tree size. Excepting N, large trees tended to have greater concentrations of

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259

Table 3 Mean percent dry weight of N, P and K in tissues (fruits, leaves and wood <1, 1±2, 2±5 and >5 cm diameter) in 10 year old trees growing at Bandia in Senegala Species

Fruit b,c

Leaf

<1

1±2

2±5

>5

1.89 (0.17) 0.10 (0.002) 0.79 (0.08)

0.80 (0.16) 0.05 (0.008) 0.19 (0.04)

0.55 (0.07) 0.03 (0.009) 0.12 (0.02)

0.47 (0.04) 0.02 (0.007) 0.07 (0.09)

0.43 (0.07) 0.01 (0.003) 0.06 (0.005)

2.0 (0.21) 0.11 (0.01) 0.66 (0.06)

0.84 (0.08) 0.09 (0.01) 0.37 (0.03)

0.75 (0.08) 0.07 (0.01) 0.19 (0.03)

0.70 (0.05) 0.06 (0.01) 0.22 (0.03)

0.75 (0.15) 0.07 (0.02) 0.25 (0.005)

2.59 (0.17) 0.12 (0.01) 1.03 (0.12)

1.02 (0.14) 0.08 (0.01) 0.46 (0.05)

0.67 (0.08) 0.04 (0.01) 0.23 (0.03)

0.56 (0.05) 0.03 (0.001) 0.23 (0.03)

0.42 (0.15) 0.02 (0.001) 0.15 (0.01)

2.13 (0.08) 0.13 (0.008) 0.46 (0.09)

0.81 (0.06) 0.05 (0.008) 0.27 (0.04)

0.54 (0.1) 0.04 (0.007) 0.23 (0.03)

0.35 (0.04) 0.02 (0.004) 0.16 (0.016)

0.31 (0.07) 0.02 (0.003) 0.11 (0.016)

A. aneura

N P K

P. cinerariab,c

N P K

P. juliflorab,c

N P K

A. niloticab

N P K

A. tortilisb

N P K

2.96 (0.28) 0.19 (0.06) 0.85 (0.28)

1.08 (0.11) 0.12 (0.02) 0.34 (0.04)

0.71 (0.06) 0.09 (0.01) 0.30 (0.05)

0.55 (0.04) 0.07 (0.008) 0.30 (0.04)

0.57 (0.12) 0.05 (0.008) 0.24 (0.04)

A. indicac

N P K

2.26 (0.10) 0.12 (0.004) 1.37 (0.16)

0.81 (0.07) 0.12 (0.01) 0.73 (0.08)

0.54 (0.01) 0.08 (0.02) 0.53 (0.09)

0.42 (0.04) 0.06 (0.008) 0.42 (0.04)

0.51 (0.05) 0.04 (0.006) 0.33 (0.06)

A. raddianab

N P K

2.79 (0.17) 0.22 (0.01) 1.14 (0.10)

1.07 (0.09) 0.16 (0.03) 0.35 (0.07)

1.05 (0.08) 0.14 (0.01) 0.28 (0.03)

0.98 (0.08) 0.11 (0.01) 0.31 (0.03)

0.85 (0.05) 0.09 (0.01) 0.30 (0.03)

E. camaldulensisc

N P K

1.29 (0.07) 0.12 (0.02) 1.25 (0.16)

0.38 (0.03) 0.09 (0.01) 0.49 (0.07)

0.34 (0.04) 0.06 (0.01) 0.30 (0.03)

0.26 (0.03) 0.06 (0.005) 0.29 (0.04)

0.24 (0.04) 0.08 (0.02) 0.34 (0.05)

2.33 (0.17) 0.26 (0.02) 1.60 (0.06)

2.09 (0.12) 0.17 (0.03) 1.15 (0.06)

a

Standard errors of means are given in parenthesis. n ˆ 4 for fruits and 5±7 for other tissues. Nitrogen ®xing. c Exotic species. b

Table 4 Mean concentrations of total C, N, P and K in the top 10 cm of soil beneath differing species of trees growing at a semi-arid coastal site in Senegala Species

C g kg

A. aneurab,c P. cinerariab,c P. juliflorab,c A. niloticab A. tortilisb A. indicac A. raddianab,c E. camaldulensisc P. chilensisb,c

9.0 13.2 12.4 12.1 7.5 12.4 11.8 11.0 6.5

a

1

(2.1) (1.4) (2.1) (1.1) (1.5) (3.1) (1.9) (3.0) (1.2)

Figures in parenthesis are standard errors of the means. Nitrogen ®xing. c Exotic species. b

N g kg

1

0.80 (0.18) 1.10 (0.12) 1.12 (0.18) 1.05 (0.09) 0.61(0.13) 1.07 (0.31) 1.02 (0.17) 0.93 (0.20) 0.57 (0.17)

P mg kg 548 460 472 386 255 309 288 308 319

(71) (47) (71) (35) (50) (47) (65) (46) (65)

1

K mg kg 812 711 881 702 439 725 552 548 553

(131) (87) (131) (65) (92) (193) (119) (115) (119)

1

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nutrients in soils beneath their leaf canopies than was found in soil supporting smaller trees (p  0:05). Nevertheless, and irrespective of tree size, there was signi®cantly less soil carbon and N beneath P. chilensis than there was beneath other tree species and provenances (p  0:05). Although not strictly comparable in statistical terms because they occupied different plots, concentrations of nutrients in soil beneath E. camaldulensis and A. indica were within the range of values found in soil beneath the N-®xing species in the main trial. Whereas C and nutrient concentrations in soil beneath E. camaldulensis approximated the middle of the ranges for soil beneath N-®xing species, excepting P, concentrations of elements in soil beneath A. indica lay in the upper end of the range for soil supporting N-®xing species. 4. Discussion 4.1. Growth biomass and survival In this study, trees of the naturalised species P. juli¯ora performed best: 98% of them survived, they accumulated more biomass than the other species and mean dbh at age 10 years exceeded that found by Ndiaye et al. (1993) for 10-year-old Casuarina equisetifolia growing on coastal sand dunes in northern Senegal. Mailly and Margolis (1992) speculated that C. equisetifolia had the potential to be the most productive source of wood in Sahelian Senegal. With about 1000 stems ha 1, C. equisetifolia produced mean annual volume increments of 2±4.2 m3 ha 1 depending on location within the dune complex (Ndiaye et al., 1993). An approximate comparison of volume production by P. juli¯ora and C. equisetifolia may be made from the biomass data. Using wood speci®c gravity for Prosopis of 0.76 (El Osta and Megahed, 1992), to estimate harvestable woody volume from biomass thicker than 5 cm, yields a mean annual volume increment by P. juli¯ora of 3.8 m3 ha 1 to age 10 years from the 394 surviving stems per hectare. That ®gure rises to 5.5 m3 ha 1 a 1 if woody biomass between 2 and 5 cm thick is included. However, this method of calculating timber volume differs from that used by Ndiaye et al. (1993) and so the comparison should be treated with caution. Nevertheless, productivity of P. juli¯ora

appears to be at least of the same order as that of C. equisetifolia. The relatively small, frequently bush-like tree species A. aneura ranked second among the N-®xing species for harvestable woody biomass. However, A. aneura is often multi-stemmed, its wood is dense (ca. 1.1 t m 3, Boland et al., 1987) and it also tends to produce large branches relative to its stem size. Consequently, its comparative biomass is greater than its small stature or basal diameter might suggest. A. aneura and E. camaldulensis produced similar harvestable biomass. The stems (thicker than 5 cm) of E. camaldulensis and A. aneura weighed about 51 and 40 kg per tree, respectively, and other harvestable woody biomasses between 2 and 5 cm thick were about 9 and 18 kg per tree, respectively. These latter data re¯ect that E. camaldulensis partitioned most of its assimilates to the stem in contrast to A. aneura, the branches of which made a large contribution to harvestable woody biomass (>30%). Similarly, branches of A. indica made a large contribution (24%) to harvestable woody biomass. Biomass production by the indigenous A. raddiana and A. nilotica although less than that of A. indica, were similar to that of E. camaldulensis and exceeded the productivity of A. tortilis. However, the performance of the individual provenances varied, demonstrating that species selection does not hold the key to productivity; there is a need to match provenance to site. Growth of all provenances of A. raddiana and A. nilotica was good, but survival of both species was best for local Senegalese provenances (numbers 7 and 29, respectively). That P. juli¯ora and A. nilotica grew well on this site was not surprising. The groundwater (about 10 m below the ground) is slightly brackish and both species are known to grow well on sodic sites. On alkaline soils, Goel and Behl (1996) and Chaturvedi and Behl (1996) found that P. juli¯ora and A. nilotica were the best performing species. However, both also grow well on non-sodic semi-arid sites. Abebe (1994) found that P. juli¯ora and A. nilotica were among the best performing species in southern Ethiopia. Despite its ``weed-like'' invasive tendencies (Muniappan and Viraktamath, 1993), the adaptability of P. juli¯ora (Jambulingam and Fernandez, 1986; Harris et al., 1998) and its good coppicing ability (Jambulingam and Fernandez, 1986; El Fadl, 1997) render it a good

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choice of species for short-rotation forestry in the semi-arid zone. A. tortilis and A. raddiana produced much less leaf biomass (<3 kg per tree) than all of the other species. Leaf biomass of E. camaldulensis and A. aneura, about 15 kg per tree, exceeded that of the remaining species by about a factor of 2. Fodder production and harvesting of leaf materials for dry season fodder is a common practise in the semi-arid zone. E. camaldulensis grows fast but its leaves are unpalatable (Neil, 1989; Burrows et al., 1990), and in India, eucalypts seriously depress growth of herbage beneath their canopies (Upadhyaya, 1996). Consequently, although it is a popular species for poles and ®rewood (Neil, 1989), its utilisation for dry season fodder could be counter productive. On the other hand, A. aneura (Mulga) which in terms of fodder biomass is the most important dry season fodder tree in Australia (Boland et al., 1987) seems capable of producing substantial amounts of fodder, as well as poles and fuelwood. However, Miller et al. (1994) assert that mulga fodder is de®cient in N, P, S and energy and that supplemental feeding is required to maintain animal condition. Data from this study (Table 3) con®rm the relatively poor quality of A. aneura fodder. Its leaves contained the smallest P concentrations and its concentrations of leaf N were smaller than those of all species except E. camaldulensis. In a separate study of six species, leaves of P. juli¯ora had the highest digestible crude protein content (Zech and Weinstabel, 1983) and in this study, P. juli¯ora produced about 8 kg leaves per tree. Consequently, P. juli¯ora has potential for use as a multipurpose timber and fodder species. 4.2. Plant nutrient concentrations and their consumption in biomass production Repeated harvesting of leaves and woody tissues from fast-growing plantations has serious implications for site nutrient budgets and long-term sustainability of production (Jorgensen and Wells, 1986; Parrotta, 1989; Toky and Singh, 1993). Consequently, nutrientuse ef®ciency (units of nutrient element consumed per unit of biomass produced) should be considered when selecting fast-growing trees for repeated harvesting. Nitrogen-®xing species tend to have large concentrations of N in their tissues. Consequently, with such species, sustainability in terms of N is less critical than

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for other elements because N removed in harvesting can be replaced by N ®xed by microsymbionts. Of the non-N-®xing species tested, E. camaldulensis had the smallest concentrations of N in leaf and thick woody tissues, but leaves and wood of the pan tropical non-N®xing A. indica contained larger concentrations of N than several N-®xing trees (Table 3). Farmers utilise the leaves of A. indica for numerous purposes (von Maydell, 1990) as well as its wood and hence both leaves and wood are frequently harvested. Phosphorus concentrations in leaves of A. indica, ranked fourth of the eight species tested, it had the second largest concentrations of P in wood and only E. camaldulensis contained larger concentrations of K in wood than A. indica. These data highlight the risk to future P and K supplies of frequent wood harvesting for both species and although E. camaldulensis leaves are not usually removed from sites, N and P tend to become immobilised beneath plantations of eucalypts (Toky and Singh, 1993). Because of the relatively large concentrations of nutrients in leaves, leaf harvesting potentially poses a greater risk to site nutrient budgets than timber harvesting (Deans et al., 1999). Results from this study indicate that harvesting all of the leaves at age 10 would remove 77, 108 and 84 kg N ha 1; 4, 5 and 4 kg P ha 1; and 47, 45 and 33 kg K ha 1 for A. indica; A. aneura; and P. juli¯ora, respectively. Such nutrient removal rates exceed Sanchez's (1976) estimates of N and K removal in grain by cereal crops. Comparable ®gures for harvesting all wood thicker than 2 cm at age 10 are 149, 103 and 196 kg N ha 1; 13, 6, and 10 kg P ha 1; and 107, 14 and 72 kg K ha 1. Thus, harvesting the ®nal year's crop of leaves would remove about 40±70, 30±80 and 45±300% of the nutrient amounts removed in harvesting all woody biomass thicker than 2 cm that had been produced in 10 years of growth for N, P and K, respectively. 4.3. Nutrients in soil beneath tree canopies There were few signi®cant differences in concentrations of nutrients in soil beneath N-®xing trees in this study. Verinumbe (1987) reported that crop production was greater on soil taken from beneath A. indica than on soil taken from beneath P. juli¯ora or E. camaldulensis. The soil nutrient status beneath trees in this study indicated that soils were as fertile

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beneath A. indica as they were beneath at least some N-®xing species. In contrast, and as in Verinumbe's (1987) study, soil beneath E. camaldulensis was less fertile than the average for soils beneath the other tested species. Despite having the second smallest concentrations of P in its leaves, concentrations of P in soil were greatest beneath A. aneura which probably re¯ects its large leaf biomass. There were no nutrient analyses for soils prior to establishment of the plantations and thus detecting change caused by the presence of trees was not possible. However, chemical analyses indicate that soils at Bandia are badly degraded, even after 10 years of tree growth. Nutrient concentrations in the top 10 cm of soil averaged only 0.092 and 0.037% for N and P, respectively. Although these ®gures correspond to about 1.4 and 0.6 t ha 1, which could provide the needs for numerous timber and fodder harvests, it seems likely that most of the material will not be available to plants. Plant available P (Olsen method) averaged only about 16.3 ppm which corresponds with about 26 kg P ha 1, i.e. enough for only about two leaf and timber harvests of A. indica, A. aneura and P. juli¯ora. Although trees through their associations with mycorrhizas may be capable of mobilising P from relatively recalcitrant pools in soil (Palm et al., 1991), Buresh and Tian (1998), Deans et al. (1999) concluded that there was little evidence for trees increasing inorganic plant available soil P on degraded sites. Consequently, long-term sustainability of biomass production seems unlikely in the absence of inputs of fertiliser, farmyard manure or other sources of organic matter. 5. Conclusions As assessed from individual trees, the exotic species P. juli¯ora, A. aneura and A. indica produced more above-ground biomass in a 10-year rotation than indigenous Sahelian species. Of the indigenous trees, A. nilotica produced about as much biomass as E. camaldulensis and was the most ef®cient user of non-renewable nutrient elements in biomass production. Differences in nutrient element concentrations in soil beneath the differing tree species depended on tree

size. Overall, soil beneath Prosopis species was the most fertile, but concentrations of P were greatest beneath A. aneura. For use as multipurpose timber, fodder and soil ameliorating trees, P. juli¯ora, A. aneura and A. nilotica appear to have the greatest potential among the tested species because of their good growth and ef®cient use of non-renewable plant nutrients. Nevertheless, in the absence of nutrient inputs, fast productivity of plantations on such degraded soils seems unlikely to be sustainable beyond about two rotations where both wood and foliage are harvested. Acknowledgements This study was partly funded via contract no. TS3CT93-0232 within the STD-3 section of the Commission of the European Communities' research programme for Cooperation with Third Countries and International Organisations. We gratefully acknowledge the ®nancial support. Appendix A Nitrogen-®xing tree provenances at Bandia Provenance Species number details 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

A. nilotica P. cineraria A. nilotica ssp. indica var cupressiformis P. cineraria A. tortilis A. aneura A. raddiana P. cineraria A. tortilis A. aneura P. cineraria A. aneura P. juliflora A. tortilis A. tortilis A. aneura

Country of seed origin

CNRF reference number

Yemen India India

84/949 84/954 84/974

India Israel Australia Senegal India Israel Australia India Australia Senegal India Sudan Australia

84/953 84/976 84/964 82/599 84/950 84/975 84/960 84/951 84/971 82/746 84/978 84/980 84/961

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Appendix A. (Continued ) Provenance Species number details

Country of seed origin

CNRF reference number

17 18 19 20 21 22

Australia Australia Australia Yemen Australia India

84/967 84/970 84/972 84/993 84/965 84/984

Australia Sudan India

84/968 84/988 84/985

India

84/986

India

84/987

Australia Senegal India India Chile India

84/966 81/443 84/952 84/977 84/947 84/983

Israel Yemen Chile Australia Australia Australia Australia

84/979 84/991 84/948 84/963 84/973 84/962 84/969

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

A. aneura A. aneura A. holosericea A. tortilis A. aneura A. nilotica ssp. indica var jacquemontii A. aneura A. raddiana A. nilotica ssp. indica var jacquemontii A. nilotica ssp. indica var vediana A. nilotica ssp. indica var cupressiformis A. aneura A. nilotica P. cineraria A. tortilis P. chilensis A. nilotica ssp. indica var jacquemontii A. raddiana A. senegal P. chilensis A. aneura A. cowleana A. aneura A. aneura

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