Species As Shade Trees Whenreplan

Potential of some Neotropical Albizia species as shade trees when replanting cacao in Ghana

G.J. Anim-Kwapong Cocoa Research Institute of Ghana, P.O. Box 8, New Tafo-Akim, Ghana; (e-mail: [email protected]; telephone: 233 27 609900; fax: 233 27 609901)

Received 12 June 2002; accepted in revised form 17 April 2003

Key words: Biomass production, Decomposition rates, Planted tree fallow, Shade trees, Soil amelioration

Abstract

The Cocoa Research Institute of Ghana has embarked on studies to support the replanting of cacao ͑Theobroma cacao L͒ in areas, which previously carried the crop but are now degraded. A key component of the studies is to identify fast growing tree species capable of ameliorating degraded soils and ultimately providing suitable shade for cacao. A screening trial involving ten Albizia species in a randomized complete block design experiment was therefore initiated in 1996 to evaluate growth rate, leaf biomass production, carbon and nitrogen contents and decomposition rates. Over a four-year period, Albizia adenocephala, A. guachapele, A. niopoides, A. plurijuga, A. saman and A. tomentosa showed promising results, with 12.2 to 14.5 m height and between 12.4 and 22.4 cm stem diameter ͑DBH͒. Crown diameter ranged between 6.1 and 10.1 m, with light transmission through crowns averaging 50-65% of full sunlight throughout the year. Half-yearly leaf biomass production ranged between 3 and 10 t ha–1, yielding between 0.07 and 0.32 t N ha–1 from each coppicing. Half-life for carbon and nitrogen release from leaves of the six species averaged 31.0 and 32.0 days respectively. This short-term release of C and N is an indication of the quality of the leaf prunings. These species can provide early ground cover, appropriate shade, N and organic matter requirements for re-establishing cacao on denuded and degraded lands.

Introduction Excessive logging and poor farmland management have resulted in the degradation and deforestation of Cacao ͑Theobroma cacao L͒ is an understorey tree most of the natural forest sites suitable for cacao cul- and in Ghana the crop is traditionally cultivated un- tivation in Ghana ͑Hepper 1986͒. In order to sustain der the shade of selectively thinned forest. The forest the cocoa industry and to halt further deforestation, a shade trees contribute to the build-up of soil organic programme of studies to support the replanting of ca- matter, carbon sequestration, nutrient recycling and cao in denuded and degraded forest areas which pre- the maintenance of biodiversity. Shade trees also re- viously carried cacao was started in 1986 by the duce evapotranspiration and hence moisture stress Cocoa Research Institute of Ghana ͑CRIG͒. The dif- during the dry season. This is essential for the sur- ficulty in re-establishing cacao on old cacao lands in vival and establishment of cacao seedlings in season- West Africa has been recognized and attributed to loss ally wet and dry environments ͑Beer 1987͒. Thus, of soil fertility and lack of appropriate vegetation especially in cases of low-input agriculture, shade cover for cacao shade ͑Adams 1962; Ayanlaja 1983͒. trees confer sustainability ͑control of insect pests and The planting of fast growing tree legumes ͑managed weeds, microclimatic stability and soil fertility main- tree fallow͒ has been identified as an option for soil tenance͒ to cacao production ͑Ahenkorah et al. 1987; fertility restoration and maintenance of degraded Beer 1987; Johns 1999͒. lands in Africa ͑Buresh and Tian 1997͒. This study 186 sought to identify and propagate fast growing tree le- Table 1. Soil properties of the study site at CRIG, Tafo, Ghana gumes capable of restoring soil fertility and provid- Soil properties Soil Depth ing shade for the re-establishment of cacao. 0-8 ͑cm͒ 8-30 ͑cm͒ Albizia is a pantropical genus ͑tribe Ingeae and pH ͑1:1 H O͒ 5.8 5.2 subfamily Mimosoideae͒ with about 150 species of 2 Organic matter ͑%͒ 2.5 0.6 which 35 occur in Asia, 48 in Africa and 35 in Organic carbon ͑%͒ 1.45 0.35 America ͑Nielson 1981͒. Most species of the genus Total N ͑%͒ 0.22 0.10 have high biomass production, a spreading crown and Ca ͑me /100g soil͒ 6.40 2.88 light feathery foliage in addition to nodulation and Mg ͑me/100g soil͒ 0.96 0.32 ͑ ͒ nitrogen fixing capabilities ͑Allen and Allen 1981͒ K me/100g soil 0.38 0.20 Na ͑me/100g soil͒ 0.22 0.20 and consequently are good shade trees for tea and Total exchangeable bases 7.96 3.62 coffee plantations. However, they remain largely un- Exchangeable acidity ͑AlϩH͒ 0.10 0.30 tested as shade trees, particularly in the Humid West CEC ͑me/100g soil͒ 8.06 3.92 African sub-region. The objective of this study there- Base saturation ͑%͒ 99.0 92.0 ͑ ͒ fore was to evaluate the potential of ten Neotropical Available Bray’s ppm P 1.25 0.37 Available Bray’s ͑ppm K͒ 109.0 64.0 Albizia species for soil amelioration and as shade Sand ͑%͒ 67.8 54.5 trees for cacao based on the following criteria: emer- Silt ͑%͒ 18.2 16.5 gence and survival ͑adaptability͒; ability to withstand Clay ͑%͒ 14.0 29.0 repeated coppicing; dry matter production; leaf carbon and nitrogen contents; leaf carbon and nitrogen release rates; and crown characteristics. A manage- Origin and identity of seeds used in the study ment option for replanting cacao, based on the potential of these species to restore degraded lands through The Albizia species seed lots used in the trial ͑Table a managed tree fallow system, is also discussed. 2͒ are part of the Oxford Forestry Institute’s collection of native Central American tree genera ͑Hughes and Pottinger 1997͒. Materials and Methods Seed preparation, planting and experimental design Site description All seeds used in the study were mechanically scari- The study was conducted at the Cocoa Research In- fied to break seed coat dormancy by clipping the seed stitute of Ghana, Tafo ͓06° 11' N, 00° 22' W, 220 m coat at one end. Scarified seeds were immersed in above sea level͔. The average annual rainfall over syrup ͑sugar solution͒ for ten seconds to provide ad- twenty years is 1461 mm. The rainfall pattern is bi- hesive surface for coating with peat-based rhizobium modal with maxima in May-June and September-Oc- inoculum ͑inoculant group H, supplied by Agrofor- tober. Variation in monthly temperature is slight. The ester Tropical Seeds, Hawaii, USA͒. Treated seeds mean monthly maximum in the hottest month ͑Feb- were sown at stake in April 1996, in a randomized ruary or March͒ is 31-33 °C; the mean monthly mini- complete block design with 10 trees per species per mum in the coldest month ͑August͒ is 19-21 °C. The line plot, replicated four times. Distance between mean monthly sunshine is longest in November trees within a line plot was 0.5 m. The distance be- ͑212.7 hours͒ and shortest in August ͑92.8 hours͒. The tween line plots was 3.0 m. An eleventh-tree line of soils are Ferric Lixisol ͑Paleustalf͒, brown to yellow- Gliricidia sepium, the most commonly planted shade ish red, well-drained soils developed in situ from tree for cacao in Ghana, was included as a standard. weathered materials of hornblende granodiorite. The site is flat ͑surrounded by gently sloping land͒. The Data collection texture of the soil ͑0-8 cm depth͒ is sandy loam with relatively low levels of N, P, K and organic carbon Data collection on seedling emergence ͑Table 1͒. The trial site previously carried cacao for Seedling emergence ͑germination͒ was assessed at over 30 years. four weeks after sowing. A seedling was considered to have emerged when it measured Ͼ 2.0 cm above 187

Table 2. Origin and identity of Neotropical Albizia species seedlots used in the study at CRIG, Tafo, Ghana Species Provenance Ident. No. Lat. N Long. W Alt. ͑m͒ Parent trees bulk sibs A. hybrid* Rio Grande Oaxaca, Mexico 38/87 15°59 97°16 10 1 1 A. adenocephala Agua Fria, Choluteca, Honduras 33/88 15°16 87°06 1100 30 20 A. guachapele Motagua Valley, Zacapa, Guatemala 10/83 14°59 89°30 200 25 Ϫ Pespire, Choluteca, Honduras 14/89 13°37 87°21 100 25 Ϫ Comayagua, Honduras 68/87 14°25 87°43 600 10 Ϫ Tosagua, Manala, Ecuador 63/88 0°45 80°10 30-60 15 15 A. niopoides Tapaire, Choluteca, Honduras 21/84 13°23 87°13 75-250 25 25 San Esteban, Chiquimula, Guatemala 46/88 14°45 89°31 400 20 20 A. occidentalis Puente Cuilala, Michoacan, Mexico 47/88 18°11 103°05 10-60 30 Ϫ Huatulco, Oaxaca, Mexico 39/87 15°43 96°12 0.100 10 Ϫ A. plurijuga Narcisco Mendoza, Chiapas, Mexico 48/88 16°33 92°59 550 15 Ϫ A. purpusii Nenton, Huehuetenango, Guatemala 42/88 16°45 92°59 550 15 Ϫ A. saman Choluteca, Choluteca, Honduras 28/84 13°25 87°12 50-150 200 Ϫ La Galera, Choluteca, Honduras 38/89 13°18 87°04 400-500 30 Ϫ A. sinaloensis El Tezal, Sonora, Mexico 106/92 27°03 108°53 430 5 Ϫ A. tomentosa Hidalgo, Oaxaca, Mexico 40/87 16°02 97°35 100 25 20 * Putative hybrids of four sympatric species with parentage not fully investigated. Source: Hughes and Pottinger ͑1997͒, Oxford Forestry Institute, U.K. ground level. Three months old seedlings ͑same age 4. No dominance, Ͻ 5 primary branches as those sown at stake in the field͒ raised in the nurs- 5. No dominance, Ͼ 5 primary branches. ery using treatments described above were trans- planted into the field to replace seeds that did not germinate. Dry matter production and leaf biomass nutrient contents Growth data collection Height and girth of seedlings were measured at age At age ten months, the middle four trees in each line four and ten months. The measurements were done on plot were coppiced ͑harvested͒ at a height of 50 cm the second, fourth, sixth and eight trees within each above ground level. Side branches below cutting line. Height was determined using a graduated pole, height were gathered vertically and cut at the same from ground level to the tip of the highest point ͑stem height for the determination of dry matter production apex͒ of each plant. Girth was measured at 10 cm per tree. After ten months of coppice regrowth, the above ground level using a pair of calipers. For a tree four middle trees were cut again at 50 cm above with multiple stems, individual stem diameter was ground level for dry matter determination. Subse- measured. The squared diameters were summed and quent harvesting of regrowth material for dry matter the square root of the sum of squared diameters ob- determination was done at six-monthly intervals for a tained as the composite diameter value ͑Stewart and period of 24 months. Leaves and soft green stem of Salazar 1992͒. Ͻ 6.0 mm diameter were separated from the woody components of prunings and oven-dried separately at Form and habit assessment 80 °C for 48 hours. Subsamples of oven-dried leaves and stems ͑ Ͻ 6.0 mm diameter͒ were ground to pass Four and ten months old seedlings were rated to as- a 1 mm mesh sieve. Subsamples were prepared and certain their form and habit based on the following analyzed for leaf carbon and nitrogen following the ratings: methods described by Anderson and Ingram ͑1993͒. 1. Dominant main stem, primary branches Ͻ 0.5 height of main stem Leaf Biomass Decomposition 2. Dominant main stem, primary branches Ͼ 0.5 height of main stem Fresh leaves harvested from the six months coppiced 3. Weak apical dominance, branches similar length to regrowth trees were air-dried. Subsamples were main stem oven-dried at 80 °C to account for moisture ͑oven- 188 dry weight͒. The equivalent of 2.0 g oven-dry weight vals under clear sky conditions around midday ͑1200 from the air-dried sample was placed in plastic mesh hours GMT͒ when the vertical projection of the litterbags measuring 20 ϫ 15 cm with a mesh size of crown on the ground is concentrated around the tree. 1mm. For each species 30 bags per plot per replica- Measurements were done in four quadrants around tion was buried in the line plots to a depth of 5.0 cm each tree at a distance of 0.5 m from the base of the to secure the litterbags and its contents in May 1999. tree. Light transmission was calculated as the ratio Six bags per plot per replication were retrieved at between sunlight intensity under tree crown and di- monthly ͑30 days͒ intervals for five months and the rect sunlight intensity in the open expressed in per- rate of loss of carbon ͑C͒ and nitrogen ͑N͒ from leaf cent. biomass were monitored. At each sampling time, soil and debris adhering on bag surfaces were carefully Data Analysis removed by gently rinsing with tap water over a fine soil sieve. Cleaned leaf samples for each species per Angular transformation of the data on the number of replicate were bulked. Bulked samples were oven- seedlings emerging at 4 weeks was performed. The dried at 80 °C for 48 hours, weighed and then ground transformed data was subjected to one-way analysis to pass a 1mm mesh sieve and analyzed for total C of variance ͑ANOVA͒. One-way ANOVA was also and N. Initial leaf biomass was also analyzed for to- performed on all other data using the Minitab Statis- tal C and N, following the method described by tical package ͑Minitab for WINDOWS Version 12͒. Anderson and Ingram ͑1993͒. The single exponential Significant difference between two means was deter- decay model was fitted to the data on percent C and mined using Tukey’s pairwise comparisons test ͑one- N remaining at each sampling time. This was used to way ANOVA multiple comparisons͒ in Minitab for calculate the decomposition rate constant k for C and WINDOWS Version 12. Natural log transformations N. Residuals were plotted against fitted values to de- were performed on the data on leaf C and N remain- termine the suitability of the model for the data. The ing at each sampling time before regression analysis. form of the model is presented in the equation: YϭA.e-kt, where Y is the percent of initial C or N remaining at sampling time t. Time t is given in days Results and k, the release rate constant, is expressed in days–1. A is the initial amount of C or N in leaf matter placed Emergence and adaptation of tree species in bag while e is the base of the natural logarithm ͑eϭ2.718͒. The half-life or time required for the loss Significant differences ͑p Ͻ 0.001͒ in seedling emer- of 50 per cent of C or N in the initial leaf biomass gence was observed among the species ͑Table 3͒ with was calculated as – ln ͑0.5͒/k ϭ 0.693/k ͑Olson 1963͒. five species ͑A. guachapele, A. occidentalis, A. plurijuga, A. purpusii and A. saman͒ recording over 70 per Continuous assessment of tree growth cent emergence as compared to 30 per cent for G. sepium, the standard. Four of these five species listed A total of eight trees per species ͑two per replication͒ for best seedling emergence also recorded the best were left uncut for continuous assessment of growth. growth, with A. guachapele being particularly out- Tree height and stem diameter at breast height ͑DBH, standing in height at age 4 months. A. purpusii on the 1.3 m͒ were measured at age two ͑June 1998͒ and other hand, germinated well but had slow initial four ͑June 2000͒ years. DBH was measured using a growth compared to A. guachapele and A. saman at diameter tape and height was measured using an Op- age four months. However, at ten months, its height tical Reading Clinometer PM-5 ͑Suunto Precision In- and girth ͑stem diameter͒ were comparable to the struments͒. In 2000, when trees were four years old, other four above-mentioned species. A. saman re- measurements of crown diameter, depth and volume corded the best growth at age ten months with girth were made using the clinometer and tape and follow- ͑6.0 cm͒ and height ͑4.7 m͒ being significantly differ- ing the methods described by Philip ͑1994͒. Light ent from all others. Girth and height of trees at age transmission through crown was assessed from May four and ten months were significantly correlated 2000 to April 2001 using a light meter ͑LUXMETER ͑rϭ0.80, p Ͻ 0.001 and rϭ0.90, p Ͻ 0.001 respec- HD 8366, Wagtech International Ltd. Berkshire, UK͒. tively͒. Measurements were carried out at four weekly inter- 189

Table 3. Seedling emergence and subsequent growth of Albizia species and Gliricidia sepium at age four and ten months at CRIG, Tafo, Ghana. Age 4 months Age 10 months ͑1st pruning͒ Species Emergence* Girth ͑mm͒ Height ͑cm͒ Girth ͑cm͒ Height ͑m͒ Total dry wt % leaf ϩ Dry wt of ͑%͒ of pruning stems of leaf ϩ per plant total dry wt. stem† prun- ͑g͒ of pruning ing ͑tha–1͒ A. hybrid 25.8af 3.0ac 24.0a 1.0a 0.9a 87.0a 45.0a 0.3a A. adenocephala 19.7cf 2.0a 29.0a 1.0a 0.9a 384.0b 58.0bc 1.5b A. guachapele 84.7bd 8.0b 71.0b 4.0b 3.6b 813.0c 58.0bc 3.3c A. niopoides 11.5c 2.0a 23.0a 2.0a 1.2ae 130.0a 56.0b 0.5a A. occidentalis 72.7b 6.0bc 41.0cd 4.0b 2.8c 483.0d 56.0b 1.9b A. plurijuga 80.8b 6.0bc 40.0ce 5.0c 3.6b 629.0e 61.0c 2.6d A. purpusii 85.4d 2.0a 28.0a 4.0b 3.8b 246.0f 61.0c 1.0e A. saman 95.3e 6.0bc 51.0d 6.0d 4.7d 881.0c 56.0b 3.4c A. sinaloensis 32.6a 2.0a 23.0a 1.0a 1.0a 57.0a 53.0bd 0.2a A. tomentosa 27.3a 3.0ac 32.0ac 2.0a 1.8e 380.0b 51.0d 1.4be G. sepium 30.0a 5.0ac 50.0de 2.0a 2.6c 821.0c 53.0bd 3.0cd ANOVA ͑p Ͻ ͒ 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 Pooled StDev 3.6 1.5 4.3 0.2 0.3 38.4 2.1 0.2 Means in a column followed by the same letter are not signiﬁcantly different at pϭ0.05 according to Tukey’s pairwise comparison test. StDev ϭ Standard Deviation; Critical value ͑33df͒ϭ4.88 * Re-transformed means. † Soft green stem to 6.0 mm diameter.

At the first pruning of trees at age ten months, G. Leaf nutrient contents and decomposition sepium ͑3.0 ton ha–1͒, A. guachapele ͑3.3 ton ha–1͒ and A. saman ͑3.4 ton ha–1͒ produced the greatest Based on the half-yearly leaf biomass production amounts of leaf biomass and these were not signifi- ͑Table 4͒, the estimates of organic carbon yield for A. cantly different from each other at pϭ0.05 ͑Table 3͒. saman, A. guachapele and A. adenocephala were 4.4, Correlations between girth and above ground total dry 3.1 and 2.7 t ha–1, respectively, compared to an esti- matter production ͑rϭ 0.70, p Ͻ 0.001͒ and height mate of 1.50 t ha–1 for G. sepium. Similarly, N yields and above ground total dry matter production were estimated at 0.32, 0.26, 0.20 and 0.12 t ha–1 re- ͑rϭ0.70, p Ͻ 0.001͒ of the species were strong and spectively. The fact that A. adenocephala now highly significant. appears as one of the most productive shows that we have to be careful in interpreting early results ͑e.g. Coppice regrowth and biomass productivity from year one͒. Leaf carbon loss, expressed as percent of initial leaf carbon content was rapid. The loss Ten months after coppicing for the first time ͑second was highest in A. sinaloensis ͑55.5%͒ and lowest in pruning͒, three species ͑A. guachapele, A. plurijuga A. tomentosa ͑34.3%͒ during the first 30 days after and A. saman) produced leaf biomass in excess of 5.0 placement of litterbags. Half-life of carbon loss was tha–1 dry weight each compared to 2.4 t ha–1 for G. 22.9 and 44.4 days respectively, for A. sinaloensis and sepium ͑Table 4͒. Girth prior to this coppicing and A. tomentosa. G. sepium had a half-life of 24.6 days. total biomass regrowth after coppicing showed posi- Similarly, leaf N loss was rapid, with leaves loosing tive and significant correlation ͑rϭ0.78, p Ͻ 0.001͒. more than 70% of initial N in 60 days. The loss rate Average leaf biomass production ͑leaf prunings͒ at of N was fastest in A. sinaloensis with a half-life of six monthly intervals for twenty-four months was 24.3 days and slowest in A. tomentosa with a half- significantly ͑p Ͻ 0.05͒ higher for six out of the ten life of 46.8 days ͑Table 4͒. The high temperature and Albizia species compared to G. sepium ͑Table 4͒. rainfall during the period of decomposition might have influenced the rapid loss of leaf C and N as ex- plained by Vanlauwe et al. ͑1995͒. In addition, the low C:N ratio of the leaves, which ranged from 9.5 190

Table 4. Coppice regrowth, average half-yearly leaf biomass and decomposition of Albizia species and Gliricidia sepium at CRIG, Tafo, Ghana. Age 10 months coppice regrowth ͑2nd Decomposition Half-life pruning͒ Species Girth ͑cm͒ Height ͑m͒ Leaf and Half-yearly C ͑%͒ N ͑%͒ Half-life C Half-life N stem prun- leaf dry ͑days͒ ͑days͒ ing dry matter pro- matter ͑t duction ͑t ha–1͒ ha–1͒ A. hybrid 2.6abc 4.5 4.2abe 4.4a 39.0acd 2.9a 25.2ad 27.5a A. adenocephala 2.0abc 3.3 3.4ace 5.8b 47.0b 3.5bd 25.3ad 27.2ab A. guachapele 4.0a 4.8 5.9b 8.5c 36.0a 3.0ab 25.2ad 27.1ab A. niopoides 1.4bc 3.7 2.7ad 2.7d 47.0b 4.6c 28.9abf 25.9bh A. occidentalis 3.4abc 3.8 4.5ab 4.4a 46.0b 3.0a 31.6b 36.1c A. plurijuga 3.3abc 4.4 5.2bc 4.8ab 48.0b 3.7de 28.6abf 28.2a A. purpusii 2.0abc 3.6 2.3ad 2.8d 40.0acd 3.3ad 29.0ab 32.4d A. saman 4.1ab 4.8 5.5bc 10.1e 44.0bc 3.1abf 36.1c 38.3e A. sinaloensis 1.2c 3.3 1.1d 4.1af 38.0a 4.0e 22.9d 24.3f A. tomentosa 2.3abc 4.1 2.3de 2.4d 48.0b 3.1ab 44.4e 46.8g G. sepium 2.7abc 3.5 2.4ad 3.3df 44.0bd 3.6def 24.6df 25.6fh ANOVA ͑p Ͻ ͒ 0.002 0.19 0.001 0.001 0.001 0.001 0.001 0.001 Pooled StDev 1.0 NS 0.9 0.4 2.3 0.2 1.8 0.6 Means in a column followed by the same letter are not significantly different at pϭ0.05 according to Tukey’s pairwise comparison test. StDev ϭ Standard Deviation; Critical value ͑33df͒ϭ4.88; NS ϭ Not significant at pϭ0.05. † Soft green stem to 6.0 mm diameter. in A. sinaloensis to 15.5 in A. occidentalis ͑Table 4͒ height of almost all the Albizia species at age 48 and the burial of leaves in the soil also contributed to months as compared to G. sepium has the advantage the short-term release of C and N ͑Mafongoya and of providing better clearance between cacao canopy Nair 1997͒. and shade trees and this will minimize any straddling effect of shade trees on cacao canopy ͑Table 5͒. Continuous assessments of tree growth Light transmission through the crown of the Albi- zia species ranged from 50 to 65 % of full sunlight All ten Albizia species were rated 1, with dominant throughout the year, except for A. guachapele and A. main stem and primary branches Ͻ 0.5 height of saman which recorded over 90% transmission in Au- main stem, indicating strong apical dominance and gust/September and February, respectively, due to leaf fast growth ͑Philip 1994͒. G. sepium was rated 3, with shedding. Light transmission through the crown of G. weak apical dominance and branches similar in length sepium, which is also deciduous, ranged between 50 to main stem at age four and ten months. Table 5 and 90 % of full sunlight throughout the year. shows that at age 48 months, all the Albizia species Leaf shedding in A. guachapele begun three years had crown lengths, which were significantly after sowing. Flowering coincided with leaf shedding. ͑p Ͻ 0.001͒ higher than that of G. sepium with the The species was leafless for a brief period between exception of A. purpusii and A. occidentalis. A. August and September. This is of interest since mean guachapele and A. saman were outstanding for monthly sunshine duration is lowest at this time of the height, DBH and crown development. Though A. year due to overcast conditions. A shade tree that plurijuga had height and DBH comparable to A. sheds its leaves within this period will intercept very guachapele and A. saman its crown dimensions were little light making more light available to the cacao significantly ͑p Ͻ 0.001͒ smaller with a crown vol- growing underneath and therefore should be a ume of 66 m3 compared to 180 and 227 m2 for A. suitable shade tree for cacao in Ghana. guachapele and A. saman respectively. However, it was significantly ͑p Ͻ 0.001͒ larger than A. purpusii ͑11.1 m3͒ and G. sepium ͑25.9 m3͒. The superior 191

Table 5. Continuous assessment of growth of Albizia species and Gliricidia sepium at age 24 and 48 months of growth at CRIG, Tafo, Ghana. Age ͑months͒ 24 48 24 48 Age 48 months Species DBH ͑cm͒ Height ͑m͒ Crown length ͑m͒ Crown diameter ͑m͒ Crown volume ͑m3͒ A. hydrid 3.3a 10.7a 4.5ab 11.9a 6.6a 4.5a 35.0a A. adenocephala 4.2bd 14.8bd 6.3a 12.8ab 7.1ae 6.7bd 83.6b A. guachapele 8.3cd 22.4c 6.9a 13.7ab 8.7b 8.9c 180.4c A. niopoides 3.1ab 12.4ab 4.0ab 12.2ab 8.3b 7.0d 106.5d A. occidentalis 4.6abd 12.3a 6.0ab 11.3a 5.0c 4.5a 26.6e A. plurijuga 6.4de 17.1de 6.8a 14.5b 6.8a 6.1d 66.0f A. purpusii 2.5ab 10.9a 4.0ab 6.9c 3.9d 4.1a 11.1g A. saman 6.5de 20.1ce 6.8a 13.5ab 8.5b 10.1e 227.0h A. sinaloensis 2.2ab 18.9e 2.9b 11.6ab 7.6e 6.0d 76.3i A. tomentosa 3.9abe 18.5e 4.8a 13.3ab 8.5b 6.2d 85.5b G. sepium 4.2abe 8.0f 3.4a 8.5c 4.9c 4.5a 25.9e ANOVA ͑p Ͻ ͒ 0.001 0.001 0.002 0.001 0.001 0.001 0.001 Pooled StDev 1.3 1.0 1.4 1.0 0.3 0.4 1.0 Means in a column followed by the same letter are not signiﬁcantly different at pϭ0.05 according to Tukey’s pairwise comparison test. StDevϭ Standard Deviation. Critical value ͑33df͒ϭ4.88

Discussion ͑1988͒ estimated the dry matter input from these sources to be between 3-14 t ha–1 year–1, containing The growth of the ten Albizia species at age four and 60-340 kg N ha–1 year–1. Though not all of this N is ten months reported here are comparable or better available for use by cacao, this magnitude gives an than those reported for species of similar age in their indication of adequacy since for replanting in native Honduras in Central America ͑Mejia 1997͒. degraded areas in Ghana, the recommended rate of The productivity of six out of the ten species is also application of inorganic N fertilizer for young cacao comparable or better than G. sepium as observed in ͑6-18 months͒ is 14.7 g N plant–1 or 23.5 kg N ha–1 the present study and for other species such Leucaena for cacao planted at a density of 1600 trees ha–1 leucocephala and Flemingia macrophylla reported ͑Anon. 1996͒. The dry matter and estimated N con- elsewhere ͑Barnes 1998͒. This together with the high tents of the leaf biomass of seven out of the ten Al- survival rate indicates the potential of these Albizia bizia species reported in this study are comparable to species in the climatic and edaphic conditions of the the estimates of Beer ͑1988͒ and these are adequate forest zone of Ghana, although longer-term trials are to provide both N and organic material for young ca- required to confirm this. cao in an appropriate cropping system. However, tree The N content of leaves of each of the Albizia spe- biomass containing sufficient nutrients to meet crop cies exceeded the 1.74% level required to avoid N demand is not enough the nutrients must be supplied immobilization when applied as mulch and indicate in synchrony to crop needs ͑Palm 1995͒. Results of the high quality of the leaf pruning of these species the N release rate of the various species in this study ͑Palm 1995͒. This coupled with the ability of the trees are therefore essential for controlling the timing and to continue to regrow vigorously following multiple addition of leaf pruning to the soil. Bulking leaf coppicing is of great importance in the management pruning of six of the species ͑A. adenocephala, A. of the trees for soil fertility improvement. Fast growth guachapele, A. niopoides, A, plurijuga with emphasis rate and high leaf biomass production as exhibited by on A. saman and A. tomentosa which had relatively the best performing Albizia species are essential for slow N release rates͒ as mulch may slow down nutri- nutrient recycling and availability of plant nutrients ent release. This together with timing of pruning and and organic matter to associated crops in an agrofor- mulch application to coincide with peak N demand by estry based cropping system ͑Palm 1995͒. young cacao may lead to an efficient utilization of Above ground pruning residues and litter-fall have mulch N by the crop ͑Young 1997͒. been recognized as a major source of transfer of N Crown size and shape ͑the ratio between crown from leguminous shade trees to associated crops. Beer length, H and radius of crown base, R͒ is one factor 192 that determines the potential shading effects of a tree seedlings. Traditionally important food crops such as species. The shaded area caused by a narrow crown cocoyam ͑Xanthosoma spp͒, yam ͑Dioscorea spp͒ is smaller and concentrated around the tree than the and cassava ͑Manihot esculenta͒ can also be grown shaded area caused by a broad crown. The shading as understorey species in partial shade and can yield effect of a crown decreases as the H/R ratio of the carbohydrate-rich produce. The shade trees and food crown increases ͑Kuuluvainen and Pukkala 1987͒. crops are initially managed to provide 60% ground Most of the Albizia species showed the tendency of cover as shade for young cacao ͑6-12 months old developing spreading crown and hence, a low H/R plants͒. Leaves and soft green stems of thinnings and ratio. This crown character of the species indicates it’s coppice trees are used as mulch for cacao seedlings. potential as a shade tree for cacao in that it is likely Subsequently, coppice trees are managed to provide to provide earlier and better ground cover than G. regrowth leaf biomass for mulching young cacao at sepium, which had a relatively narrow crown that six monthly intervals till cacao canopy closes. The could be described as columnar. rest of the established trees, providing 30% ground Cacao has a low light saturation point ͑LSP͒ of 400 cover ͑25 to 30 trees ha–1 based on the crown char- ␮ Em–2 s–1 and a low maximum photosynthetic rate acteristics of A. saman͒ are left to provide permanent ͑7mg dm–1 h–1͒ at light saturation ͑Hutcheon 1981͒. overhead shade for cacao and for continued soil fer- The photosynthetic rate of the crop decreases if the tility improvement through natural leaf fall. photosynthetic apparatus is exposed to light intensities exceeding 60% of full sunlight that is 1800 ␮mol m–2 s–1 ͑Galyuon et al. 1996͒, while prolonged expo- Acknowledgement sure to high light intensities damages the photosynthetic mechanism of the leaves ͑Raja Harun and The author gratefully acknowledges Dr A. J. Pot- Hardwick 1988͒. Low light intensities however sup- tinger, formerly of the Oxford Forestry Institute, UK, press flower production with light levels less than for providing the seeds and the rhizobium inoculum. 1800 hours year–1, having a considerable depressing Mr. Agyenim Boateng and staff of the Soil Science effect on production ͑Asomaning et al. 1971͒. There- Laboratory of CRIG are also acknowledged for pro- fore, openness of shade to allow the passage of abun- viding technical assistance. This paper is published dant sun fleck, as observed for the Albizia species in with the permission of the Executive Director of this study, apparently make the species desirable for CRIG. cacao shade.

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