Ecophysiology of indigenous trees in agroforestry systems in the semi-arid tropics
Increasing demand for timber, fuelwood and other forest products has outstripped production in many areas of the semi-arid tropics, leading to deforestation and land degradation resulting from erosion and nutrient depletion. Agroforestry offers the potential to provide forest products, improve productivity and reduce soil erosion and environmental degradation. However, as previous reports have shown that competition between trees and crops for water in semi-arid areas adversely affects crop yields, attention has turned towards studies of the existing practice of boundary planting, where low tree planting densities and the associated benefits of land demarcation and soil stabilisation increase the viability of incorporating trees into crop land. The aim is to select and manage tree species in ways that limit their negative effect on crop yields and improve the overall value of the system. The present study was carried out at Machakos (1° 33' S, 37° 14' E, altitude 1660 m) in the Kenyan highlands, where the bi-modal annual rainfall of c. 740 mm is divided approximately equally between two rainy seasons (short rains, October-February, long rains, March July). The experiment was set up in April 1993 to examine the influence of tree/crop interactions on system productivity. Each 18 x 18 m plot, except for the sole crop plots, contained a central row of trees planted at 1 m spacing. Four overstorey agroforestry treatments were examined between March 1996 and March 1998; these included two indigenous species, Croton megalocarpus and Melia volkensii, and two exotic species from Central America, Senna spectabilis and Gliricidia sepium. Beans (Phaseolusulgaris) and maize (Zea mays) were grown during the short and long rains respectively. M. volkensii and S. Spectabilis exhibited similar leafing phenology patterns, losing almost all leaf cover during the long dry season (July-October) and flushing before the ensuing rains. During the short dry season, S. spectabilis lost few leaves, whilst M. volkensii lost some leaves before flushing prior to the onset of the long rains. M. volkensii lost a large proportion of its leaf cover during the 1997/98 short rains due to the unusually high soil moisture content. C. megalocarpus although predominantly evergreen, lost a large proportion of its leaves during dry periods, whereas leaf area increased rapidly under wetter conditions. G. sepium had one annual period of low leaf cover during the long dry season and did not regain full leaf cover until mid-way through the short rains. The three-dimensional model of canopy photosynthesis and transpiration, MAESTRA, was parameterised for C. megalocarpus and M. volkensii using existing models to describe the response of photosynthesis to light and temperature and stomatal responses to light and vapour pressure deficit. The photosynthesis model fitted the experimental data well, but stomatal conductance in C. megalocarpus, although showing responses to light and vapour pressure deficit, was not closely correlated with ambient environmental conditions. M. volkensii had higher leaf area than C. megalocarpus for most of the 18 month simulation period, comprising three rainy and three dry seasons; modelled assimilation for this period was 49 % greater in M. volkensii, while canopy water use efficiency and transpiration were respectively 35 and 11 % higher. These differences accounted for the more rapid growth rate and greater competition with adjacent crops associated with M. w1kenrii relative to C. megalocarpus. Above-ground woody biomass production was greatest in M. volkensii, followed by S. spectabilis, C. megalocarpus and G. sepium; production during the fourth and fifth years after planting ranged between 2.8 and 4.9 t ha-¹ yr¹. Crop production in the agroforestry treatments was always lower than in sole crops due to below-ground competition for water and, in seasons with higher water availability, shading by the trees. Of the agroforestry systems examined, seed production for beans was highest under M. volkensii and G. sepium, followed by C. megalocarpus and S. spectabilis. Grain production in maize was greatest under C. megalocarpus, followed by G. sepium, S. spectabils and M. volkensii. Mean annual aboveground biomass production including maize grain and stover, bean seed, woody biomass and tree leaves in the M. volkensii treatment exceeded that for the sole crop plots, even though rainfall during 1996 and 1997 was only 61 and 95 % of the long term average. Although the biomass production of leaves was not estimated for S. spectabilis and G. sepium, the results obtained suggested that biomass production was greater than that obtained under sole crop cropping. The inverse correlation between tree and crop yield suggests that the value of the tree products must exceed the associated crop losses if benefits are to be obtained from agroforestry. M. volkensii is valued in areas of Kenya where markets for its products exist and therefore shows great promise for extension in semi-arid areas; where necessary, pruning may be used to reduce competition with crops and increase the length of clear bole. C. megalocarpus is widely used as a shade tree in East Africa and seems well suited for this purpose as its impact on adjacent crops was least of all the tree species examined. S. spectabilis, although having straight unbranched stems, exhibited a level of competition with adjacent crops that would necessitate a high value for its timber products to warrant its adoption. The least suitable tree species of those examined was G. sepium, whose poor form and susceptibility to attack by fungal pathogens and insects severely undermined its potential value for use in agroforestry systems.