Gas exchange and water use efficiency of trees and maize in agroforestry systems in semi-arid Kenya

Improved understanding of the impact of leafing phenology and soil water supplies on the gas exchange and water use efficiency (WUE) of trees in agroforestry systems in the semi-arid tropics is essential to optimise tree/crop interactions and the use of scarce natural resources, particularly in view of recent initiatives to use carbon credits to reward subsistence farmers who plant and manage trees effectively. Understanding of the relationships between photosynthetic and transpiration rates and chlorophyll content may help to explain variation in growth rate, productivity and WUE between species and growth environments. Such information is lacking for the species examined here, the evergreen Grevillea robusta (A. Cunn), semi-deciduous Alnus acuminata (HBK) and deciduous Paulownia fortunei (Hemsl.). Although differences in leafing phenology influence the extent of competition or complementarity between trees and crops in agroforestry systems, and hence crop yield, farmers may deliberately plant fast-growing, competitive tree species if they provide attractive economic returns or carbon credit subsidies. The objectives were to characterise: (1) the extent of variation in foliar gas exchange and instantaneous water use efficiency (WUE i ) between trees with contrasting leafing phenologies; (2) the influence of leaf age and available soil moisture on gas exchange and WUE i of trees; and (3) chlorophyll and foliar nitrogen (N) concentrations and gas exchange parameters for maize in agroforestry and sole crop systems. Foliar chlorophyll and N concentrations, assimilation (A) and transpiration rate (E) were much greater in P. fortunei than in A. acuminata or G. robusta. A, E and WUE i were greatest in young mature leaves of all three species but declined as they senesced. WUE i increased as soil moisture content declined and was similar in all species, suggesting their differing leafing phenologies were responsible for observed differences in long-term water use and growth. A and WUE i were greater in maize than in tree species, consistent with its C4 photosynthetic pathway. The implications for tree and crop growth, yield and acceptability to subsistence farmers in the semi-arid tropics are discussed.