Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth

The work reported here formed part of a multidisciplinary project to examin e the changing tree/crop interactions, expressed in terms of resource captu re, tree and crop growth and system performance, that occur as the trees in semi-arid agroforestry systems establish and mature. Grevillea robusta (A. Cunn.; grevillea) trees were grown in a dispersed planting arrangement, ei ther as sole stands or in combination with cowpea (Vigna unguiculata L.) or maize (Zea mays L.). Allometric approaches were used to monitor leaf area and leaf, branch and trunk biomass over a 4.5-year period. Seasonal and ann ual growth increments and the corresponding mean daily growth rates were ca lculated; crop growth, development and yield were also determined over nine growing seasons. Tree height, leaf area and trunk and branch biomass were reduced in the agr oforestry treatment (CTd) during the first 600 days after planting as a res ult of competition with the associated crops. Tree height subsequently incr eased more rapidly in the agroforestry treatment (CTd) and regained parity with the sole grevillea treatment (Td) by ca. 820 days after planting. Howe ver, although seasonal and annual biomass increments were comparable in bot h treatments once the trees became the dominant component of the system, ab ove-ground biomass in CTd grevillea never recovered fully to Td values duri ng the observation period; trunk volume and biomass were consistently lower in CTd than in Td grevillea, reducing their economic value. Tree growth co ntinued throughout the dry season, sometimes at rates similar to or greater than observed during the cropping seasons, indicating that the trees were able to utilise residual soil moisture or deep reserves within the profile. This finding is discussed in relation to recent interpretations of niche s eparation in savanna vegetation. The factors responsible for the observed r eductions in tree growth in the agroforestry system are examined; effects o n crop yields and system performance are considered in part II. (C) 2000 El sevier Science B.V. All rights reserved.