Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth
Abstract
The work reported here formed part of a multidisciplinary project to examine the changing tree/crop interactions, expressed in terms of resource capture, 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, either 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 annual growth increments and the corresponding mean daily growth rates were calculated; 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 agroforestry treatment (CTd) during the first 600 days after planting as a result of competition with the associated crops. Tree height subsequently increased more rapidly in the agroforestry treatment (CTd) and regained parity with the sole grevillea treatment (Td) by ca. 820 days after planting. However, although seasonal and annual biomass increments were comparable in both treatments once the trees became the dominant component of the system, above-ground biomass in CTd grevillea never recovered fully to Td values during the observation period; trunk volume and biomass were consistently lower in CTd than in Td grevillea, reducing their economic value. Tree growth continued 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 separation in savanna vegetation. The factors responsible for the observed reductions in tree growth in the agroforestry system are examined; effects on crop yields and system performance are considered in Part II.
Citation
Lott, J.E.; Howard, S.B.; Ong, C.K.; Black, C.R. Long-term productivity of a Grevillea robusta-based overstorey agroforestry system in semi-arid Kenya: I. Tree growth. Forest Ecology and Management (2000) 139 (1-3) 175-186. [DOI: 10.1016/S0378-1127(00)00266-8]