Competition in tree row agroforestry systems. 1. Distribution and dynamicsof fine root length and biomass

Complementarity in the distribution of tree and crop root systems is import ant to minimise competition for resources whilst maximising resource use in agroforestry systems. A field study was conducted on a kaolinitic Oxisol i n the sub-humid highlands of western Kenya to compare the distribution and dynamics of root length and biomass of a 3-year-old Grevillea robusta A. Cu nn. ex R. Br. (grevillea) tree row and a 3-year-old Senna spectabilis DC. ( senna) hedgerow grown with Zea mays L. (maize). Tree roots were sampled to a 300 cm depth and 525 cm distance from the tree rows, both before and afte r maize cropping. Maize roots were sampled at two distances from the tree r ows (75-150 cm and 450-525 cm) to a maximum depth of 180 cm, at three devel opmental stages. The mean root length density (L rv) of the trees in the up per 15 cm was 0.55 cm cm(-3) for grevillea and 1.44 cm cm(-3) for senna, at the start of the cropping season. The L rv of senna decreased at every dep th during the cropping season, whereas the L rv of grevillea only decreased in the crop rooting zone. The fine root length of the trees decreased by a bout 35% for grevillea and 65% for senna, because of maize competition, man ual weeding, seasonal senescence or pruning regime (senna). At anthesis, th e L rv of maize in the upper 15 cm was between 0.8 and 1.5 cm cm(-3). Maize root length decreased with greater proximity to the tree rows, potentially reducing its ability to compete for soil resources. However, the specific root length (m g(-1)) of maize was about twice that of the trees, so may ha ve had a competitive uptake advantage even when tree root length was greate r. Differences in maize fine root length and biomass suggest that competiti on for soil resources and hence fine root length may have been more importa nt for maize grown with senna than grevillea.