Exploring below ground complementarity in agroforestry using sap flow and root fractal techniques

Indices of shallow rootedness and fractal methods of root system study were combined with sapflow monitoring to determine whether these 'short-cut' me thods could be used to predict tree competition with crops and complementar ity of below ground resource use in an agroforestry trial in semi-arid Keny a. These methods were applied to Grevillea robusta Cunn., Gliricidia sepium (Jacq.) Walp., Melia volkensii Gurke and Senna spectabilis syn. Cassia spe ctabilis aged two and four years which were grown in simultaneous linear ag roforestry plots with maize as the crop species. Indices of competition (sh allow rootedness) differed substantially according to tree age and did not accurately predict tree:crop competition in plots containing trees aged fou r years. Predicted competition by trees on crops was improved by multiplyin g the sum of proximal diameters squared for shallow roots by diameter at br east height(2), thus taking tree size into account. Fractal methods for the quantification of total length of tree root systems worked well with the p ermanent structural root system of trees but seriously underestimated the l ength of fine roots (less than 2 mm diameter). Sap flow measurements of ind ividual roots showed that as expected, deep tap roots provided most of the water used by the trees during the dry season. Following rainfall, substant ial water uptake by shallow lateral roots occurred more or less immediately , suggesting that existing roots were functioning in the recently wetted so il and that there was no need for new fine roots to be produced to enable w ater uptake following rainfall.