Hydraulic conductivities of competing root systems of Grevillea robusta and maize in agroforestry

Models of water uptake in mixed stands of vegetation commonly assume that water is partitioned among competing root systems in proportion to relative root length densities. Such an approach assumes implicitly that roots of different species have equivalent hydraulic properties. This was tested for root systems of Grevillea robusta A. Cunn. and maize (Zea mays L.) at a semi-arid site in Kenya. The hydraulic conductances for roots of both species were measured in situ at the scale of the whole root or root system using a high pressure flow meter (HPFM). Hydraulic conductivities (kappa(r)) were expressed per unit root length. Root lengths were estimated for maize plants by soil coring and for G. robusta using a fractal branching model calibrated against soil coring. Mean kappa(r) was 1.88 x 10(-7) +/- 0.28 x 10(-7)kg s(-1) MPa-1 m(-1) for G. robusta and 1.25 x 10(-7 +/-) 0.13 x 10(-7)kg s(-1) MPa-1 m(-1) for maize. Values of kappa(r) were not significantly different (P < 0.05), suggesting that the assumption of hydraulic equivalence for root systems of the two species may be valid, at least when hydrostatic gradients are the major driving force for water uptake. Differences in conductivities between these species could arise, however, because of variation in the hydraulic properties of roots not accounted for here, for example because of root age, phenology or responses to the soil environment.