Competitive interaction of trees and crops for water by agroforestry systems

• University of Western Australia, Australia
• Agriculture, Western Australia, Australia
• CSIRO Division of Plant Industry, Australia

Trees on farms play an important role in increasing water use and restoring land degradation in Australia, and improving nutrient cycling, water use and sustainability of resource poor farmers in East Africa. A study on the water use by shallow and deep roots and rooting patterns of tree species commonly used on farms in Australia and East Africa will be examined in a collaborative project between CSIRO, Agriculture Western Australia (AWA) and the University of Western Australia (UWA) in Australia; and ICRAF and the University of Nairobi, in Kenya. The project will build on and link existing projects in both countries. Water use will be measured on individual tree roots using the heat pulse technique. The accuracy of the fractal branching technique to estimate the density and distribution of roots in the soil profile will be assessed, and the relationship between rooting pattern and the source of water for the trees will be established. The information will be used to scale up information at the plot level to the catchment level using hydrologic models developed in Australia.

In field studies undertaken in both Machakos in Kenya and Katanning in Western Australia the project demonstrated for the first time, using heat pulse probes, the redistribution of water from the surface to deeper soil horizons by the root systems of Grevillea robusta and Eucalyptus camaldulensis. This is the reverse of 'hydraulic lift' which is the redistribution of soil water from deeper in the profile to dry surface horizons by the root systems during the dry season. The results have been published in an international journal, Oecologia, and reported to several international meetings. Important physiological consequences result in terms of the affects on root growth and viability as well as plant nutrient and water status in tree-crop interactions. Water use of three contrasting species, monitored by CSIRO Forestry and Forest Products at Katanning, indicated large differences in the ability of the tree species in accessing deep water and their potential impact on catchment hydrology. Training was provided to Dr David Mungai, and in conjunction with the Crawford Fund, to Dr Francis Gichuki of Laikipia Research Program, Kenya, to attend a TOPOG moddeling workshop in Canberra. Difficulties were encountered in adapting TOPOG to the large watershed of Naro Moru (147 square km) of Laikipia, but model adaption was continuing after the completion of the project. Importantly, the project created an excellent opportunity for collaboration between ICRAF and Laikipia Research Program on watershed management, which is now a new research priority for ICRAF's hydrological research in Africa. It is also linking the partners of Laikipia Research Program with partners of the Alternative to Slash and Burn (ASB) policy research for sustainable upland systems in Southeast Asia.