Water use in a Grevillea robusta–maize overstorey agroforestry system in semi-arid Kenya

Novel approaches involving a combination of sap flow measurements of transpiration and allometric estimates of biomass production were used to determine seasonal water use by trees and crops in agroforestry systems. The results were used to test the hypothesis that agroforestry may improve productivity by capturing a greater proportion of annual rainfall than annual crops. Grevillea robusta A. Cunn., which is reputed to have a deep rooting habit, was grown in semi-arid Kenya either as sole stands or in combination with maize (Zea mays L.). Water use by individual trees and maize plants was determined using constant temperature heat balance gauges and scaled to provide stand-level estimates of transpiration based on linear relationships (r²>0.70) between sap flow and leaf area across a range of tree ages and environmental conditions. Maximum stand-level transpiration rates for grevillea ranged from 2.6 to 4.0 mm per day, consistent with previous studies in similar environments. Biomass production by grevillea was closely correlated with stand-level transpiration (r²>0.69–0.74), suggesting that non-destructive estimates of biomass increments can be used to provide reliable estimates of seasonal transpiration. Cumulative water use by grevillea over the 4.5-year observation period was comparable in the sole tree and agroforestry treatments, reaching a maximum utilisation of annual rainfall of 64–68% 3–4 years after planting. Approximately 25% of the water transpired by the trees was used during the dry season, indicating that they were able to utilise off-season rainfall, comprising 16% of the total annual rainfall, and residual water remaining in the soil profile after the cropping period. During the 1995 long rains, when 221 mm of rain was received, transpiration by sole maize was <50% of precipitation, compared to ca. 85% by the trees in the sole grevillea and agroforestry treatments. These results confirm that agroforestry systems may greatly increase rainfall utilisation compared to annual cropping systems. However, careful consideration of the tradeoffs between the loss of crop production and the additional value provided by tree products is essential.