Abstract

In alley cropping, woody perennials are combined with agricultural crops or grassland in order to benefit from favourable interactions between trees and crops. Trees influence growth and senescence processes in grassland as well as plant chemical composition and plant water use. In a field experiment with grassland and alley-cropped willow coppices, we analysed the impact of distance and position relative to the tree line on grassland herbage biomass, proportion of dead herbage and herbage quality in two successive growth periods of two experimental years. The experimental years were chosen so that growth periods among years showed a similar rainfall pattern. Relative ET (evapotranspiration) as defined by actual divided by potential evapotranspiration reflected the severity of water shortage with a moderately dry (relative ET ~ 0.5) first growth period with increasing dryness in the second growth period of each year. We determined bulk stomatal conductivity of the grass shoot by measuring δ¹³C (carbon) signatures. Stomatal conductivities were higher close to the trees in drier periods (i.e. more negative δ¹³C values). Herbage biomass decreased with decreasing relative ET, but showed no systematic influence of the tree line. We found that vegetation composition changed with distance to the tree line. The proportion of dead herbage was equal at all distances from the tree line under very dry conditions, but smaller close to the trees under moderately dry conditions. Herbage quality, as approximated by crude protein and ADF concentrations in live biomass, was higher closer to the tree line. We conclude that the tree line does not adversely affect grassland productivity under drought and improves the plant water status as reflected by the δ¹³C signature.