Hydrological and pedological effects of combining Italian alder and blackberries in an agroforestry windbreak system in South Africa

Abstract. The Western Cape in South Africa is a water scarce region which under forecasted climate change scenarios may receive less rainfall and higher air temperatures. The integration of trees within agricultural systems provides an effective measure for improving water retention on agricultural land. Studying an established irrigated agroforestry system (AFS) combining alder (Alnus cordata (Loisel.) Duby) as a linear windbreak with a blackberry (Rubus fructicosa L.) crop, we explore the water use dynamics of the intercrop as influenced by the windbreak element by combining methods from hydrology, soil science and forestry. We also aim to evaluate whether the proposed experimental design is sufficient to capture the water balance and the underlying controls.

Due to the irrigation the AFS is no longer a water- but rather an energy-limited system. During the measurement period 13 rainfall events were recorded delivering 5.5–117.6 mm of rainfall with an intensity of 0.4 to 5.7 mm hr^-1. Root water uptake and event analysis show infiltration to likely occur via macro-pore flow with root water uptake occurring in two depth zones corresponding to different plant communities. Soil water content varied by depth and was influenced by physical and environmental factors, but was generally higher in the intercrop zone than within the windbreak influence zone. Soil moisture did not fall below the water content at the permanent wilting point (<-1500 kPa). Values corresponding to soil water tensions above 1000 kPa were recorded on several occasions, these were mitigated by irrigation, and thus, did not result in water stress. Nutrient distribution and soil physical properties differed near the windbreak in comparison to the blackberry crop and the carbon sequestration potential is great in comparison to monoculture farming.

The interdisciplinary work explored numerous aspects of AFS and acquired different perspectives, confirming hypotheses through cross-method analyses.