Combining field survey and time series data to learn about plant water usage in a South African agroforestry system

Agroforestry systems (AFSs) are associated with many benefits such as augmented soil fertility or enhanced biodiversity. However, in water-limited areas the competition over water resources between trees and crops can reduce the productivity of the crop component. We want to share some of our results gained from in-depth analyses of time series (October 2019 to March 2020) and campaign (September 2019) data of soil moisture and matric potential in a South African AFS.

Soil water content was measured in a soil profile at two locations within an AFS plot: alongside a windbreak consisting of Italian Alders (Alnus cordata) and amongst the crop i.e. within blackberry rows. Matric potential time series are only available at the windbreak soil profile. Surficial soil samples taken along transects perpendicular to the windbreak were analysed for physical properties (e.g. texture, water retention curve).

Based on extracted water retention curves and matric potential time series, we found no evidence for plant water limitation during the measurement period (summer months) within the field site. Estimated root water uptake indicated that the trees take water from a greater range of depths, including deeper layers, than the blackberry plants. We observed divergent hydrological behaviour of the soil at the two locations during precipitation events, potentially resulting from dissimilar storage capacities and runoff formation potentials. Furthermore, the matric potential revealed hydrological information on plant water usage that was not as obvious from the soil moisture data.