The Influence of Woody Plant Thinning on Soil Hydrological Processes: A Paired Plot Experiment in A Semi-Arid Savanna Affected by Woody Thickening

During the past 150 years, grassy biomes in drylands across the globe have undergone a shift from grass to woody dominance, a phenomenon commonly termed woody thickening or woody encroachment. The hydrological implications are of concern because a change in the dominant plant functional type can alter rainfall interception, plant water uptake, and soil hydrological processes, with potentially significant implications for streamflow and groundwater recharge at the landscape scale. Removal of woody plants has long been proposed as a management strategy for increasing water yields, despite a lack of empirical evidence to prove its effectiveness. The current study investigated how woody plant thinning influenced soil hydrological processes in a semi-arid savanna in South Africa affected by woody thickening. Over a two-and-a-half-year period, a field-scale paired-plot experiment was conducted, with soil water content, soil temperature, and evapotranspiration (ET) measured in plots that had been thinned of the dominant woody plant species (Colophospermum mopane) and adjacent woody-thickened plots. Surface infiltration tests were also carried out. Thinning had minor effects on soil water in the soil profile (mean difference of < 0.05 mm³ mm⁻³ between paired plots) and soil temperature, and no pronounced effect on daily ET. Only one set of infiltration tests indicated a significant increase in infiltration following thinning. This contradicts the results of some similar studies in dryland savannas but is consistent with others. Whether thinning can increase the production of surface runoff or groundwater recharge over a longer time period in this study system requires further investigation.