Expanding Woody Encroachment - Is it a Concern for Water Yield? (SAEON Seminar, March 2025)

Описание: Presenter: Dr. Michele Toucher (NRF-SAEON Grasslands-Wetlands-Forests Node)

Title: Expanding Woody Encroachment - Is it a Concern for Water Yield?

Abstract: Grasslands and savannas are being significantly altered by woody encroachment. Woody encroachment is the gradual spread and thickening of indigenous trees and/or shrubs in a grassland or savanna, within or beyond their natural ranges. These species displace grasses and herbaceous vegetation eventually leading to a closed canopy resembling that of a dense shrub or forest. Although there is consensus on the drivers of encroachment, there remains uncertainty and concern surrounding the influence of encroachment on hydrology.
To expand the understanding of woody encroachment on hydrology paired “plot” experiments were set up in a semi-arid savanna and mesic grassland. Monitoring of evapotranspiration (ET) was undertaken simultaneously over an encroached and non-encroached area at both sites. The mesic grassland site were long-term paired catchments, a fire-adapted grassland that senesces in winter and an evergreen tree encroached catchment. The savanna site was a long-term woody plant clearing trial, the first plot was a dense stand of Mopane while the second plot was cleared annually of mopane trees. Ancillary measurements of climate, soil water and streamflow were taken.

ET at the mesic grassland site was markedly higher over the encroached catchment, particularly in the dry winter months due to the evergreen nature of the trees in comparison to the senesced grassland. The tree canopy intercepted 29 % of event rainfall. Over time, the runoff ratio of the encroached catchment decreased in comparison to that of the grassland catchment. Similarly, the ET was higher over the encroached mopane plot relative to the open savanna. However, it was the wet seasons and above average rainfall years where the increases were more evident. With increasing spatial extent of mopane encroachment, the influence on ET became more apparent. Encroachment regardless of the climate appears to negatively influence water yields, highlighting the need to actively manage encroachment especially in the light of global change and increasing water scarcity.