The Potential Impact of Woody Encroachment on Evapotranspiration Losses in South Africa's Savannas: A combined Systematic Review and meta-Analysis Approach

Abstract

Woody vegetation cover in South Africa has increased over the past 100-150 years owing to the establishment of commercial forestry plantations, the spread of alien invasive plants (AIPs) and indigenous woody encroachment (WE). Extensive research conducted over the past 50 years has shown that AIPs can lead to dramatic declines in catchment water yields as a result of their high evapotranspiration (ET) rates. This has raised concern that WE may also be responsible for increasing ET losses and adversely impacting the country's limited water resources. In this paper, we used a combined systematic review and meta-analysis approach to explore trends in the water use of different vegetation types located in various climates across South Africa, to ultimately evaluate the likelihood of WE increasing ET losses in South Africa's savannas. This study revealed mixed support for whether WE in South Africa's savannas is increasing ET losses. On one hand, the fact that WE species replace grasses and form dense thickets indicates that there is high potential for WE to increase ET losses. On the other hand, rainfall, appears to be a primary factor limiting ET in semi-arid climates, indicating little potential for WE to have any effect on ET, unless there is an above-average rainfall year, or the vegetation has access to an additional water source. This study justifies the need for additional ET monitoring in South Africa's savannas in order to determine whether large-scale WE control should be implemented to conserve water resources in one of the country's driest regions.