Comparing the differential effects of canopy shading by Dichrostachys cinerea and Terminalia sericea on grass biomass

Woody encroachment is occurring globally; however, the tropical grassy ecosystems, which dominate Africa, are particularly vulnerable to this phenomenon. Encroached ecosystems experience significant changes in biodiversity, ecosystem functioning and service provisioning. One such vital ecosystem service is the provisioning of grazing. As encroachment can reduce grass production through the shading out of shade-intolerant grasses, there is a need to better predict these impacts particularly, since multiple woody species act as encroachers and little is known on their differential effects. Encroacher species may affect grass production differently, depending on the architecture of their canopies. This study therefore aimed to compare the effects of two structurally different encroaching species (a shrub vs a tree) (Dichrostachys cinerea and Terminalia sericea) on grass biomass. We tested the effects of canopy architectural traits of these two species on grass biomass using field sampling in a savanna conservation area in southern Zimbabwe. Canopy cover was a strong predictor of grass biomass, where increasing canopy cover significantly reduced grass biomass under both species. However, the architecture of the shrub D.cinerea, typified by a large crown diameter and crown height and a small crown base-height, created more shade, which had a significantly greater negative effect on grass biomass than the canopies of T.sericea at the same stem density. Architectural traits are therefore considered useful in predicting impacts of encroaching species on grass biomass. The findings show that encroachment of D.cinerea holds the potential to cause a regime shift from an open system to a closed system as a result of its extensive canopy cover.