The influence of grass competition and soil moisture on the growth and phenology of Karoo shrubs in the Eastern Upper Karoo

Abstract

Rainfall in the Nama-Karoo is sporadic and a plant’s ability to access and utilise available soil moisture will determine its ability to persist in the system and to successfully recruit. When summer rainfall in the Eastern Karoo increases, grassiness increases at the expense of shrub canopy cover. This may drive a transition from shrubland toward grassland. The effect of grass–shrub competition on shrub growth and phenology in the Eastern Karoo is not adequately explained by the root–niche separation hypothesis or the succession hypothesis. In an experiment, we measured shrub growth and phenology in the absence (clipping treatment) and presence (control treatment) of grass over the 2017/2018 rainfall season. Grass clipping changed grass species composition over this period, but grass clipping did not benefit shrubs. We found no competitive effect of grasses on shrubs. However, shrub canopy cover showed a strong linear response to soil moisture, as did canopy cover to temperature. Similarly, soil moisture and temperature were important for stem growth and shrub phenology. Patterns of shrub growth and phenology in response to summer and winter rainfall provide important insight for land managers for optimising production while enabling phenological processes that allow recruitment to take place.