Climate–soil interactions drive environmental filtering of herbaceous mountain plant communities in Griqualand West, South Africa

Environmental filters shape herbaceous plant community composition and structure. Climate is the primary driver, followed by geological heterogeneity influencing plant–soil relationships. The mountains of Griqualand West (GW), a centre of plant endemism in South Africa, host diverse herbaceous communities, yet vegetation–environment relationships remain poorly understood. This study investigated how climatic and geological factors affect herbaceous composition and structure across four mountain ecosystems in a semi-arid Kalahari savanna landscape. Multivariate analyses revealed distinct plant communities, primarily driven by 14 environmental variables. Key soil factors (Ca, Mg, P, Fe and % silt) and bioclimatic variables related to precipitation and temperature (bioclimatic variables 5, 11, 12, and 18), strongly influenced community composition. Herbaceous plant communities differed markedly among mountain ranges. Plant density was lowest on the Langberg and highest on the Ghaap Plateau, where plants were generally shorter. Vegetation structure also varied, with higher grass cover in the Kuruman Hills and increased bare soil with reduced rock cover on the Ghaap Plateau. While the frequency of grasses and lignified forbs did not differ among regions, herbaceous forbs were more abundant in the Kuruman Hills and Ghaap Plateau. Generalised linear models (GLMs) showed that combinations of local soil chemistry and regional climate gradients drive structural variation. These findings highlight how complex climate–soil interactions shape mountain vegetation and offer insights into semi-arid vegetation responses to changing rainfall patterns and soil degradation under climate change.