Drought responses of forb and grass communities in communal and protected rangelands

Abstract

The structure, growth, dynamics and productivity of herbaceous vegetation in semi-arid savannas are strongly limited by rainfall variability and nutrient availability. These factors interact with other drivers of savanna vegetation structure and function, of which herbivory remains among the strongest disturbance agents. Herbivore pressure is applied through grazing and/or browsing by multiple herbivore guilds of which their effects vary among rangeland types and herbivore densities. The spread of African pastoralism along with the predicted increase in frequency and severity of drought events highlights the need to improve our scientific understanding of herbaceous community responses to changes in rainfall variability and rangeland type. While grasses are generally the principal component in rangeland productivity and herbaceous dynamics studies, forbs are disproportionately underrepresented, whilst hosting the most diverse components of rangeland plant communities. Since African savanna vegetation structure and function have co-evolved with native herbivores, the interactive effects of land-use change towards single-species pastoralism and frequent or extensive drought events are expected to prompt significant changes in the herbaceous layer of semi-arid savannas. Grass community responses to changes in rangeland type and rainfall variability are well documented, although forb community dynamics have largely been neglected in the past. Forbs comprise an important part of the herbaceous layer as they significantly contribute to savanna ecosystem diversity and function. Plant functional traits adapted to tolerate disturbances are considered central to the functioning of herbaceous vegetation. Global climate and land use change necessitate the identification of drought- and herbivore tolerance traits to understand the functioning of the complete herbaceous component. The drought of 2014 – 2016 provided a unique opportunity to investigate the effects of a severe drought event on herbaceous community responses to distinct rangeland land-use practices in a nutrient-poor semi-arid Lowveld savanna of South Africa. The study aimed to evaluate and compare the drought responses of forbs and grasses on community- and functional level within two contrasting rangelands (wildlife and livestock). Sampling of herbaceous vegetation data were conducted towards the end of the drought (October 2016), and repeated several months after significant rainfall (January 2017). Field surveys took place in two different rangelands in the Gazankulu area, South Africa. The two rangeland types included a protected area, hosting a diverse community of large indigenous wildlife, and a communal rangeland with a long history of cattle grazing. Sampling of floristic data was conducted per herbaceous life form, permitting comparisons of forb and grass community responses to rainfall variability and rangeland type. Species abundances, basal cover per species and plant functional traits were recorded in a total of 120 fixed 1m2 plots across rangeland types. Results revealed life-form specific responses to rainfall and rangeland type. Post-drought herbaceous communities were significantly different from drought-communities, although grass assemblages in the communal rangeland were not affected by drought release. Forbs responded significantly to rainfall variability and rangeland type on community and functional level. For all the species richness and diversity indices forbs responded significantly within the protected area, except for species evenness. The communal rangeland did not respond significantly for either life form in any of the richness and diversity indices. Grasses responded significantly in species abundance and Shannon-wiener diversity index, however forbs had the greatest interaction with rainfall variability and rangeland type. Plant functional types did not respond as expected. In the drought year perennial forb and grass plant functional types dominated the herbaceous layer. Post-drought conditions were characterised by unpalatable perennial forbs dominated and palatable perennial grass plant functional types. Forbs had higher functional diversity with the most plant functional types within both rainfall years. The communal rangeland functional and compositional characteristics remained constant across rainfall years which could indicate drought-tolerance for this community. However, the protected area did show resilience through high plant functional type abundance