How land management and water availability control ecosystem-atmosphere carbon exchange in the Karoo, South Africa

South African ecosystems are highly vulnerable to the effects of climate change, such as increasing temperatures, modifications in rainfall patterns, increasing frequency of extreme weather events and fire, and increased concentration of atmospheric carbon dioxide (CO2). At the same time, ecosystems are impacted by livestock grazing, cultivation, fuelwood collection, urbanization and other types of human land use. Climatic and land management factors, such as water availability and grazing intensity, play a dominant role in influencing primary production and carbon fluxes. However, the relative role of those parameters still remains less known in many South African ecosystems. Investigation of the carbon inter-annual variability at dwarf shrub Karoo sites will assist in understanding savanna dynamics and in constraining climate change scenarios as basis for climate adaptation strategies. This research is part of the EMSAfrica (Ecosystem Management Support for Climate Change in Southern Africa) project, which aims at producing data and information relevant to land users and land managers such as South African National Parks (SANParks). A particular focus is given on the importance of carbon cycling in degraded vs. intact systems. We investigate the impacts of climate parameters and diverse land management on ecosystem-atmosphere variability of carbon fluxes, latent and sensible energy. Long-term measurements were collected and analyzed from two eddy-covariance towers in the Karoo, Eastern Cape, South Africa. Study areas had almost identical climatic conditions but differ in the intensity of livestock grazing. The first site represents controlled grazing and comprises a diverse balance of dwarf shrubs and grasses, while the second site is degraded through overgrazing in the past (rested for approximately 8 years) and mainly consists of unpalatable grasses and short-lived species. These ecosystems are generally characterized by alternating wet (December to May) and dry seasons (June to November) with the amount and distribution of rain (average 373 mm per year) and soil moisture as the main drivers of carbon fluxes. We observed peak CO2 uptake occurring during the wet season (January to April) and a progressive decrease from wet to dry periods being highly controlled by the amount of precipitation. At the end of the observation period (November 2015 - November 2019), we found that both study sites were considerable carbon sources, but during wet periods 'overgrazed in the past' site had stronger carbon sequestration compared to 'controlled grazing' site. The higher carbon uptake could be an indication that resting of the highly degraded site for a long period may improve carbon uptake in the Karoo ecosystems. Our study shows that CO2 dynamics in the Karoo are largely driven by water availability and the effects of grazing intensity on above-ground biomass.