Under fire and heat: Managing mountain grasslands in a hotter world

Abstract Fire is a key driver of montane grassland biodiversity and ecosystem functioning, where rising temperatures linked to climate change are predicted to change the structure and functioning of the plant community. Despite the central role of fire in these systems, the extent to which fire interacts with warming to shape biodiversity and ecosystem functioning remains poorly understood. In 2017, we established a 7‐year full‐factorial warming experiment using open‐top chambers within a long‐term fire experiment in the Drakensberg Mountains of South Africa. This represents one of the first in situ warming experiments in an African montane grassland. Across warming and fire treatments, we measured microclimate, biomass and species composition. Our results show that the experimental warming significantly increased average air temperature by 0.6°C (± 0.07°C; p < 0.001), with hourly maximum temperatures differing by up to 4.4°C. However, soil and surface temperature, as well as soil moisture, varied between combinations of fire frequency and warming treatments, indicating that vegetation can act as a mediator between macroclimatic (i.e. warming treatment air temperature) and microclimatic conditions. Consistent with expectations of improved plant growth under elevated temperatures, we observed increased plant biomass in warming treatments. We recorded 35 native plant species in the experiment. Their compositional disparity was better explained by fire than by warming, and there was no significant species turnover induced by the 7 years of warming. Synthesis and applications. Our findings suggest that, although plant community composition appeared resistant to direct warming, warming increased biomass, with potential implications for fuel accumulation and fire severity. Differences in biomass and vegetation among fire frequencies mediated the effects of warming on near‐surface microclimate, including soil and surface temperature and soil moisture content. By linking vegetation structure, above‐ground biomass accumulation and microclimatic buffering, we show that fire interacts with warming to shape ecosystem exposure to climatic extremes. These results indicate that pyrodiversity (the diversity of fire effects over space and time) may enhance resilience by balancing biomass accumulation with microclimatic stability, supporting adaptive fire management under a warmer climate. Read the free Plain Language Summary for this article on the journal's blog .