Seasonal fire impact on ecosystem characteristics and surface energy in Southern Africa savanna

Abstract:

Fire is a primary disturbance to the distribution and ecological properties of the world’s major biomes. Every year in the dry season, wildfires burn about 400 Mha of land vegetated areas, leaving behind numerous scars in the landscape, which influences the surface fluxes and climate through vegetation-climate interactions. Fire models have been developed to explicitly describe the burned area, fire emissions, and fire disturbance on terrestrial ecosystems. Although most fire models accurately simulate annual fire regimes, they have deficiencies in capturing the fire peak month and tend to overestimate the fire season length. Therefore, the seasonal fire impacts on the ecosystem's carbon budget, surface energy balance, and water cycle, have not been quantified in fire modeling studies.

In this study, we develop a fire model of intermediate complexity, SSiB4/TRIFFID-Fire model, which includes fire impact on the terrestrial carbon cycle by updating fire-induced carbon loss every 10 days. The new fire model is shown to reproduce the observed fire regimes at the global and regional scale. Specifically, SSiB4/TRIFFID-Fire captures the fire season length and peak fire months in global major fire regions. It is therefore applied to provide a quantitative assessment of seasonal fire impact on ecosystem carbon budget, energy balance, and water cycle in the largest fire region, the Southern African savanna. The results show that fire can reduce LAI and GPP by 0.4 m² m^-2 and 200 g C m^-2 year^-1 separately. The removal of vegetation and exposure of bare ground cause an increase in albedo, a decrease in surface fluxes, and an increase in surface temperature. The changes in surface energy budget and water cycle are found in both the fire season and the following growing season, which may influence regional climate and large-scale circulation through biosphere-atmosphere interactions.