Coupling a biochemical vegetation dynamic model with landscape evolution for climate change impacts assessment

Vegetation plays an important role in protecting soil against erosion by intercepting rain, increasing flow resistance, promoting soil infiltration and improving soil strength. Erosion can deplete and redistribute soil carbon and nutrients, affect carbon sequestration and change the amount of carbon stored in the soil. Traditional erosion and landform evolution models tend to oversimplify the role of vegetation on erosion and its effects on the carbon cycle, failing to take into account the dynamic character of biomass and effects of hydrological and erosion/deposition processes. This work centred on the development of COPLAS, a new model that couples hydrological, geomorphological, biogeochemical and carbon processes and their dynamic interactions and effects. The model includes a coupled photosynthesis-stomatal conductance representation to model vegetation growth in response to climatic inputs and water availability. COPLAS was calibrated using historical hydrologic and energy data, erosion measurements and carbon pools estimations for Howard Springs (Northern Territory, Australia). The model was used to investigate the relative importance of changes in soil protection provided by leaves, roots, litter and soil carbon on catchment erosion rates, as well as erosional responses, soil carbon redistribution and carbon fluxes under projected future changes in precipitation, temperature and CO2 concentrations. Results showed that protection against erosion at different times of the year is provided by various vegetation parts, being leaves and roots the most important protection mechanisms. When leaves do not fully recover after the dry season, the protective effect of roots, litter, and to a lesser extent the soil carbon, turn out to be determinant. Additionally, it was found that the combination of increased temperature and rainfall produces more erosion and carbon emissions, while elevated CO2 conditions tend to reduce erosion and increase carbon sequestration. Overall, the fertilization effect of the CO2 was more significant than the precipitation and temperature effect for our study case. Finally, the model predicts that under increased rainfall, temperature and CO2 concentration conditions, soil carbon is redistributed in the catchment, creating areas of low and high soil carbon. These findings highlight the importance of a robust vegetation and carbon representation in landscape models to broaden our understanding of how climate change will affect vegetation, erosional/depositional processes and carbon redistribution.