Interactions Between Climate and Vegetation Regulate Lateral Carbon Export from Terrestrial Ecosystems

Abstract:

Lateral hydrologic export from terrestrial ecosystems transports carbon (C) to adjacent aquatic ecosystems, where it can be stored in sediments or mineralized and emitted to the atmosphere. Although aquatic emissions are a mechanism for terrestrial C loss to the atmosphere, terrestrial C budgets typically do not account for lateral C export and likely overestimate terrestrial C storage. Current theoretical understanding of lateral C export is limited, but identifying key drivers is necessary for increasing representation in terrestrial C budgets. Precipitation is undeniably a primary driver of lateral C export because it facilitates connectivity within the landscape and provides a physical pathway for C movement. However, lateral C export is co-limited by hydrologic export and C supply and, therefore, increases in precipitation may not translate to increases in lateral C export. We suggest that interactions between precipitation, vegetation, and C supply likely modulate lateral C export. To investigate the role of these feedbacks, we used a simple linked terrestrial-aquatic hydrology and C process model applied at the catchment scale. Our model identifies interactions between temperature, precipitation, and plant traits that drive response of lateral C export to climatic variation. We found that lateral C export increased with precipitation and that mean annual temperature strongly influenced the magnitude of increase when total annual precipitation exceeded about 75 cm year^-1. Increasing mean annual temperature alone increased lateral C export at a given total annual precipitation when coupled with an increase in terrestrial net primary production (NPP). Plant functional type further modified lateral C export responses to temperature and precipitation by constraining NPP response. Our results suggest that both precipitation and temperature are important climatic drivers of lateral C export and that vegetation strongly modulates the conditions at which highest lateral C export can be expected. Thus, we recommend that efforts to evaluate changes in terrestrial C cycling due to global environmental change account for lateral C export as a dynamic flux that may impact estimations of spatial and temporal variability in terrestrial C storage.