Contributions of Different Ecosystem Components to the Carbon Balance of an Encroaching Savanna

Woody encroachment in savanna ecosystems is occurring in extensive areas around the globe. This change in ecosystem structure is believed to be an important carbon sink, but the sink remains poorly quantified. To study the effect of woody encroachment on the ecosystem carbon balance at the local scale, we used continuous eddy covariance measurements in a central Texas savanna encroaching with Ashe juniper and Honey mesquite. To better understand the physiological processes underlying the overall carbon exchange, we estimated the contributions of the different ecosystem components to the overall carbon flux. Monthly soil respiration measurements, leaf level gas exchange and NEE chamber measurements were combined with a scaling approach. Soil respiration measurements, under different vegetation types (herbaceous layer, Ashe juniper, Honey mesquite) scaled to the ecosystem level, show a remarkable similarity to ecosystem respiration derived from tower measurements and suggest ecosystem respiration is dominated by below-ground processes. Yearly sums of net ecosystem exchange show that our site acts as a net carbon sink, although the size of the annual sink is dependent on the amount of precipitation. During the winter months (October-March), Ashe juniper stays active and offsets the carbon source stemming from the inactive herbaceous layer, keeping the system carbon neutral. The herbaceous layer contributes up to 30 percent of the carbon sink during favorable conditions in the spring, but reverts back to a carbon source during drought conditions in the summer. Carbon uptake by Ashe juniper and Honey mesquite persists throughout the summer and is responsible for a small net carbon uptake of the entire system during drought conditions. Our results show the dominant role of the encroaching trees in the carbon balance of this savanna site and the potential of woody encroachment to be a significant carbon sink. However, the magnitude of the sink remains dependent on the climatic conditions at our site, especially the amount of summer precipitation.