N20 Fluxes Under Managed Pine in Response to Throughfall Exclusion: Study Initiation.

In the southeastern U.S., forests occupy 60% of the land area, with a large fraction dominated by the genus Pinus, about half naturally regenerated and half planted with genetically improved seedlings. Because of their large area and high productivity, southern forests are a significant portion of the U.S. carbon budget, containing 12 Pg of C, 36% of the sequestered forest carbon (C) in the conterminous United States. Forests in the region annually sequester 76 Tg C, equivalent to 13% of regional greenhouse gas emissions, and have the potential to sequester more through reforestation, afforestation, and improved forest management.  Climate change, however, will likely have important impacts on planted southern pine systems. The SE U.S. is expected to experience continued increases in the rate of warming through the end of the century, with a rise in average temperature of 2.5 to 5 C by the 2080s, a magnitude of temperature increase that is predicted to decrease loblolly pine productivity by at least 10%. The greatest temperature increases are expected to occur in the summer, with the number of very hot days to increase faster than the rise in average temperature. Precipitation predictions for the region are less certain, but generally indicate that summertime precipitation will decline by 10% to 30%.  The goals of the PINEMAP project are to create, synthesize, and disseminate the knowledge necessary to enable southern pine landowners to harness forest productivity to mitigate atmospheric CO2 and to more efficiently utilize nitrogen and other fertilizer inputs, and to adapt their forest management approaches to increase resilience in the face of changing climate.  A primary component of the study is a factorial throughfall exculsion experiment that combines throughfall exclusion with forest fertilization. Although N2O accounts for only about 6% of anthropogenic greenhouse gas emissions, N2O is a potent greenhouse gas. It is nearly 300 times more powerful in its greenhouse effect than is CO2.  Recently, it has been questioned whether N2O emissions are large enough in highly fertilized agricultural systems to offset C benefits of increased crop productivity. Unfortunately, much less is known about N2O in fertilized forest systems. Given these responses and uncertainties, measurements of N2O fluxes after fertilization and under throughfall exclusion have been initiated in a pine forest in the piedmont of Georgia.  Early results are presented here.