Fire disturbance and nitrogen deposition impacts at the watershed scale in southern California

Abstract

The mountains of southern California have some of the highest rates of atmospheric deposition in the world. Over the last two decades a consistent picture of unusually high levels of nitrate in streams has been painted using field sites at the San Dimas Experimental Forest (SDEF) and across southern California. Within the context of southern California, three major processes appear to drive the observed variability in nitrate concentrations in streams. First, chronic atmospheric nitrogen deposition leads to high nitrate concentrations. Second, the profound inter-annual variability in rainfall drives nitrate concentrations in streams. Wet years produce a more thorough flushing of nitrate from the watershed resulting in higher nitrate concentrations compared to dry years. Third, fire is a critical process for most ecosystems in southern California; its impacts vary from vegetation and hydrologic consequences to more subtle impacts on biogeochemical fluxes. The results of a prescribed fire experiment in 1984 at the San Dimas Experimental Forest indicate that fire initially causes elevated stream nitrate concentrations but that over the longer term watersheds with more frequent fire have lower nitrate export. This longer term decrease after fire is likely due to losses of nitrogen to the atmosphere during burning and to surface water export in sediment and dissolved load in the immediate post-fire environment. These losses are then followed by rapidly aggrading vegetation during the ensuing years of post-fire recovery that provides a sink for nitrate originating from either atmospheric deposition or N mineralization/nitrification.

Citation

Meixner, Thomas; Fenn, Mark E.; Wohlgemuth, Peter M. 2003. Fire disturbance and nitrogen deposition impacts at the watershed scale in southern California. In: Renard, Kenneth G.; McElroy, Stephen A.; Gburek, William J.; Canfield, H. Evan; Scott, Russell L., eds. First Interagency Conference on Research in the Watersheds, October 27-30, 2003. Washington, DC: U.S. Department of Agriculture, Agricultural Research Service: 637-642.