Temporal Trends in Stream N Concentrations and Responses to Disturbances in US Forested Basins

USFS Experimental Forests and Ranges have been collecting stream hydrologic and chemistry data in reference and disturbed forested basins across the country for several decades. These sites are located across a gradient of climatic conditions and are uniquely positioned to examine stream chemistry trends during the last decades and to study water quality responses to natural disturbances and forest harvest. To increase our understanding of stream chemistry trends at sites that are considered reference, sites that have not had anthropogenic impacts in the last 70 years other than atmospheric deposition, we synthesized stream N data collected over 12 to 43 years from 22 forested reference basins from seven USFS Experimental Forest Research sites . Results show high temporal and spatial variability in stream water N concentrations and trends. Some reference basins within a single Experimental Forest displayed temporal N concentration trends in opposite directions, suggesting that local factors (i.e., aspect of the basin, precipitation, vegetation, geology and soil type, and moisture) may be more relevant than regional factors. Additionally, we found trends in streamwater N concentrations changing direction over time. These findings highlight the importance of long-term stream water chemistry studies with continuous records encompassing wide range of climatic variability since controls of N exports from headwater streams may vary at both fine and long temporal and spatial scales. With a better understanding of trends in reference basins, we are beginning to examine stream chemistry responses to multiple types of disturbances including forest harvesting, insect outbreaks, fires, and hurricanes. We analyzed data from a total of 33 disturbance events on 8 Experimental Forests to quantify both the magnitude and duration of stream N responses. This analysis will increase our understanding of water chemistry responses to disturbance across biomes.