Using tree rings to better understand the role of biological demand in the inter-annual variability of stream nitrate concentrations at the Fernow Experimental Forest, West Virginia

Background/Question/Methods

Long-term stream-water chemistry records from three reference watersheds at the Fernow Experimental Forest display synchronous, inter-annual oscillations in stream water nitrate concentrations that are relatively large (variations of ~27%), and that appear to reoccur about every 7 years. These variations need to be understood in order to detect the onset of changes to longer-term trends that may be related to alterations in nitrogen deposition.

We hypothesized that the observed oscillations are caused by reoccurring changes in climatic factors that can influence the growth, and thus the demand for nitrogen, by forest trees. To investigate the role biological demand has in this variability, we cored ten Acer rubrum, Liriodendron tulipifera, and Betula lenta trees (n=30) from one of the reference watersheds (WS10).  Ring width measurements from the cores were cross-dated, converted to basal area increment (BAI), and averaged. A cross-correlation analysis between BAI, stream water nitrate, and climatic variables was used to determine if significant temporal relationships existed between tree growth, across all species, and stream water nitrate levels.

Results/Conclusions

Cross-correlation analysis revealed that higher mean maximum temperatures were strongly associated with greater tree growth after a lag of 1-year (R²=0.34). Greater tree growth, in turn, had a strong association (R²=0.49) with lower stream nitrate concentrations after a1-year lag. Thus, temperature has a strong, negative association (R²= 0.58) with stream nitrate concentrations over a 2-year lag. These results document a recurring temporal pattern of events that begins with favorable climate for trees and ends with decreased nitrate concentrations in stream water. Understanding the nature of the variability in stream water chemistry will aid in detecting long-term trends, which at the Fernow Experimental Forest, are linked to both rainfall chemistry and ultimately to the upwind emissions of  industrial pollutants. Further research into synchronous variations in climate could reveal a more distal connection between stream water chemistry, biological demand, and quasi-periodic climate phenomena such as the El-Nino Southern Oscillation.