Assessing the Linkages between Tree Species Composition and Stream Water Nitrate in a Reference Watershed in Central Appalachia

Abstract

Many factors govern the flow of deposited nitrogen (N) through forest ecosystems and into stream water. At the Fernow Experimental Forest in WV, stream water nitrate (NO3−) export from a long-term reference watershed (WS 4) increased in approximately 1980 and has remained elevated despite more recent reductions in chronic N deposition. Long-term changes in species composition may have altered forest N demand and the retention of deposited N. In particular, the abundance and importance value of Acer saccharum have increased since the 1950s, and this species is thought to have a low affinity for NO3−. We measured the relative uptake of NO3− and ammonium (NH4+) by six important temperate broadleaf tree species and estimated stand uptake of total N, NO3−, and NH4+. We then used records of stream water NO3− and stand composition to evaluate the potential impact of changes in species composition on NO3− export. Surprisingly, the tree species we examined all used both mineral N forms approximately equally. Overall, the total N taken up by the stand into aboveground tissues increased from 1959 through 2001 (30.9 to 35.2 kg N ha−1 yr−1). However, changes in species composition may have altered the net supply of NO3− in the soil since A. saccharum is associated with high nitrification rates. Increases in A. saccharum importance value could result in an increase of 3.9 kg NO3−-N ha−1 yr−1 produced via nitrification. Thus, shifting forest species composition resulted in partially offsetting changes in NO3− supply and demand, with a small net increase of 1.2 kg N ha−1 yr−1 in NO3− available for leaching. Given the persistence of high stream water NO3− export and relatively abrupt (~9 year) change in stream water NO3− concentration circa 1980, patterns of NO3− export appear to be driven by long-term deposition with a lag in the recovery of stream water NO3− after more recent declines in atmospheric N input.