Complex response of the forest nitrogen cycle to climate change

Climate exerts a powerful influence on biological processes, but the effects of climate change on ecosystem nutrient flux and cycling are poorly resolved. Although rare, long-term records offer a unique opportunity to disentangle effects of climate from other anthropogenic influences. Here, we examine the longest and most complete record of watershed nutrient and climate dynamics available worldwide, which was collected at the Hubbard Brook Experimental Forest in the northeastern United States. We used empirical analyses and model calculations to distinguish between effects of climate change and past perturbations on the forest nitrogen (N) cycle. We find that climate alone cannot explain the occurrence of a dramatic >90% drop in watershed nitrate export over the past 46 y, despite longer growing seasons and higher soil temperatures. The strongest climate influence was an increase in soil temperature accompanied by a shift in paths of soil water flow within the watershed, but this effect explained, at best, only similar to 40% of the nitrate decline. In contrast, at least 50-60% of the observed change in the N export could be explained by the long-lasting effect of forest cutting in the early 1900s on the N cycle of the soil and vegetation pools. Our analysis shows that historic events can obscure the influence of modern day stresses on the N cycle, even when analyses have the advantage of being informed by 0.5-century-long datasets. These findings raise fundamental questions about interpretations of long-term trends as a baseline for understanding how climate change influences complex ecosystems.