Shooting at a moving target: Evaluating ecosystem response to extreme events in a changing world

Abstract:

Evaluating ecosystem response to extreme events is extraordinarily challenging, requiring understanding of interactions of multiple factors. These evaluations are greatly complicated if background conditions such as climate, land use or atmospheric deposition are changing. At the Hubbard Brook Long Term Ecological Research (LTER) site in New Hampshire, USA, long-term changes in climate and in atmospheric deposition of nitrogen, sulfur and acidity have caused fundamental changes in ecosystem structure, function and response to disturbance. In this talk we review how long-term changes in climate have led to reductions in nitrogen cycling and availability which have in turn influenced ecosystem response to soil freezing events. Snow manipulations to induce soil freezing and natural soil freezing events in the 1990s induced significant increases in nitrogen cycling and losses but similar manipulations and events in the 2000s have not produced similar responses. These differences appear to be driven by decreases in nitrogen mineralization rates associated with climate change over this time period. Dissolved organic carbon response to soil freezing also appears to have changed over this period and appears to influence the nitrogen response. In addition to climate, atmospheric deposition has changed over time at Hubbard Brook, with marked decreases in deposition of sulfur and acidity since the 1960s, when a series of extreme clear cut events occurred at the site. We hypothesize that response to similar cuts would produce very different responses due to the loss of base cations associated with 50 years of acid rain. These examples illustrate the need to account for long-term changes in ecosystem structure and function when evaluating response to extreme events.