Rapid ecosystem shifts are triggered by interactions of severe landscape disturbance and climate change in the southwestern US

Background/Question/Methods: Many studies predict changes in species distributions in response to changing climate. At local scales, community composition reflects changes in the suitability of existing habitat for species persistence. Both modeling and empirical studies suggest that such changes due to climate alone are likely to be expressed at multi-annual to decadal time scales. In contrast, severe large-scale disturbances can reorganize ecosystems on much shorter time scales of days to months. We posit that it is the combination of climate change and severe disturbance that is most likely to trigger abrupt ecosystem transitions into novel configurations, rather than either factor acting separately. These new configurations can be resilient in their new state, and resistant to return to pre-disturbance conditions. In addition to ecological effects, these transitions can include potentially persistent alterations to geomorphic, soil, hydrological, and biogeochemical systems. Such abrupt transitions are predicted to become more common under conditions of altered future climate and amplified disturbance regimes: climate provides the envelope within which these dynamics occur, but disturbance provides the trigger for abrupt system reorganization.

Results/Conclusions: We are studying the impacts of multiple successive fires and post-fire succession over a 35 year period in the Jemez Mountains in northern New Mexico, USA, one of the best monitored and instrumented ecosystems in western North America. The most recent event is the 63,400-ha 2011 Las Conchas Fire, the largest fire in New Mexico history, which burned into the perimeters of several prior major fire events from 1977, 1996, and 2000, leaving large areas of landscape with nearly total tree mortality. Burn severity data will allow us to partition out the highest severity areas, which have the highest potential for undergoing abrupt ecosystem transitions. In areas of repeated burns, particularly at high severity, we predict the most rapid triggering of ecosystem tipping-point behavior. Preliminary observations indicate large-scale type conversions, specifically from forested to forest-shrub and interior chaparral vegetation types following multiple severe fires with overlapping perimeters during a period of extended drought conditions.