Tuesday, November 8, 2005
Nonlinear interactions between climate, landscape structure, and plant migration
The majority of ecological studies have focused on inherent biological complexity and cross-scale interactions as a primary cause of nonlinear ecosystem dynamics (e.g. threshold effects, cascading events, etc.). However, as shown in a series of studies on the late-Holocene migration of plant species in western North America, exogenous, non-biological factors that drive many ecological processes may also behave in a nonlinear manner, thereby promoting or amplifying nonlinear ecosystem change. Long-duration tree-ring chronologies and other high-resolution proxies suggest that the climate of western North America is characterized by highly non-stationary behavior. More specifically, western climate tends to switch between persistent hot/dry and cool/wet regimes over decadal to centennial timescales. When information on Holocene plant distributions from fossil woodrat middens and other biogeographic archives is combined with these paleoclimatic records, we see that persistent climatic regimes paced plant migrations throughout the Holocene via their role in promoting or suppressing disturbances. Moreover, decadal to centennial variability modulates the frequency of dispersal events and the distance they cover, as well as probability of a seed reaching a suitable location on the landscape. Switching between different climatic regimes may also alternately increase or decrease the probability of survival and reproduction after arrival in a new habitat. The resulting progression of Holocene plant migrations was often one of “fits and starts” marked by multiple long-distance dispersal events and backfilling rather than a steady wave-like progression. Overall, regime-like behavior in the climate system leads to spatial and temporal nonlinearity in migration processes that may, in turn, be compounded by cross-scale interactions and inherent biocomplexity. If we hope to predict the outcome of future plant migrations, we must first develop forecast models that can accommodate non-stationary climate and potential interactions between habitat structure and climate over landscape to regional scales.
Steve Gray is a plant ecologist at the U.S. Geological Survey-Desert Laboratory in Tucson, Arizona who studies ecosystem and landscape responses to climate variability over decadal to multidecadal time scales. As part of this work Steve has developed a network of millennial-length tree-ring chronologies from sites throughout the Interior West. These tree-ring records provide a long-term (100s to 1000s of years) perspective on droughts and precipitation variability in the region. Steve's work also explores the interplay between climate variability and land-use practices in natural resource management. As a National Research Council Post-Doctoral researcher Steve focuses on using information from paleoecological and palecoclimatic archives to help develop and test predictive models for ecosystem and watershed responses to climate change.