Disequilibrium vegetation dynamics in response to climate change: Lessons from the late glacial no-analog communities
Climate change in the next century is expected to drive disequilibrium dynamics in biotic communities, particularly for long-lived species such as trees, that may exhibit lagged responses. In many systems, we are already beginning to see the fingerprints of historic and ongoing climate change or other anthropogenic impacts, and so many communities that appear to be in equilibrium may actually be in a transient state. As a result, conservation strategies that rely on static restoration or management targets are unlikely to be successful. The paleoecological record may provide a promising way forward in understanding disequilibrium dynamics, as the increased spatiotemporal resolution of pollen data now allows us to reconstruct vegetation dynamics at sub-centennial scales. Additionally, the rate and magnitude of forecast climate change is predicted to approach that observed during the last deglaciation. Plant associations without a modern analog are a well-recognized feature of those late glacial landscapes, and their formation has been attributed to no-analog climates and the extinction of Pleistocene herbivores. While the establishment of no-analog plant communities took less than a century, the interval of high vegetation dissimilarity from present lasted nearly 1000 years, providing an opportunity to explore disequilibrium dynamics in tree communities in response to climate change.
We conducted a nonmetric multidimensional scaling analysis of seven sites in the upper Midwest, to reconstruct their trajectory in ecological space through time. The formation of the late glacial no-analog plant associations was driven not only by the individualistic response of taxa to extrinsic external abrupt forcing, but also by lagged responses driving disequilibrium dynamics during an interval of continuous environmental variability that lasted over a millennium. This is expressed by abrupt changes in vegetation and increased variability within and between sites during the no-analog interval, caused by the differing response times of taxa to climate change relative to the timescale of climate variability. We use the range of vegetation responses observed during the last deglaciation to inform a conceptual model of ecological disequilibrium in response to the climate changes exhibited during deglaciation (e.g, gradual and directional, abrupt and directional, and abrupt and variable).