Paleoecology provides many examples of abrupt ecological change. Indeed, the post-glacial period of the last 11,000 years, the Holocene Epoch, began after a rapid climate warming event at the end of the cold Younger Dryas period, which transformed ecosystems across the northern hemisphere and much of the world. Later events included range-wide abrupt species declines and rapid vegetation state changes at landscape scales. Here, I contrast two examples to consider the potential roles of climate and disturbance as stressors or triggers of abrupt declines in the abundance of dominant tree taxa. Both declines were studied by comparing fossil pollen data with independent paleoclimate and disturbance datasets.
Results/Conclusions
The first case, the rapid decline of Tsuga canadensis (eastern hemlock) across its range in eastern North America at ca. 5000 years before present, has been widely studied because of its abruptness, range-wide extent, and apparent involvement of only a single species. Statistical models of Tsuga’s climate niche reveal a complex relationship between abundance and multiple climate variables, which when combined with the multivariate history of temperature and precipitation from the northeast U.S. accurately predicts the decline without invoking additional dynamics or disturbance agents. The second case reveals how at finer, landscape scales, disturbances can interact with climate change to produce abrupt state changes. In this example from the Park Range of northern Colorado, pollen, charcoal, and paleoclimate data reveal that extensive fires helped to rapidly and permanently reduce sub-alpine forest cover after the Medieval period 1000 years ago. Statistical relationships indicate that climate was a strong driver of the sub-alpine forest changes, but that fires likely accelerated the transition from closed to open forest types. A simple model indicates that the response to fire depended upon the direction of climate change with both facilitating reduced tree cover. Taken together, these Holocene examples from the Younger Dryas to the Medieval period indicate that climate changes often induce rapid ecosystem responses, and that the structure of species’ climate niches and interactions with disturbances play important roles in determining the outcomes.