The International Commission on Stratigraphy recently ratified three ages within the Holocene epoch (Greenlandian [11,700 years ago], Northgrippian [8,326], Meghalayan [4,200]). Each was preceded by a presumably global climatic event that is widely recognizable in the stratigraphic record, and which precipitated biologic changes that are evident in a range of paleontologic proxies. However, the magnitude of biotic change and long-term repercussions have largely been studied on only a local scale. In light of current climate change, this begs the question: what are large-scale consequences of abrupt climate change on biotic communities? We address this question using pollen data from the Neotoma database, and assess whether past global climatic events that define the Holocene’s boundaries led to abrupt and/or persistent vegetational changes across North America. We evaluate whether vegetation changes in the last ~100 years--representing the beginning of the proposed Anthropocene epoch--are comparable to past levels of vegetation change over the last 14,000 years. We use two methods for quantifying abrupt vegetation change: bayesian change point (BCP), and stratigraphically constrained cluster analysis (CONISS). We incorporate age model uncertainty and account for differences in sample size to generate a conservative measure of past rates of abrupt change.
Results/Conclusions
Regardless of the metric used (BCP or CONISS), the highest number of abrupt changes occurred within the last 250 years. Though the magnitude of vegetation change during the Anthropocene is comparable to the magnitude of change at the end of the Pleistocene, the rate of increase was markedly faster in the Anthropocene. This signal, however, is largely driven by the eastern half of the continent. In the western US and Canada, rates of vegetation changes were not significantly elevated during the last 250 years relative to previous 14,000 years. The large number of abrupt changes within the last ~250 years is likely a product of direct anthropogenic modification of vegetation (logging, land conversion for agriculture, etc.) as well as climate change and introduced species. The relative magnitude of these drivers differs across the continent, which may explain the observed differences between western and eastern North America. Overall, we found that vegetation in North America did not always respond in a synchronous manner to global climatic change. However, during the transition from the Holocene to the Anthropocene, we find a clear increase in abrupt change in vegetation, which is likely a product of multiple human-induced stressors on the landscape.