Past and future changes in forests, insights through time and across space

Background/Question/Methods

Climate change is rapidly altering conditions across the globe and will dramatically modify species assemblages through local extirpation and species range shifts. Many US national parks are already outside their historical climate space, and the prospect of dramatic ecological responses is a challenge to an agency with the mission to “[preserve] unimpaired the natural and cultural resources and values of the national park system.” Effective forest stewardship in this era of continuous change will require understanding of anticipated changes. Insights can be gained from examining how potential changes compare to past dynamics and how projected future forests compare to today’s forests. We explored projected forest community change in 12 eastern national parks by examining 1) local forest composition history obtained from the North American Pollen Database and 2) projected forest composition based on the USFS Tree Atlas. We used rate-of-change statistics based on the Bray-Curtis dissimilarity measure to provide 1) a temporal comparison of projected rates of forest community change from 1990-2100 with their rates of change over the Holocene and 2) a spatial comparison of projected 2100 forest community composition under two greenhouse gas emissions scenarios with composition of forested ecological landscapes of today.

Results/Conclusions

The paleorecord indicates rapid forest community change during the early Holocene – soon after the Last Glacial Maximum (17-10 thousand years ago) – and again during the period since European settlement. In fact, half of all studied parks show evidence of rapid rates of change following European arrival. Projected rates of 21^st-century forest community change exceed the full Holocene range in about a quarter of parks under a low-emissions scenario based on a “cool” model (PCM-B1) and half of parks under a high-emissions scenario based on a “warm” model (Hadley A1FI). Projected rates in the remaining parks generally match maximum Holocene rates. Spatial comparison shows that projected forest composition for each park under the low emissions scenario is generally highly similar to an ecological region within 50-300 miles (similarity values of 0.70-0.85), but under high emissions is most similar to regions >800 miles away and similarity is substantially lower (<0.5). Change has been a constant and periodically rapid feature in forests of the eastern US, and insights through time and across space indicate potentially substantial future changes to park landscapes.