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Climate Change and Red Oak Expansion

Many scientists have predicted that as the climate warms, red oak trees will gain a significant amount of suitable habitat in the Northeast and perhaps become a dominant species in northern New England hardwood forests. But Rebecca Stern isn’t convinced. By studying the tree rings of mature red oaks in Vermont and correlating their growth patterns with historic climate data, the recent PhD graduate of the University of Vermont found that the story isn’t as simple as it may seem.

Red oaks are at the northern edge of their range in Vermont and are most abundant in the western valleys of the Green Mountains. That’s where Stern cored 213 trees from 11 different sites. She found that red oaks put on more growth in years when there was higher precipitation, and they grew less in years when summer temperatures were particularly hot. She also observed that when there was an extended growing season, especially at lower elevations, red oaks benefited.

Given the forecast for increased moisture and a longer growing season in the future as the climate changes, one would expect that red oaks would receive a significant boost and potentially become a dominant species. But Stern said timing is everything.

“Red oak is growing really well in Vermont overall, and that’s good news for us when we want to capture more carbon in the atmosphere,” she explained. “But when warmer temperatures and increased moisture happen is important. Really hot summer temperatures aren’t beneficial to red oaks, and the season that temperatures are projected to increase the most is in the summer. And we’re projected to get more precipitation than in the past, but it will be more episodic, with giant rainfalls spread out with droughts in between. The increased precipitation will be in the winter and spring and less in the summer when the trees need it the most.” As a result, while red oaks are likely to benefit somewhat from the changing climate, Stern’s findings suggest that the inopportune timing of changes to weather patterns will counterbalance the positive factors.

Whether red oaks become a dominant species in the region in the future is still an open question, even if existing canopy trees get a boost. “That has everything to do with regeneration on the forest floor, and there isn’t much data about regeneration,” Stern said. “Anecdotally, there are reports of plenty of oak seedlings, but in areas farther south where there is heavier deer browse; oak seedlings are tasty to deer, and that’s a potential problem.”

To get a more complete picture of how northern forests are likely to change, Stern is comparing her data about red oaks with similar data she collected about maple, beech, and birch trees to determine how they are likely to fare in the future. She is also studying two conifer species, which can photosynthesize longer each year due to their evergreen needles, to see if that will give them an advantage over hardwoods.

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