In Forests, Climate Change Brings Complicated Consequences


Climate change is creating new challenges for forests and forest managers. Dartmouth researchers have responded with a resource that can help those managing forest pests and pathogens—an extensive review of nearly 500 papers going back to 1665.

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Postdoctoral Researcher Aaron Weed, at left, and Professor of Biological Sciences Matt Ayres inspect native hemlock for the hemlock woolly adelgid, an invasive insect that feeds by sucking sap from hemlock trees. (Photo by Eli Burakian ’00)

“This is a comprehensive cross-disciplinary synthesis of how climate affects North American forests by changing patterns of pestilence” says Dartmouth Postdoctoral Researcher Aaron Weed, lead author on the paper, which was published in the journal Ecological Monographs.

Professor of Biological Sciences Matt Ayres co-author of the study, adds that the review deals not only with the impact of climate on forest pestilence, but also the consequences for the forests and for humans.

“Epidemics of insects and diseases, events that we think of as pestilence, are the most important agent of disturbance in North American forests,” says Ayres. “Since 2000, the area infested by tree-killing bark beetles alone is more than the area impacted by wildfires.”

He notes that while bark beetles have been killing pine forests for centuries, climate change is rapidly altering the extent and impact of tree-killing pests.

Weed, Ayres, and Jeffrey Hicke of the University of Idaho conducted the synthesis as an invited contribution to the 2014 National Climate Assessment (NCA). The NCA is a product of collaborations involving 13 federal science agencies and 240 authors drawn from academia; local, state, tribal, and federal governments; the private sector; and the nonprofit sector. This is the third NCA. The first assessment was in 2000 and the second in 2009.

“Predictions from the first NCA of expansions in forest disturbances from climate change have been upheld—in some cases more rapidly and dramatically than expected,” the authors write.

Furthermore, the first NCA’s detailed predictions, such as how far native bark beetles would expand northward and how much higher in the mountains, have been upheld. Warming of the coldest night of the winter has been a crucial enabler.

“There has been good progress in the theory and data that connect climate, physiology, and insect population dynamics,” says Ayres. “Climate metrics are as numerous as baseball statistics, so it has been helpful to learn that one—the coldest night of the winter—is particularly important.”

This, he says, influences the distribution of many plants and plant-eating insects, but the insects respond almost instantly. The pests are now in forests that are especially vulnerable, forests that developed under different environmental conditions and may not have fully adapted to the new climate.

A recent example cited in the paper is the New Jersey Pinelands, where there is an unprecedented epidemic of southern pine beetles. The coldest night of the winter now averages 7 degrees Fahrenheit warmer than 50 years ago. “Otherwise we would not have a beetle epidemic there,” Weed says. Over the same time, average temperatures increased by less than a degree. (Milo Johnson ’13, who was a research assistant in Ayres’ laboratory as an undergraduate, produced the documentary A Story About a Beetlefocusing on the biology and management of the southern pine beetle.)

Weed says that the hemlock woolly adelgid, an invasive insect that has been killing hemlock trees in Connecticut and Massachusetts, is now similarly moving into New Hampshire and Vermont, where they were previously excluded by cold winter nights.

There may be two sides to the pest picture, however. As predicted by the first NCA, it seems there are also regions where forest pestilence has declined. The report highlights the extensive and highly productive pine forests of East Texas and Louisiana, places ravaged in the past by the southern pine beetle. In these forests, the insect is now rare, due, in part, to the warming.

Ayres adds that “climate change is increasing tree growth in most of North America, so forests grow faster and have more to give, but it is important to recognize that forests can only mature slowly while infestations can be fast. They can be affected almost instantly by pests.”

The authors reason that maintaining North American forests under a changing climate should include expanding forests in regions where conditions are becoming more favorable. They argue that climate change heightens the need for ever-stronger collaboration between scientists and managers.

Ayres says that he hopes their published synthesis will be widely read and will make a contribution toward meeting a very large challenge.

Joseph Blumberg