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Trees on the move

Written by Katie Renwick, SoGES 2013-2014 Sustainability Leadership Fellow, and PhD Candidate in the Department of Ecosystem Science and Sustainability and Graduate Degree Program in Ecology

Throughout the past decade, an unusually severe outbreak of the mountain pine beetle has affected millions of acres of forest across western North America. The mountain pine beetle is a native insect that affects several species of pine. Beetles burrow into trees to lay their eggs just beneath the bark, and trees are ultimately killed by a fungus that the beetles carry.

To a forester, the mountain pine beetle may be seen as a pest that destroys valuable timber resources. To tourist visiting Rocky Mountain National Park, the millions of dead trees may be seen as an eye sore. As a scientist, I view the mountain pine beetle outbreak as an opportunity to learn more about the impacts of forest disturbances.  

Disturbances can create a sort of natural experiment when different parts of the landscape are affected to different degrees, and can help us answer questions that cannot be addressed experimentally. My research focuses on understanding how beetle outbreaks may interact with climate change to alter the composition and structure of forests. Drought stress can make trees more susceptible to insect attacks, and rising temperatures allow more beetles to survive the winter. As a result, mountain pine beetle outbreaks are expected to become more frequent and severe as the climate warms. Understanding the implications of this interaction between climate change and forest disturbances will become increasingly important for land managers tasked with maintaining diverse and productive forest ecosystems in the future.

By studying how forests located at different elevations respond to this current outbreak, I can learn how the impact of insect outbreaks may differ in relation to temperature. One question that I am particularly interested in is whether or not the beetle outbreak will facilitate tree migrations. Many species are expected to migrate towards higher (cooler) elevations as a result of climate change. Trees are immobile, and so in forests this process occurs through the death of old trees at lower elevations and an increase in the number of seedlings at higher elevations. As a result, tree “migrations” are slow and can lag behind the rate of climate change. Insect outbreaks can accelerate the migration process by killing old trees that might have persisted for many decades and reducing competition so that new seedlings can become established. The mountain pine beetle outbreak may consequently allow trees to “migrate” faster than would otherwise be expected.

 

 

 

 

 

Visitors at Rocky Mountain National Park are confronted by vast hillsides of gray, needless trees killed by the mountain pine beetle.

 

 

 

 

 

Mountains create sharp climatic gradients that allow us to study how temperature may affect forest recovery following the mountain pine beetle outbreak.

 

 

 

 

 

New seedlings at the upper elevation limit of lodgepole pine indicate that the species is already starting to migrate in response to climate change.

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