Study shows how aspen forests maintain the diversity needed to adapt to changing environments

Olivia Cope, the lead researcher, keeps two aspen trees from her experimental plots. Although the trees are identical in age, the larger tree is more likely to grow faster than its smaller counterpart. The forest's genetic structure was changed by the faster-growing trees over time. Credit: Rick Lindroth / UWMadison
It is not as exciting to watch paint dry than it is to see a forest grow.

This long wait has made it difficult to study how forests respond to environmental fluctuations. It's even more difficult to predict how they will fare under new pest pressures or changing climates.

A new study, spanning a decade, by University of WisconsinMadison researchers has revealed how aspen stands alter their genetic structure as they grow and compete for sunlight. Aspen is North America's most widely distributed tree species and an indicator species of how forests will adapt to changes in the environment.

Trees are genetically predisposed towards achieving superior growth when faced with fierce competition. However, the survivors are less able to deal with damaging insects.

This experiment shows how evolutionary forces can rapidly shape entire forest stands. This experiment also shows that different environmental factors can keep forests diverse and able to respond to different stressors.

These findings are important for conservation biologists, who seek to conserve diverse forest ecosystems against global warming, invasive species, and other environmental changes.

Rick Lindroth, UWMadison's professor of entomology, said that the study demonstrated how important traits like growth and defense can be combined and how genetic diversity will enable populations to adapt to changing stresses.

Lindroth, the lead author, was a former student at UWMadison, Olivia Cope, and her colleagues published their findings Sept. 6, in Proceedings of the National Academy of Sciences.

Aspen are known to colonize disturbed areas, such as the barren landscapes created by wildfires like those that occurred in western North America recently. A small area will have thousands of trees germinate, and they begin to grow taller than their neighbors. This intense competition quickly determines winners and losers.

Scientists created this environment by planting young seedlings near the UWMadison campus in dense stands. They then removed three-quarters the seedlings from half of the plots to reduce the competition for sunlight. The result was two types of tree stands, one with high competition and one without.

Some trees are genetically predisposed for growth while others focus their resources on protecting insects and mammals. Cope began to track the growth of the trees at five years old and which trees survived the next five.

Scientists discovered that trees were shorter if they focused more on defense. As they were shaded by taller trees, the shorter plants were more likely die. The tallest trees grew to more than 40 feet by the end of the study; the smallest trees that survived were only seven feet tall.

Lindroth says that plants can grow exponentially so a slight difference in height in the beginning allows them to capture more sunlight and this difference can magnify over time.

The genetic structure of the forest has changed as a result of the death of highly protected trees more frequently. Fast-growing trees became dominant, particularly in densely planted and competitive plots. This divergence led to genetic differences between the low-competition and high-competition stands over time.

The trees didn't suffer much insect damage during the study period. However, Lymantria dispar, an invasive moth, ate almost all of the leaves from a nearby experimental stand of aspen trees during the study period. Researchers expect periodic pest attacks like these to reward aspen forests that provide enough defense and growth.

This balance act will help to create a forest that can meet changing threats.

Lindroth says that there is a shifting dynamic due to a changing environment which ultimately selects for diversity preservation within a population. "If this diversity has a genetic foundation, it is possible to maintain it because under certain conditions one trait might be beneficial while under other circumstances it may not."

These decade-long experiments seem rare. Lindroth says that they are becoming more important in an age of environmental change. This is particularly true in the west where record wildfires have decimated large swathes of land, altering forest composition.

After a failed previous study that examined the effects of deer browsing on aspens, the lab decided to conduct this study. The serendipitous repurposing trees and new funding provided the timeframe necessary to discover evolutionary forces at play in long-lived aspens.

Lindroth says, "It takes time to understand the ecological reactions of trees. "We were blessed with that opportunity."

Continue reading Drought can affect aspen survival for decades, new study shows

More information: The growthdefense trade-offs influence population genetic composition of an iconic forest tree species, Proceedings of the National Academy of Sciences (2021). Information from the Journal: Proceedings of the National Academy of Sciences. Growthdefense trade-offs influence population genetic composition of an iconic forest species. (2021).