Watching paint dry has nothing on watching a forest grow. That achingly long wait has always made it challenging to study how forests adapt to environmental fluctuations. As a result, it’s hard to predict how forests will fare, for example, in a changing climate or under pressure from new pests.

That’s why a group of CALS research-ers took the long view. Their recently concluded study — 10 years in the making — reveals how aspen stands change their genetic structure over time as trees balance pest defense with growth.

When faced with stiff competition, trees genetically predisposed to prioritize growth fare better as they win the battle for sunlight. But the survivors are less equipped to handle damaging insects. The experiment demonstrates how evolutionary forces can quickly shape entire forest stands. It also suggests that a litany of environmental changes can promote diverse forests capable of responding to different stresses.

“What this work has done is show how key traits, like growth and defense, can be coupled together and how genetic diversity will allow populations to adapt to new stresses,” says Rick Lindroth, a professor of entomology who supervised the study.

Aspen is the most broadly distributed tree species in North America, and it’s a bellwether for how forests will adapt to an onslaught of human-influenced environmental changes. The species often colonizes disturbed environments, including the barren landscapes that appear after wildfires, such as those in western North America in recent years. Thousands of trees will germinate in a small area, and the race begins to grow tall enough to escape the shade of their neighbors. This intense competition quickly selects for winners and losers.

Lindroth, along with collaborators Eric Kruger, professor of forest and wildlife ecology, and Ken Keefover-Ring, professor of botany and geography, simulated this environment by planting young seedlings in dense stands at UW’s Arlington Agricultural Research Station. Then, they removed three-quarters of the seedlings in half the plots, which produced two types of tree stands: one with high levels of competition for sunlight and one with low levels.

Some trees were genetically predisposed to prioritize growth, while others put their resources into producing protective chemicals that deter attacks by insects and mammals. When the trees were five years old, Olivia Cope, a doctoral student in the Department of Integrative Biology at the time, started tracking how fast the trees grew and which survived for the next five years.

The scientists saw that the more trees focused on defense, the less they grew. The shorter plants were more likely to die as they were shaded out by their taller neighbors. By the end of the study, the tallest trees towered more than 40 feet; the shortest surviving trees were just seven feet tall.

“Because plants grow exponentially, a little bit of difference in height early on allows them to capture more light, and that difference in height can magnify over time,” says Lindroth.

Because the defense-focused trees died more often, the genetic structure of the forest stands changed over time. Trees with fast-growing genetics came to dominate, especially in the densely planted and highly competitive plots.

Over time, this divergence meant the low-competition stands and the high-competition stands developed different genetic structures.

During the study period, there was little insect damage on the trees. But during 2021, the invasive moth Lymantria dispar ate through almost all the leaves of a nearby experimental aspen stand. The researchers expect that similar periodic waves of pests would reward those aspen forests that balance growth with sufficient defense. This balancing act should help create a diverse forest capable of meeting changing threats.

“You have this shifting dynamic because of a changing environment that ultimately selects for the maintenance of diversity within a population,” says Lindroth. “If that diversity has a genetic basis, the reason it can be maintained is that under some conditions one trait may be beneficial whereas under other conditions it may not be.”

The study’s findings are valuable for conservation biologists who want to preserve diverse forest ecosystems in the face of global warming, invasive species, and other environmental changes.

This work was supported by the U.S. Department of Agriculture (Grant WIS01842) and the National Science Foundation (Grants DEB-1456592 and DGE-1747503).