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Forest census over 96 years shows loss of tree diversity and resilience

Researchers at the University of Illinois Urbana-Champaign have analyzed 96 years of forest census data to better understand ecological changes and inform management practices.

Their , published in Forest Ecology and Management, reveals concerning homogenization trends. This means the forest has become less diverse over time, losing trees that played a critical role in its ecosystem.

The researchers analyzed census data from Trelease Woods, which the university acquired in 1917. Homogenization was linked to the spread of the emerald ash borer, an invasive beetle, and Ophiostoma fungus, which causes Dutch elm disease.

Many deciduous forests like Trelease Woods are losing diversity, co-author Jennifer Fraterrigo said. Fraterrigo is professor of natural resources and environmental sciences. She worked on the study with her former graduate student, Jennifer Álvarez, who is currently an environmental assessment researcher at the Illinois State Geological Survey, part of the Prairie Research Institute at Illinois. Integrative biology professor James Dalling and former NRES forest ecologist John Edgington were also co-authors on the study.

Dutch elm disease and the emerald ash borer have significantly decreased the abundance of elm and ash trees in Trelease Woods and across North America, Fraterrigo said. However, diseases and pests failed to fully explain homogenization trends: suppression of forest fires, whitetail deer overpopulation, and the introduction of invasive plants can also drive homogenization, she said.

"The study was focused on biotic disturbances—disease, pests, and invasives—because we have data from before they arrived, so that's something we knew was affecting the forest," Álvarez said. "But we can't isolate any single factor."

Less diversity means less resilience

While the study focused on changes caused by Dutch elm disease and the emerald ash borer, these were not the only factors affecting the forest, and elm and ash were not the only species whose relative abundances changed.

When elm and ash die, they leave gaps in the forest, which other species compete to fill in. That's when other factors come into play—fire suppression, for example. Wildfires also create space in the forest for new trees to grow. Without fire, forests become shadier and wetter, a process known as mesophication.

So instead of a wide variety of species replacing declining ones, only a handful of plants spread—mostly sugar maple and Ohio buckeye, which are well adapted to the new conditions. Oak trees, which don't do well in the shade, have declined.

"This can make the forest less resilient to future disturbances," Fraterrigo said. "If one species makes up 70% of a forest, and then a pest or disease targeting that species is introduced, then 70% of the forest trees would be gone, likely leading to functional collapse."

Broad implications from a unique case

The study is unusual in that it follows a single forest, Trelease Woods, over a long period of time. Most forests in the Midwest have been clearcut at least once or have sustained other significant forms of damage and degradation. But Trelease has survived with relatively little human interference and related physical damage, and scientists have been collecting data from the site for over a century. It's a massive group effort, Fraterrigo said. Hundreds of NRES and integrative biology students have surveyed the site for class credit over the years.

"Most studies use space-for-time substitution to understand how ecosystems respond to disturbance over time. Researchers might not have long-term data for a specific site, so they'll compare many sites that have been exposed to the same kind of disturbance but at different times. We'll assume that any observed differences reflect how an ecosystem responds over time," Fraterrigo said. "But that approach has its problems. Having the Trelease Woods data allowed us to directly investigate how the forest responded to ecological changes."

Research at Trelease might help inform forest management globally, as the exceptional site was added to the Forest Global Earth Observatory Network in 2018. ForestGEO, as it's called, helps researchers standardize forest data and collect it in one place. As of today, the network monitors roughly 7 million trees comprising nearly 13,000 species, connecting insights from individual scientists to reveal broader trends.

"Trelease is a relatively undisturbed forest, yet we see this mesophication trend," Fraterrigo said. "More active management might be necessary to conserve certain species, even in places with little human activity."