By hogging sunlight, invasive shrubs harm native plants in US forests

3 September 2021

Photo: Pixabay/BjoernSchrempp

When most people in the United States think of invasive species, they might picture Burmese pythons in Florida or cane toads in Hawaii. Few Americans may think of shrubs as invasive species, yet these plants too can wreak havoc with the country’s forests.

By sprouting leaves earlier in the spring than native plants and keeping them longer in the fall, invasive shrubs can hog more sunlight, which is usually in short supply near the forest floor to begin with. At the same time, their foliage can lower air temperatures for other plants living underneath them in a double whammy.

These features likely give invasive shrubs a competitive advantage that enables them to replace many native plants over time in the understories of temperate, deciduous forests in the eastern part of North America.

Scientists at Penn State University have reached this conclusion after a three-year study they conducted to examine the effects of invasive shrubs in Hartley Wood, a 43-acre tract of forest owned by the university.

The researchers found though various measurements that invasive shrubs limited the infiltration of sunlight to the forest floor by nearly 27 percent compared to native shrubs. The differences were the starkest in the spring when plants need plenty of sunlight to grow.

“Think of it this way: it’s like a 60-watt lamp in every 10 square feet of the forest is removed by the presence of invasive shrubs, on average, across the growing season,” said Erynn Maynard-Bean, a scientist at Penn State who led the research.

In addition, invasive shrubs also tend to reduce temperatures beneath them, which means that their effect on plants growing in the understory is even more marked, the scientists have discovered.

“Light and temperature influence ecosystem processes, likely slowing the growth of native plants,” Maynard-Bean explained. “We saw a pattern of reduced maximum air temperature in the presence of invasive shrubs during the growing season of most years.”

And it isn’t just native plants that are negatively influenced by invasive plants hogging the sunlight. Bees, butterflies and amphibians living in forest understories are also affected, the scientists say.

“Those are exothermic species, meaning their activity and development is highly influenced by light and temperature in the understory,” Maynard-Bean notes. “So this could help explain why we don’t find as many butterflies, bees and amphibians in invaded understories as we do in uninvaded understories.”

Lower light conditions on the forest floor could harm the quality of litter and decomposition rates. In addition, invasive shrubs are making forests in the eastern region of the country more resistant to forest fires. That may sound like a good thing, but it isn’t so.

“The darker, cooler conditions caused by the invasion of nonnative shrubs have the potential to exacerbate the mesophication — the transition of a fire-adapted community to a fire-intolerant one — of Eastern deciduous U.S. forests,” the scientist said.

“Decades of forest fire suppression have allowed mesophytic, or shade-tolerant, species to shift the understory toward more cool, damp and shaded conditions resistant to burning.”

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