They cleared invasive shrubs from a Pennsylvania forest; What grew back surprised researchers

Beneath a forest’s tall trees, invasive shrubs can grow into a dense layer that leaves little room for native plants. Pulling them out looks like the obvious way to start restoring a woodland. What happens next is less obvious. Native plants might move in on their own, or the cleared ground might need replanting.

Researchers at Pennsylvania State University put that question to the test in Hartley Wood, a mature woodland at the university’s Arboretum. In a paper in Invasive Plant Science and Management in 2019, the researchers reported that native vegetation recovered better than they had predicted.

According to Science Daily in 2009, the team began repeatedly removing a group of 18 invasive shrub species from five treatment plots, each paired with a control plot for comparison. This was no single clean-up. Over the following years, they kept removing regrowth and new seedlings.


Seven years in, native understory plants, small trees and other vegetation had returned to the cleared plots. Native cover grew by more than the researchers measured in unmanaged forest where invasive shrubs were thin on the ground.

The invaders had occupied the good spots

One might expect the areas occupied by invasive shrubs to be poor places for native plants to grow.

Ecologist Erynn Maynard-Bean and associate professor Margot Kaye suggested a different reading. The shrubs may have taken hold in places with relatively good sunlight, water and soil nutrients. Once the shrubs were controlled, native plants could draw on the same resources.

In its account of the research, Penn State’s own account of the work said regeneration in the treated plots was stronger than comparison with less-invaded forest would have led the team to expect.

The point is that an invaded patch is not necessarily empty of native species. Some native species may already be present, waiting for conditions to improve. Reducing that competition can give them more opportunity to regenerate.

The team stopped short of saying every removal project would go the same way. Theirs was one mature woodland in central Pennsylvania, and a forest’s response depends on its plants, soil, light and other conditions.

Why the finding matters to people managing forests

Restoration takes years. Clearing shrubs is only one part. They also have to decide whether to plant natives, keep watching new growth and also stop invaders from coming back. That can be costly, especially for a large site. Where local ground level plants already survive under or around invasive shrubs, natural regeneration could help to restore parts of a woodland without planting each replacement by hand.

Maynard-Bean said removal can be worthwhile even when managers have no budget for a separate programme to re-establish natives. The experiment did involve repeated attention to regrowth, though, so it should not be taken as proof that one clearance guarantees recovery.

Native herbs are part of the habitats for birds, insects and amphibians, also a dense layer of non-native shrubs can alter the conditions in which these animals feed, reproduce and shelter.

The study did not mention whether amphibians or birds' numbers recovered. It tracked a change in plants that form part of their habitat. That is a step forward, but it does not show that wildlife gained.

Clearing is only the beginning

This took place in Hartley Wood , a 16 hectare woodland that contained mature oak, maple trees and hickory. The forest had escaped much of the historical logging that altered other areas, but shrubs had spread through its understory, in parts because of its closeness to the landscaped residential area.

The findings were published in 2019, offering evidence that a forest with a long history of invasion could still respond when those shrubs were removed repeatedly. They also explain why monitoring matters; it helps researchers track the vegetation over several years rather than just judging success immediately after clearing.

Since there is no universal timetable for forest recovery, and the experiment did not establish that mature canopy trees would quickly be replaced. In fact, the study found that no change in the regeneration of taller canopy trees during the examined period.

But the forest floor had a useful story to tell. Once the invasive shrubs were brought under control, native plants moved in to take advantage of the open space and resources. That has a practical lesson for restoration teams looking to figure out where to spend scarce dollars. Sometimes the first thing to do is not plant more, but give the plants already there a chance.