Commentary: Shedding light on non-native, invasive plants

By Dr. Erynn Maynard-Bean

If you stroll through a hardwood forest after the spectacle of leaf drop this fall, you can soak up some sunshine.

Two months prior, when the canopy was full, the same walk was notably darker. Many forest species, such as tree saplings, bees, butterflies and frogs, are adapted for higher light in the spring and fall. An extreme example are spring ephemerals — popular forest wildflowers that put out leaves and showy flowers and begin to die back again, all before tree canopy closure.

Enter the antagonists of this story: nonnative, invasive ornamental shrubs, including bush honeysuckles, privets, burning bush and barberry.

At this point, it is important to clarify some terminology. Nonnative species are from a different part of the world — usually transported by humans further than they would have otherwise been able to move. By federal definition, the subset of nonnative plants that are very successful in their new range — spreading into natural areas and causing harm to native species and ecosystems — are called invasive species. It should also be noted that some people use “invasive” to refer to undesirable or weedy native species, which is not how I am using it here.

The abundance and the number of invasive shrub species is increasing in hardwood forests across the eastern U.S. Dispersed long distances by birds that naively eat their nutritionally devoid fruit, invasive shrubs succeed in forest understories. They tend to grow more densely than native forest shrubs and have leaves earlier in spring and keep leaves longer in fall. This produces shade in the understory when native species need open conditions.

My research has shown that invasive shrubs have leaves about 47 days longer than native shrubs within each growing season in central Pennsylvania. So it is not surprising that in the absence of invasive shrubs in the forest, I found significantly increased light availability, native plant diversity and tree seedlings. Because leaf timing differs across a region, it is difficult to know how these findings apply to other forests.

Fortunately, there is a group of shining protagonists in this story! Hundreds of citizen scientists across the northeastern U.S. made observations that I used to characterize differences in leaf timing. I found that the difference in leaf timing observed in Pennsylvania will increase to the south and decrease to the north by about 5.1 days per degree latitude. In Delaware, that means invasive shrubs have leaves from about 52 to 57 days longer than native shrubs! The negative impacts to light and to native species are even more extreme here. As you can see, research on invasive shrubs anywhere in the eastern U.S. can now be understood in the context of the whole region. This information can also be used to help land managers detect and remove invasive shrubs while native species are dormant.

So often the fight against an invasive species is something intangible. For example, what can one person do to stem the tide of an invasive fish like the northern snakehead or a winged tree pest like the emerald ash borer?

With invasive shrubs, the most important part is not removing vast swaths of them from the landscape, but creating safe harbors for native plants and the animals that need them to continue being successful.

What can you do? Consider replacing invasive shrubs in your own landscaping. Seek out one of the growing numbers of garden stores specializing in Delaware’s beautiful native plants! Or inform yourself before your next purchase at a traditional plant store where invasive plants are often not labeled. There are many commonly available native plants and even nonnative species that do not readily invade natural areas. A quick Google search can help plants and animals in our native forests immensely.

Dr. Erynn Maynard-Bean is a scientist doing research surrounding forest ecology in the Ecosystem Science and Management Department at Penn State University.