Invader Advantage

The rapid pace with which invasive shrubs infiltrate forests in the northeastern United States makes scientists suspect they have a consistent advantage over native shrubs, and the first regionwide study of leaf timing, conducted by Penn State researchers, supports those suspicions.

With the help of citizen scientists spread over 150 sites in more than 20 states, researchers collected thousands of observations over four years of exactly when both invasive and native shrubs leaf out in the spring and lose their leaves in the fall. The study area was expansive, stretching from southern Maine to central Minnesota, south to southern Missouri, and east to North Carolina.

"Eastern North America is the recipient of more invasive shrub species into natural areas than any other geographic region of the world," said Erynn Maynard-Bean, a postdoctoral researcher in the college working under the guidance of Margot Kaye, associate professor of forest ecology. "Invasive shrubs are growing in both abundance and the number of species established at the expense of many types of native species."

The researchers reported in Biological Invasions that invasive shrubs can maintain leaves 77 days longer than native shrubs within a growing season at the southern end of the area studied. The difference decreases to about 30 days at the northern end of the study area. At the southern end of the study area, the time when invasive shrubs have leaves and native shrubs do not is equally distributed between the spring and the fall; in the northern reaches of the study area, two-thirds of the difference between native and invasive growing seasons occur in the fall.

The longer period with leaves gives invasive plants an advantage in acquiring more energy from sunlight, and their leaves create shade in the early spring and late fall that may limit growth of native species such as forest ephemeral wildflowers, Maynard-Bean said. "This helps explain their negative impact on native tree regeneration, plant diversity, and abundance. But invasive shrubs also have a negative impact on communities of animal species sensitive to light and temperature, such as bees, butterflies, and amphibians."

"With the help of citizen scientists watching plants with us from around the eastern United States, we found a pattern of greater extended leaf phenology as you move south," she said. "This provides a unified framework for connecting local-scale research results from different parts of the eastern United States that had previously not agreed with one another."

--Jeff Mulhollem