Staging an invasion

Biology Professor Janet Morrison’s six-year study will measure the effects of invasive species on the region’s forests.

Words by Victoria Bartek

Carrying fences in one hand and a notepad in the other, Professor Janet Morrison leads students into Rosedale Park, in Hopewell, NJ, plodding carefully through the foliage carpeting the soft woodland floor. She pauses occasionally, readjusting the fencing digging uncomfortably into her palm, until finally she reaches her destination. Forty square plots lie in front of her, each one 16 meters wide and marked in the corners with PVC pipe. She examines each as students aid her in setting up fences around the plots.

Morrison has set up 120 such plots in six forests throughout Mercer County, NJ, on properties controlled by the county’s Department of Parks and Friends of Hopewell Valley Open Space, a nonprofit land trust. It is here that she conducts her research on invasive species; she is currently creating a “staged invasion” of Japanese stilt grass and garlic mustard. By manipulating plant growth in the plotted areas, Morrison can examine the effects of deer browsing, co-invasion (when these two species grow together), and competition on biodiversity. “These are clearly driving forces in these forests, yet they haven’t been tested experimentally,” explains Morrison. “What actually are deer doing? What actually is competition from these invasive species doing? How are deer affecting the actual invasion of the invasive species? Through the new grant from the National Science Foundation, we’re really going to try to tease this stuff apart.”

Morrison explains the study from her office, a white room with vines snaking artistically on the walls. “Half of our plots are fenced, and half aren’t. Deer will not have access to these fenced plots for six years, and the unfenced plots they will have access to. Within each one of those groups [caged and uncaged], there are four other treatments that are imposed. So a quarter of them are going to have nothing else done to them, just the deer being kept out or let in. To a quarter of them [in each group], we’re adding garlic mustard at a low constant density; a quarter of them we’re adding Japanese stilt grass, again at the same low, constant density; and to a quarter of them, both are being added. Then we’re going to follow what happens in those herb layer communities for six years. We’re going to be looking at deer herbivory and at how much the populations of garlic mustard and Japanese stilt grass grow. In other words, how much they’re invading in these plots.”

In order to reverse any adverse effects planting invasive species might cause, Morrison will undertake a complicated removal plan once her research concludes. “We’ll have to take out the cages and do the removals [of invasive species] in all the plots. I’ll organize some invasive species removal days every fall or spring, and get students and community members from local conservation groups to come and help. That will be an opportunity for us to teach people.”

Months after receiving the grant from the NSF, Morrison has begun setting up fences and germinating seeds for stilt grass and garlic mustard, preparing for the years ahead with fervent anticipation. Over the course of the study, she hopes to better understand the effects of garlic mustard, Japanese stilt grass, and deer herbivory on forest biodiversity, and plans to use her findings to implement strategies for invasive species removal and to educate the public.

“I always thought there was a big missing piece in the invasion biology world,” she said. “Land reserves often implement deer hunting or invasive species removal, but what is most effective? This study is a really powerful experimental approach to understanding these ecological interactions that, I think, are highly responsible for restructuring the plant community as we see it now.”