In January, Jason Fridley is headed to France to study how invasive ragweed is responding to climate change in that country’s northernmost region. “Ragweed is a good old American species,” says Fridley, a professor of biology in the College of Arts and Sciences at Syracuse University whose six-month research project will be supported by the Hauts-de-France Regional Award from the Fulbright U.S. Scholar Program. Although present in Europe since the 1800s, ragweed spread widely in France during World War I with the American cavalry, whose horses and hay most likely laid the groundwork for it to seed, grow and spread across the countryside. Today, the hay fever-inducing invader is abundant in the Mediterranean and other parts of Europe and creeping northward, stirring public health concerns, Fridley says. “There are just hints of it in that area of France, but it looks like it’s at the cutting edge of a front that’s moving.”
Fridley plans to explore ragweed’s basic functions and how its growth may be influenced by increasing levels of atmospheric carbon dioxide, which is absorbed by plants as part of the photosynthesis process that triggers growth. With carbon dioxide and other greenhouse gases rising to unprecedented levels in the atmosphere, climate change can be a catalyst for the spread of invasive species, creating warmer and longer growing seasons. Studies have shown that ragweed “responds favorably to elevated carbon dioxide,” Fridley says. “If so, we might be able to make some predictions about which plants are going to continually be those that are most abundant or start to spread more rapidly because carbon dioxide is increasing so fast.”
For more than two decades, Fridley has studied the ecology of plant communities, often focusing on the role of climate change. He’s among a group of researchers who have studied hillside pasturelands at the Buxton Climate Change Impacts Laboratory in Derbyshire, England—now in its 25th year and considered one of the longest running climate experiments in the world. He has examined how a warming climate can influence the transition of old fields in the Eastern United States into forests. He’s also conducted research in the Great Smoky Mountains National Park, studying how warmer temperatures affect plants at higher elevations.
Fridley and his research teams have studied upwards of 100 different species of plants. “We take species we think are important and use them as models to ask global-change questions,” he says. One species they’re currently studying is Japanese knotweed. In a comparative study supported by a $628,000 grant from the National Science Foundation , Fridley and his Syracuse University research team are collaborating with researchers in Europe and Japan to explore whether some invasive species are “pre-adapted” to invade areas because they come equipped with attributes that bolster their success in new habitats, or if they change through time in invaded territory to better compete against native plants. By analyzing the plants in both their native and invaded ranges and through laboratory tests of leaf chemistry and structure, they hope to answer several questions, including how plants with varying photosynthetic capacities allocate nitrogen—a key plant nutrient—for different leaf functions, such as light harvesting and cell wall structure. Along with determining whether the plants shift their resource allocation to better perform in a new environment, they want to learn whether the invaders already possessed photosynthetic advantages through natural evolution in their home ranges. “We’re also testing whether photosynthetic performance is the principal driver of competitive dominance in invaders,” Fridley says.
This summer as part of the study’s field research, team members Julie LeVonne, a biology doctoral student, and Noelle Stevens, a senior conservation biology major at SUNY College of Environmental Science and Forestry, are collecting and analyzing samples of woody understory species of plants in Central New York and the Hudson Valley region, testing how they react to various levels of light and carbon dioxide. “I enjoy species composition and biodiversity, so it’s interesting to see how an area can have this incredible biodiversity and then one invader can come in and drastically alter the levels of biodiversity,” LeVonne says. “With climate change, we’re going to see different environmental factors and this will influence how both native and invasive species interact. We’ll be able to use that information to see what invasive events might happen in the future.”
Biogeography—the geographic distribution of plants—plays an important role in Fridley’s research, as knowing plant histories and where they live now can offer insights as to where they may thrive in the future as climate change alters environments. Consider the impact of shifts in temperature, rainfall, soil fertility, the length of the growing season and the presence of encroaching invaders. In the Buxton experiment, for example, the researchers learned that tiny plants amid pasture and rock outcrops slowly migrated over a 20-year period. “Now, however, because the growing season is longer than when we began, things are starting to move faster. The bullies of the plant world are pushing everything out.”
Defining Invasive Species
In the tussle for space in the plant kingdom, those bullies are often invasive species. But determining what’s classified as invasive is no easy chore. Typically, the invaders are non-native species introduced into a new environment, either purposefully, say as an ornamental shrub, or accidentally (such as ragweed in France). But depending on the scope, native plants can also be considered invaders, Fridley says. He points to the black locust tree, a native of the Southeast U.S. that vastly expanded its range and is viewed as a non-native scourge by many in New England. Non-native plants can also become naturalized species, making themselves at home in a new area and not causing problems. What distinguishes invasive plants from their competitors is their aggressive behavior—they move in, spread like wildfire and take over. “Among scientists there is no universally agreed upon definition of what invasive means,” Fridley says. “Usually what it comes down to is whether some government agency is paying money to get rid of it, then we call it invasive.”
Here in Central New York, plenty of invasive plants have infiltrated the landscape: bush honeysuckle, Japanese knotweed, black swallow wort, purple loosestrife and giant hogweed to name a few. “The vast majority of these are here to stay,” Fridley says.
Surging in the Central New York Shade
While Fridley recognizes the frustration of property owners battling invaders and that research can help steer land management practices, he is most fascinated by what he learns from how plants function and optimize their attributes. For instance, what do successful invasive plants in Central New York share in common? “In our environment that often means dealing with a seasonally shaded system, which for a plant is stressful,” Fridley says. “The way a leaf works is radically different in the sunshine and in the shade.” Most of the invaders in Central New York forests grow fast in high light, possessing leaves that are conducive to light, Fridley says, but they also seem to be shade tolerant. They not only grow well in spring before forests leaf out, but they also continue to grow once the forest canopy limits light reaching the understory. As an example, Fridley draws a contrast between the native spicebush and the bush honeysuckle, an invasive deciduous shrub. Highly shade tolerant, the spicebush doesn’t leaf out on the forest floor until the forest canopy unfolds. Then, it may start to senesce its leaves as soon as late July. “Whatever it’s evolved to do, it’s evolved to do it in a short period of time with minimal resources,” he says. Meanwhile, the bush honeysuckle takes off early, grows fast and flourishes more than spicebush in the same low-light environment, often keeping green leaves through the fall, Fridley notes.
Many of Central New York’s forest invaders are from Japan, Fridley says. While CNY’s forest flora endured ice age after ice age and millions of years of brutal winters, Japan and most of Asia escaped the glaciers, leading to flora of dense forests that was strongly shade tolerant, Fridley says, turning again to spicebush as an example. “Is it any surprise that its modern ecology is so restricted?” he says. “It’s so afraid of what will happen in our unpredictable spring and fall seasons that it has this really short growing season—the plants of Japan don’t experience that, so that may mean they’re pre-adapted to invade here and replace our more risk-averse plants.”
Roots and Shoots in Action
When it comes to using their resources, invasive plants seem to employ what Fridley calls a “fast and loose strategy.” Typically, before native plants drop their leaves, they conserve nitrogen, phosphorus and other nutrients, pulling them from the leaves back into the stem or down into the roots. “Invaders, however, are wasteful,” Fridley says. “When you measure the nutrients during growing season and then after its leaves have fallen to the forest floor, it turns out that their nutrient contents aren’t that different.”
In the last few years, Fridley has expanded his research to investigate what’s going on below ground, where plant roots interact with microbes to gather nutrients. As Fridley explains, plants have a carbon-nitrogen exchange system. “They harvest and use carbon largely for energy, but to do that they need materials, nitrogen in particular,” he says. “The carbon comes from above ground and the nitrogen from below ground—essentially the roots and shoots trade those essential elements. Roots and shoots work together to make babies (seeds) as efficiently as possible.”
By synthesizing the two processes in the lab, Fridley hopes to gain more understanding of how plants become successful—and whether invaders benefit from the work of soil microbes. It is another piece of the puzzle that ultimately could reveal why invasive plants come to dominate particular environments. “We want to understand what the actual biological constraints are when evolution is building a plant—what are the limits of what a single plant can do,” Fridley says. “Invaders may tell us that better than native plants that aren’t actively spreading.”