Wildflower is published quarterly by the Wildflower Center. Its content is national in scope with articles about the conservation and use of native plants as well as news from the Wildflower Center. A subscription is provided to Wildflower Center members as a benefit of membership.
How invasive plants behave in a warming world
By Bibi Wein
You may be able to observe some of the effects of climate change wherever you garden or enjoy the green world. I have. One example: in fall/winter 2009, we had no snow until December 9 in the Adirondack foothills of upstate New York, where I’ve gardened for 22 years. In at least 20 of those years, snow began in October or early November, often not melting until May. So which plants profited from this season’s reprieve from the bitter winter to come? Perennial weeds: red clover, bird vetch, oxeye daisy – all aliens. Untouched by the hard frosts that killed off everything else, they got a head start for spring, remaining green – and photosynthesizing – as much as two months longer than the natives they compete with in the flowerbed. And though the late snowfall gave me a larger window for pulling weeds, I couldn’t get them all.
To compound the problem, with invasives already on the increase, climate change has put more pressure on native plants by altering habitat conditions and growing seasons. Two separate issues for conservationists? In fact, they’re tightly linked.
“Invasive species generally thrive in stressed conditions such as those brought about by climate change,” says Camille Parmesan, Ph.D., assistant professor of Integrative Biology at The University of Texas at Austin. “By altering the competitive balance to favor invasive species, climate change may further disrupt ecological systems.” But, as Parmesan asserts, we’re not merely talking about the future. “Significant studies now exist to conclude that the consequences of climate change are already detectable within U.S. ecosystems,” she reported with co-author Dr. Hector Galbraith in an investigation of more than 40 studies for the Pew Center on Global Climate Change.
An invasive plant is, by definition, a species that is non-native in a particular ecosystem and whose introduction causes or is likely to cause harm to the economy, the environment or human health. At latest count, 1,026 species have been reported to be invading natural areas in the U.S. Their success is usually due at least in part to the fact that, lacking the natural enemies that kept them in check at home, they proliferate in menacing proportions. As world population, international travel and trade increase, fresh crops of exotic newcomers continue to take root in American soil as they have done for centuries.
“Climate change is also creating new areas for invasive plants to colonize,” adds biologist David Inouye, Ph.D., professor of biology at the University of Maryland. “Places that were too cold for some species are now okay. As growing seasons lengthen, warmer temperatures open up new areas where invasives can grow.”
In the Rocky Mountain alpine zone of Colorado, where Inouye has been investigating the growing patterns of plants for 36 years, the longer growing season and warmer temperatures have not been a boon to fragile native wildflowers like the green gentian (Frasera speciosa), tall larkspur (Delphinium barbeyi) and tall bluebell (Mertensia ciliata) that he studies. “The phenology has been changing,” he says. “The timing of snow-melt sets the clock. We’ve observed snow melting earlier, less snow over the winter, warmer springs, a typical growing season starting earlier – but at the same time, the date of last frost hasn’t changed much. With earlier growing seasons, there are more plants with buds developed by the time the last frost occurs. The obvious result: fewer flowers. “If they don’t make flowers, then there are no seeds and they’re not replacing themselves,” explains Inouye. This means even fewer food sources for pollinators, which can in turn mean fewer plants.
Warmer temperatures may encourage some plants to expand or change their range to higher altitudes. “But they are joining species that are already there, so there is more competition [for resources],” adds Inouye. Some of those species could be exotic invasives already established.
“An exotic doesn’t become invasive unless the climate is similar to its own climate,” Parmesan points out. “As the climate shifts, the native, especially if it’s an endemic, is stressed already. Exotics will take over if they find the climate suitable, where the native no longer does. If you stress the competing native, exotics will win.”
Just as it opens the door for invasives, climate change also can reinforce the destructive power of some troublesome exotics in areas where they are already established. One toxic alien, the common reed (Phragmites australis), appears to take advantage of one climatic factor that can increase with global warming.
Delaware Biotechnology Institute plant and soil scientist Harsh Bais discovered that invasive Phragmites secretes gallic acid to kill off neighboring plants and take over new turf. In a study published in 2009, Bais and his team reported that UV rays degrade the gallic acid to produce an additional toxin, mesoxalic acid. This second poison triggers what Bais has referred to as a “cellular death cascade” in its victims, destroying structural protein in the roots within minutes of exposure. The photo-degraded toxin is particularly potent in invasive forms of Phragmites australis. Among those it threatens are the endangered U.S. native Phragmites, as well as wild rice, cattails and wetland orchids, often turning diverse wetland communities into monocultures with little food or shelter for wildlife.
It’s important to note, however, that it is not known whether climate change always will exacerbate invasive plant problems. Some of the newest research uses climate-matching models to evaluate where invasives are now as well as where they might enter or retreat from in the future.
At University of California-Davis and the California Invasive Species Council, research scientists Elizabeth Brusati and Joe DiTomaso are using such modeling and have so far collected data on 36 of the 200 species known to be invasive in California.
They study invasives in their current location without climate change and, by looking at what the plants need to survive in their native range and climatic expectations for different areas, are able to predict how well an invasive plant will do in a particular part of the state. The team hopes that the models someday will allow county personnel throughout California to answer questions about the presence of invasive species using an online database.
This could be good news for at least some fragile native species but not likely to resolve what is clearly a huge global problem. The 2009 International Red List of Threatened Species claims that 70 percent of the world’s wild plants assessed so far are under threat (for all reasons). The North American environments that are home to the species most threatened include those at high altitude; small unusual habitats like the scrub of Lake Wales Ridge, Florida; and some small fens. In addition, endemic cactus species throughout the Southwest are severely threatened by a number of invasive grasses and the fires these grasses both encourage and survive.
“The drought in the West is so severe, it’s actually a state change in climate,” warns Parmesan. “We are anticipating mega-droughts, and invasive species may thrive in them. Throughout the Southwest, many cactus species are endemic. There’s a big worry about fire coming in because of the cheatgrass and other African annual grasses. With fire, cheatgrass just takes over. And these cactus species are not fire-adapted. They don’t recover.”
But that’s not all. Cheatgrass is also one example of an invasive species that can actually cause climate change. According to a report from the Department of the Interior, in cheatgrass-invaded areas of the Great Basin, the fire cycle has changed dramatically, from infrequent to frequent, radically altering ecosystem processes including carbon dynamics. This has helped cheatgrass change about 20 million acres from a net carbon sink to a net carbon source; frequent fires release carbon dioxide into the atmosphere, thus contributing to the increase in global warming. According to the same report, other invasives change water dynamics.
“The fate of a species lies in the net effect of all stressors combined,” says Parmesan. “In some cases it may be easiest to reduce the overall stress on a species by mitigating the non-climate stressors. For example, if both climate change and invasive species threaten a valued resource, it may be most cost-effective to focus attention on reducing the incursions of the invasive species.”
This suggests that, along with doing our part to reduce carbon dioxide in the atmosphere by restricting our use of fossil fuels, we can all make some contribution to protecting biodiversity by fighting invasives wherever and whenever we can. Our small efforts may be more potent than one might think. Consider this example: Pull and destroy a single stem of purple loosestrife and you potentially prevent up to 3 million tiny seeds – with an astonishing germination rate of about 90 percent – from taking root somewhere.
Bibi Wein is the author of The Way Home: A Wilderness Odyssey (Tupelo Press), an environmental memoir about the Adirondacks.