Not all species flee rising temperatures. As the mercury has inched upward across western North America over the last 40 years, many plant species have moved downhill, toward—not away from—warmer climates, according to the results of a new study. The finding adds to growing evidence that temperature isn’t the only factor influencing how Earth’s life will respond to climate change.
“This is a very cool study and demonstrates what many of us have been saying—that we will get surprises,” writes Camille Parmesan, a climate change biologist at Plymouth University in the United Kingdom, in an e-mail to Science. She was not involved with the study.
Like animals, plants require specific environmental conditions—such as the right temperature, moisture, and light levels—in order to thrive. Even small changes in environmental parameters can affect the reproduction and survival of a species. As global temperatures rise, both animal and plant populations are projected to gradually shift toward northern latitudes and upward to higher elevations where temperatures are cooler in order to stay within their ideal range of environmental conditions.
In an effort to understand how plants may cope with changing climates, researchers at the University of Washington, Seattle, compiled geographic coordinate data for the locations of nearly 300 plant species within seven topographically distinct regions across western North America, ranging from the western Sierra Nevada mountain range in Nevada to the eastern Rocky Mountain Foothills of northern Canada, spanning the last 40 years. They then compared these findings with changing climate conditions, such as temperature, rain, and snowfall. The study is the most extensive of its kind to date.
The results of the analysis were unexpected. More than 60% of plants shifted their distributions downward, toward warmer, lower elevations—despite significant climate warming across the regions under study, the team reported online on 24 July in Global Change Biology. Even more striking, all plants within a region—regardless of species—moved in the same direction.
“Initially, we thought there was something wrong with our analysis—species distributions are expected to shift upward, not downward,” says team leader and plant ecologist Melanie Harsch. “But we redid the analysis and we got the same results.”
A closer look revealed that the downhill movement of plants was likely driven by the changes in precipitation that accompanied warming temperatures. Those regions that experienced less rain and snow at high elevations were those with plants shifting toward lower elevations with wetter climates. “Less snow in winter translates into less water in summer, resulting in water-stressed plants and downward shifts,” Harsch says.
Although plant populations are shifting downward toward greater water availability, they will also have to contend with an increasingly warming climate. “It’s a double-edged sword,” Harsch states, “as temperatures rise, water needs will also increase.”
Although previous, smaller studies have also noted downhill movements in relation to water availability, others report uphill movements in relation to temperature, suggesting the direction of species movements is dependent on local environmental conditions as well as the types of species present. “These studies highlight the importance of understanding the complexities not only of future climate change but the climatological requirements of individual species,” says Anne Kelly, a plant ecologist at the Catalina Island Conservancy in Avalon, California, who was not involved in the work.
Future climate changes are projected to intensify precipitation patterns in western North America, leading to more pronounced shifts in plant distributions and potential subsequent effects on the wildlife that depend on them for food and habitat. “How we decide where to allocate limited resources such as money and manpower to conserve species in the face of long-term global warming is a primary concern right now,” Harsch notes. “We can’t monitor all species everywhere, but, by identifying the factors responsible for environmental changes, we can begin to predict effects and prioritize conservation management choices.”