When humans wander the planet, they carry their plants along, often inadvertently. For example, Plantago major earned the common name ‘white man’s footprint,’ because it hitchhiked to the U.S. with European settlers and began growing along trails and roads. It is a very common species in the Southeast and has naturalized all over the globe.
“Wherever you find a lot of humans, you typically find a lot of non-native species,” says USDA Forest Service research ecologist Qinfeng Guo.
People tend to live, travel, work, and farm at lower elevations. One implication of this is that remote mountaintops have fewer non-native species, as Guo and his colleagues confirmed. Guo recently led a study on diversity and distribution patterns of native and non-native plants at different elevations. The study was published in the Journal of Biogeography.
Relative to the studies on native species, fewer have considered diversity and distribution patterns of non-native plants at different elevations.
Guo and his colleagues identified 65 research studies about non-native plant diversity along elevational gradients. The studies were conducted on mountains all over the globe, and their designs typically feature transects that run up mountains. Plots are located along those elevational gradients, and plants in each plot are documented.
The 65 case studies also included 32 mountain ranges where both native and non-native plants were sampled.
Their analysis shows that, in most cases, native plants peak in richness at mid-elevations. However, non-native plants are mostly concentrated at lower elevations.
“Our observations constitute a snapshot of ongoing, long-term invasion processes,” says Guo. The patterns among non-native plants will change as they continue to spread and new nonnative species are being introduced.
Middle elevations usually get more rain than lower elevations, making them prime plant habitat. “That’s likely one of the major reasons why native plant richness is highest at middle elevations,” says Guo.
High elevations are harsh environments – cold, with short growing seasons. Many plants, whether native or not, simply cannot survive there. Native plant diversity tapers off at high elevations, as the study shows. But plants that are adapted to high elevations are often found nowhere else.
“Mountains are hotspots of biodiversity,” says Guo. Many plants on mountaintops are rare, endemic, or endangered. Appalachian avens (Geum radiatum) is all three – it has been listed under the Endangered Species Act since 1990 and only grows in 11 sites in the mountains along the Tennessee-North Carolina border.
Appalachian avens is also only found at elevations higher than 4,367 feet (1,310 meters).
As the climate warms, native plants at the highest elevations will have nowhere to migrate to – they are already at the mountain top. Meanwhile, non-native plants press up from lower elevations.
“We have clear evidence that plant species are moving higher, to mountaintops,” says Guo. “There’s no question, non-native species will continue to increase and invade higher elevations.”
For more information, email Qinfeng Guo at email@example.com.