The towering saguaro cactus may be the icon of the American Southwest, but an invasive plant is steadily encroaching into desert ecosystems. The interloper, a knee-high species of grass known as buffelgrass, will likely become even more of a presence in arid landscapes in the future, new research has revealed. That’s because buffelgrass weathers increased temperatures and drought conditions—two hallmarks of climate change—more readily than its native brethren. According to the researchers, arid environments are slated to experience pronounced changes in vegetation in the coming decades, a shift that will have far-reaching implications not only for desert ecosystems themselves but also for human-built infrastructures.
Guaranteed from the Start
Buffelgrass (Pennisetum ciliare) was first introduced to North America from Africa in the 1930s. The tough grass was originally intended as food for foraging cattle. Like other plants such as kudzu that have thrived in their nonnative environments, buffelgrass’s biological success was just about guaranteed from the start: Its seedlings survive at high rates, it can rapidly colonize bare soil, it makes efficient use of water, and it’s capable of tolerating extreme drought.
Today buffelgrass is a common sight in the vast Sonoran Desert, which spans the southwestern United States and northwestern Mexico. But it’s an unwelcome guest—buffelgrass has been labeled a “noxious weed” by the Arizona Department of Agriculture, and the National Park Service regularly hosts “buffelgrass pulls.”
“It invades deserts and crowds out native plants,” said Perry Grissom, a restoration ecologist at Saguaro National Park in Tucson who was not involved in the research and who has led many buffelgrass pulls. “It’s better adapted to our desert than our plants that are endemic.”
Biodiversity to Monoculture
Buffelgrass’s bad reputation is well earned, said Sujith Ravi, an environmental scientist at Temple University in Philadelphia and lead author of the study. It slashes ecosystem biodiversity by outcompeting native grasses, leading to landscapes that are veritable monocultures, he said. “Whereas there used to be a mixture of different communities, now it’s more of a single-community landscape.”
That’s bad news, because biodiversity has been shown to make ecosystems more stable and resilient to potentially adverse changes. And when an inevitable “crash” occurs—when essentially all vegetation dies off for a period of time—the soil that’s exposed is readily eroded by wind and water. “There’s an irreversible loss of resources from the system,” explained Ravi. Furthermore, when buffelgrass thrives, the thick vegetation facilitates the spread of fire in an otherwise patchy landscape, and larger fires are more likely to affect human-built infrastructure.
With climate models predicting increasing temperatures and more frequent droughts in arid landscapes, an open question is how well buffelgrass will fare in the future compared with native plants. Several years ago, Ravi and his colleagues began an experimental investigation of buffelgrass and its native counterpart, tanglehead (Heteropogon contortus), in the glass-walled Biosphere 2 research facility in southern Arizona.
A Harbinger of the Future
Biosphere 2 is an ideal laboratory for studying the effects of climate change because it can be tuned to create different environmental conditions. The facility, which tops 3 acres, reproduces several of the planet’s major biomes—including the ocean, wetlands, rain forest, savannah, and desert. “It’s like a field experiment because it’s so huge,” said Ravi.
The team grew hundreds of buffelgrass and tanglehead plants and divided them between Biosphere 2’s savannah biome, maintained at ambient conditions, and its desert biome, which is warmed by roughly 5°C. The idea was to repeat the experiments in two conditions to mimic the effects of climate change, said Ravi.
After watering the plants regularly for a few months, the researchers then withheld irrigation from half of the plants for several months, effectively exposing them to drought-like conditions. The water-starved grasses responded as they would in nature: They went dormant. The team accordingly irrigated the plants again the following spring before finally quantifying what fraction of grasses of each species, exposed to each set of temperature and moisture conditions, survived.
Ravi and his colleagues found that grasses of both species rallied after experiencing drought-like conditions at ambient temperatures. But the combination of warmer temperatures and lack of moisture killed 100% of the native tanglehead plants compared with only roughly 80% of the invasive buffelgrass plants. That’s a significant difference in mortality, said Ravi. “If something is going to come back, it’s going to be the invasive grass.”
This finding wasn’t wholly unexpected given the nature of buffelgrass, said Grissom. “After seeing how it behaves, I’m not surprised. It’s really tough.”
These results are a harbinger of what’s to come in arid regions, the researchers suggested. Drought- and heat-adapted invasive plants like buffelgrass will increasingly gain a toehold, at the expense of native species. Climate change and biological invasions work in tandem to alter desert landscapes for the worse, said Ravi. “They can synergistically act to drive landscapes into degradation.”
—Katherine Kornei (@KatherineKornei), Science Writer