Invasive plants and climate change will alter desert landscapes
The towering saguaro cactus may be the icon of the American Southwest, but an invasive plant regularly encroaches on desert ecosystems. The intruder, a species of knee-high grass known as bufflgrass, is likely to become even more prevalent in arid landscapes in the future, new research has found. This is because the buffalo withstands high temperatures and drought conditions – two hallmarks of climate change – more easily than its native brethren. According to the researchers, arid environments are expected to experience pronounced changes in vegetation over the next few decades, a shift that will have profound implications not only for desert ecosystems themselves, but also for human-built infrastructure.
Guaranteed from the start
Buffalograss (Pennisetum ciliare) was first introduced to North America from Africa in the 1930s. The tough grass was originally intended as livestock feed. Like other plants such as kudzu that have thrived in their non-native environments, wheatgrass’ biological success was pretty much guaranteed from the start: its seedlings survive at high rates, it can quickly colonize bare soil, it uses water efficiently and is able to tolerate extreme drought.
Today, buffalo weed is a common sight in the vast Sonoran Desert, which spans the southwestern United States and northwestern Mexico. But he’s an unwelcome guest – buffalo grass has been labeled a “noxious weed” by the Arizona Department of Agriculture, and the National Park Service regularly holds “buffalo grass raffles.” .
“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 bush prints. “It is better adapted to our desert than our endemic plants.”
From biodiversity to monoculture
Buffelgrass’ bad reputation is well deserved, said Sujith Ravi, an environmental scientist at Temple University in Philadelphia and lead author of the study. It reduces the biodiversity of ecosystems by crowding out native grasses, leading to landscapes that are veritable monocultures, he said. “While there used to be a mix of different communities, now it’s more of a single-community landscape.”
This is bad news, as biodiversity has been shown to make ecosystems more stable and resilient to potentially harmful changes. And when an inevitable “crash” occurs – when virtually all vegetation dies for a time – the soil that is exposed is easily eroded by wind and water. “There is an irreversible loss of system resources,” Ravi explained. Additionally, when buffalo grass thrives, the thick vegetation makes it easier to spread fire across an otherwise uneven landscape, and larger fires are more likely to affect human-built infrastructure.
With climate models predicting rising temperatures and more frequent droughts in arid landscapes, an open question is how well wheatgrass will fare in the future compared to native plants. Several years ago, Ravi and his colleagues began an experimental investigation of Buffalo grass and its native counterpart, tanglehead (Heteropogon contortus), in the glass-walled Biosphere 2 research center in southern Arizona.
A harbinger of the future
Biosphere 2 is an ideal laboratory to study the effects of climate change as it can be tuned to create different environmental conditions. The facility, which spans 3 acres, replicates several of the planet’s major biomes, including ocean, wetlands, rainforest, savanna and desert. “It’s like an on-pitch experience because it’s so huge,” Ravi said.
The team grew hundreds of bush and tanglehead grasses and split them between Biosphere 2’s savannah biome, maintained at ambient conditions, and its desert biome, which is warmed by around 5°C. . The idea was to repeat the experiments under two conditions to mimic the effects of climate change, Ravi said.
After watering the plants regularly for a few months, the researchers then suspended irrigation of half the plants for several months, exposing them to near-drought conditions. The water-hungry grasses reacted as they would in nature: they went dormant. Accordingly, the team irrigated the plants again the following spring before finally quantifying the fraction of grasses of each species, exposed to each set of temperature and humidity conditions, that survived.
Ravi and his colleagues found that grasses of both species recovered after experiencing drought conditions at ambient temperatures. But the combination of warmer temperatures and a lack of humidity killed 100% of the native tangled head plants, compared to only about 80% of the invasive bush grass plants. That’s a significant difference in mortality, Ravi said. “If anything is going to come back, it will be the overgrown grass.”
This finding wasn’t entirely unexpected given the nature of bush grass, Grissom said. “After seeing how he behaves, I am not surprised. It’s really difficult.
These findings are a harbinger of what is to come in arid regions, the researchers suggested. Invasive plants adapted to drought and heat, such as grassroots, will increasingly take over, at the expense of native species. Climate change and biological invasions work in tandem to worsen desert landscapes, Ravi said. “They can act synergistically to drive landscape degradation.”
—Katherine Kornei (@KatherineKornei), science writer