It all comes down to the roots.
Woody plants, such as sagebrush and juniper, have roots that dive deep into the soil and can grow to where the water is. But the roots of most grasses are stuck near the surface.
As climate change alters the way rain falls in Utah — stronger storms that happen less often — the more adaptable woody plants could one day overtake grasses.
New research from Utah State University measured exactly how much water different root systems can absorb and how that might transform landscape ecosystems in a changing climate.
“All over the world, we've seen woody plants have become more abundant in the past 50 years,” said Andrew Kulmatiski, an associate professor with the USU Ecology Center and Department of Wildland Resources. “It's definitely a problem that's happening, and my research is providing an explanation for why it's happening.”
For the study, Kulmatiski and his team injected isotope tracers at various soil depths. As the plants grew, they could tell exactly how deep each plant was drawing its moisture from.
The team also used sprinklers to simulate different precipitation scenarios — from historical rain averages to potential future climates — at roughly 200 plots across the Intermountain West.
“If we know where plants are getting their water from and how they use their roots, we can use that to predict which species will do well and which species will fail in future climates.”
The study found that the simulations that mimicked larger, less frequent rain events increased the growth of pretty much everything but grass. That’s contrary to long-held ideas about root performance.
Historically, Kulmatiski said scientists have thought that grasses had an advantage because their roots are closer to the surface — and closer to where the precipitation is coming from.
“It's actually the opposite. … It kind of flips what we've always thought about grasses and woody plants on its head.”
They found that even though grass roots have the first crack at the water, the deeper roots of woody plants actually absorb more of it.
One reason is that grass’ shallow roots have to fight evaporation. Especially in arid places like southern Utah, a big chunk of precipitation disappears into the atmosphere before it seeps into the soil.
“Having your roots really shallow means you have to compete with air,” he said. “And air is always going to win.”
Heavier rain events push water deeper into the soil. And grass can’t absorb all the water as quickly as it’s passing by. So more of it gets down deep where the woody plant roots are.
While that may be good news for shrubs, it poses some grim ramifications for Utah’s plant landscape. For instance, a future with more shrubs and less grass could make it harder for ranchers who rely on pasture land for their grazing cattle.
Deeda Seed, a senior campaigner for the Center for Biological Diversity in Salt Lake City, said this shift in the ecosystem should be viewed as an alarm bell pushing society to reduce the carbon emissions driving climate change.
“This is an example of what happens when you don't,” Seed said. “You've got these extreme weather events, and then you've got complete disruption of ecosystems that have been in place for millennia.”
The good news, she said, is that the technology for renewable energy has caught up and made it possible to address climate change in ways that she could only imagine 20 years ago. And individuals can make small changes to boost local ecosystems where they live by planting more native species around their homes.
But without a significant transition away from fossil fuels, she said, this type of ecological shift could not only imperil the grasses but also all the bugs, birds and animals that depend on grass for food and habitat.
“What happens when you completely disrupt and change an ecosystem in this way? … We don't know the answer to that question.”
