Although all ecosystems are affected by a changing climate, the impacts can take a while to appear. Changes in forest biodiversity, for example, are known to lag behind changes in a habitat's temperature and precipitation.
Grasslands, on the other hand, are responding to climate change almost in real time, according to a new study published in the journal Nature Ecology & Evolution. Put another way, forests accumulate climate debt, while grasslands are paying as they go, say the study's authors.
To demonstrate this concept, a team of researchers from more than a dozen institutions amassed data from years of observations across grassland communities located in what's known as the California Floristic Province. Within this biodiversity hotspot that stretches along the U.S. West Coast, the team documented trends for 12 sites observed over decades.
The researchers found that, as the climate in the region became hotter and drier, species that preferred those kinds of conditions became more dominant in plant communities. The team also included results from long-term global change experiments in the region, which demonstrated that climate change is capable of driving the types of changes observed.
The study’s lead authors, Associate professor Kai Zhu and postdoctoral fellow Yiluan Song of the University of Michigan, began the project several years ago, while Zhu was a faculty member at UC Santa Cruz and Song was a doctoral student. UCSC professors Karen Holl and Michael Loik and several UCSC doctoral alumni are all coauthors on the new paper, which built upon several prior research efforts in the Environmental Studies Department.
The study incorporates data from long-term California grassland datasets collected by Holl’s lab at several locations near Santa Cruz. Established in 1999 with help from doctoral alumnus Grey Hays, these sites were originally intended to study the effects of mowing and cattle grazing on coastal prairies. But subsequent research by doctoral alumna Josephine Lesage demonstrated that data from the control plots could be combined with other long-term grassland datasets to study whether species composition was shifting toward species usually found in warmer and drier regions.
The new paper expanded upon this basic premise by incorporating more long-term datasets and conducting many additional analyses.
“This paper shows the importance of long-term datasets for documenting and understanding the change around us and the power of combining multiple datasets across a region to assess whether trends are consistent,” Holl said.
Another important element of the new research was that it brought in data from experimental sites. Those sites included several on the UC Santa Cruz campus managed by Loik as part of the International Drought Experiment. Rainfall manipulation structures had been installed at Younger Lagoon Reserve on the Coastal Science Campus, near the UCSC Arboretum and Botanical Gardens, and on UCSC Natural Reserve lands on upper campus to experimentally reduce the amount of naturally occurring precipitation available to ecosystems for at least a full growing season.
Doctoral alumnus Justin Luong’s measurements of resulting changes to plant community composition at these UC Santa Cruz experimental sites were used within the new paper to help demonstrate that experimentally-imposed drought leads to changes in the plant community that are remarkably similar to what researchers saw in long-term observational data from the region.
“I found the consistency between the observational and experimental data to be quite striking,” said Loik. “This paper is a great demonstration of the power of combining evidence from multiple sources.”
The new paper characterized the climate preferences or niches for various species in the region, and researchers could then quantify shifts in plant communities in direct relation to temperature and precipitation changes.
"We know correlation doesn't imply causation, but the experimental data allow us to attribute the causality,” Zhu explained.
This approach yielded a clear, consistent conclusion across the studied observational and experimental sites, which Zhu and Song said is uncommon for an ecological study like this.
But what stood out even more was the pace of the ecological change, they said. It was fast and comparable to the observed rate of changes in climate. And the researchers stressed that this rapid shift in plant communities should not be seen as adaptation—at least not without further studies.
"To me, adaptation gives a positive impression that the system is changing to counter some of the negative effects of climate change," Song said. "The rapid shifts in grassland communities involve not only the gain of some hotter, drier species but also the loss of some cooler, wetter species. These shifts might have negative consequences such as [increased] dominance by non-native species and loss of biodiversity."
The research team hopes their work will help the scientific community better understand and predict the impacts of climate change and provide key insights for the restoration of grassland vegetation.
"If you want to restore grasslands, you have to ask what types of species you will plant," Zhu said. "In order to answer that, you need to at least take climate change into consideration."
Although their study focused on a single region, Zhu and Song believe the results will hold in other grasslands, provided they're interpreted in the context of a given region's climate dynamics. For example, if the climate is trending warmer and wetter, species more at home in those conditions will likely start taking over at a speed that matches the changing climate.
"I would hypothesize that we may see an even greater response to climate change in other grasslands around the world," Zhu said.
Researchers from California Polytechnic State University, Clark College, the East Bay Regional Park District, the University of Oregon, the University of Washington, the University of Western Australia and Stanford University also contributed to the study. The team also included members from several University of California institutions, including UC Berkeley, UC Davis, UC Riverside, and UC Santa Barbara.
