10-year research project show short-term warming boost, with subsequent degradation of ecosystem
By Bob Berwyn
SUMMIT COUNTY — Global warming can boost short-term plant growth, but over time, warming temperatures disrupt the nitrogen cycle and quickly lead to the deterioration of dry grassland ecosystems, according to a 10-year study done by Northern Arizona University scientists.
The researchers subjected four grassland ecosystems to simulated climate change during the decade-long study. Plants grew more the first year in the global warming treatment, but this effect progressively diminished over the next nine years, and finally disappeared.
The grasslands studied were typical of those found in northern Arizona along elevation gradients from the San Francisco Peaks down to the great basin desert.
“We were really surprised by the pattern, where the initial boost in growth just went away,” said Zhuoting Wu, NAU doctoral graduate in biology. “As the ecosystems adjust, the responses changed.”
To carry out the long-term project the researchers simply picked up sections of grasslands and moved them downhill to warmer elevations to simulate global warming impacts.
“One thing that changes with elevation is rainfall,” said NAU Biological Sciences professor Bruce Hungate, lead author of the paper. Hungate explained that the study team used interceptors and collectors to adjust for the differences in moisture to prevent the results from being skewed.
The researchers found that long-term warming resulted in loss of native species and encroachment of species typical of warmer environments, pushing the plant community toward less productive species.
The warmed grasslands also cycled nitrogen more rapidly, Hungate said.
“A lot of people expected that to be good for plants,” he said, adding that there’s at least one study from the Northeast showing beneficial effects to plants from a faster nitrogen cycle. “But we’re seeing really different responses in semi-arid western ecosystems.
Instead of making the nitrogen more available for plants, it was lost, converted to nitrogen gases that dissipated to the atmosphere or leached out with rainfall washing through the soil.
Long-term warming resulted in a loss of native species and encroachment of species typical of warmer environments, pushing the plant community toward less productive species.
“Faster nitrogen turnover stimulated nitrogen losses, likely reducing the effect of warming on plant growth,” Hungate said. “More generally, changes in species, changes in element cycles—these really make a difference. It’s classic systems ecology: the initial responses elicit knock-on effects which here came back to bite the plants. These ecosystem feedbacks are critical. You just can’t figure this out with plants grown in a greenhouse. ”
The findings caution against extrapolating from short-term experiments, or experiments in a greenhouse, where experimenters cannot measure the feedbacks from changes in the plant community and from nutrient cycles. The research will continue at least five more years with current funding from the National Science Foundation and, Hungate said, hopefully for another five years after that.
“The long-term perspective is key. We were surprised, and I’m guessing there are more surprises in store.”
Additional coauthors include George Koch, NAU professor of Biological Sciences, and Paul Dijkstra, assistant research professor of Biological Sciences. The study is being published this week in Nature Climate Change.