The highest-elevation site, a mixed conifer meadow. Credit: Michael Allwright

Global warming may initially make the grass greener, but not for long, according to new research results.

The
findings, published this week in the journal Nature Climate Change,
show that plants may thrive in the early stages of a warming environment
but then begin to deteriorate quickly.

“We
were really surprised by the pattern, where the initial boost in growth
just went away,” said scientist Zhuoting Wu of Northern Arizona
University (NAU), a lead author of the study. “As ecosystems adjusted,
the responses changed.”

Ecologists subjected four grassland ecosystems to simulated climate change during a 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
research shows the long-term effects of global warming on plant growth,
on the plant species that make up a community, and on changes in how
plants use or retain essential resources like nitrogen.

“The
plants and animals around us repeatedly serve up surprises,” said Saran
Twombly, program director in the National Science Foundation (NSF)’s
Division of Environmental Biology, which funded the research.

“These
results show that we miss these surprises because we don’t study
natural communities over the right time scales. For plant communities in
Arizona, it took researchers 10 years to find that responses of native
plant communities to warmer temperatures were the opposite of those
predicted.”

The
team transplanted four grassland ecosystems from a higher to lower
elevation to simulate a future warmer environment, and coupled the
warming with the range of predicted changes in precipitation–more, the
same, or less.

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.

The
researchers found that long-term warming resulted in loss of native
species and encroachment of species typical of warmer environments,
ultimately pushing the plant community toward less productive species.

The
warmed grasslands also cycled nitrogen more rapidly. This should make
more nitrogen available to plants, scientists believed, helping plants
grow more. But instead much of the nitrogen was lost, converted to
nitrogen gases in the atmosphere or leached out by rainfall washing
through the soil.

Bruce
Hungate, senior author of the paper and an ecologist at NAU, said the
study challenges the expectation that warming will increase nitrogen
availability and cause a sustained increase in plant productivity.

“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 can’t figure this out with plants
grown in a greenhouse.”

The
findings caution against extrapolating from short-term results, or from
experiments with plants grown under artificial conditions, where
researchers can’t measure the feedbacks from changes in the plant
community and from nutrient cycles.

“The long-term perspective is key,” said Hungate. “We were surprised, and I’m guessing there are more such surprises in store.”

Co-authors of the paper include George Koch and Paul Dijkstra, both at NAU.

Source: National Science Foundation