warming may initially make the grass greener, but not for long, according to
new research conducted at Northern Arizona University.
study, published this week in Nature Climate Change, shows that
plants may thrive in the early stages of a warming environment but begin to
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.”
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.
research reports the long-term effects of global warming on plant growth, the
plant species that make up the community, and the changes in how plants use or
retain essential resources like nitrogen. The team transplanted four grassland
ecosystems from 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.
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, an effect that should make more nitrogen available to
plants, helping them grow more. But instead much of the nitrogen was converted
to nitrogen gases lost to the atmosphere or leached out with rainfall washing
through the soil.
Hungate, senior author of the study and NAU
biological sciences professor, said the research findings challenge the
expectation that warming will increase nitrogen availability and cause a
sustained increase in plant productivity.
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.”
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
long-term perspective is key. We were surprised, and I’m guessing there are
more surprises in store.”
coauthors include George Koch, NAU professor of biological
sciences, and Paul Dijkstra, assistant research professor of
biological sciences. Wu completed the study as part of her doctoral thesis in
biology and earned her degree in 2011.