Researchers with the Boston-Area Climate Experiment are using techniques in their plant-warming experiments designed to better reflect natural daily and seasonal temperature cycles. This experiment is too new to have been in included in this study, but its design addresses some of the concerns raised by the study. Credit: Jeff Dukes, Purdue University

Plants
are leafing out and flowering sooner each year than predicted by
results from controlled environmental warming experiments, according to
data from a major new archive of historical observations assembled with
the help of a NASA researcher.

Researchers
use experiments that manipulate the temperature of the environment
surrounding small plots of plants to gauge how specific plants will
react to higher temperatures. The observed plant responses can then be
incorporated into models that predict future ecosystem changes as
temperatures around the globe continue to rise. But when a group of
scientists compared these results to a massive new archive of historical
observations, they found that the warming experiments are dramatically
underestimating how plants respond to climate change.

The
results were published online in the journal Nature on May 2. In
addition to quantifying how a broad collection of plant species have
responded to date to rising temperatures, the study suggests that the
way warming experiments are conducted needs to be re-evaluated.

“This
suggests that predicted ecosystem changes—including continuing advances
in the start of spring across much of the globe—may be far greater than
current estimates based on data from warming experiments,” said
Elizabeth Wolkovich, who led the interdisciplinary team of scientists
behind the new research while she was a postdoctoral fellow at the
University of California, San Diego. “The long-term records show that
phenology is changing much faster than estimated based on the results of
the warming experiments. This suggests we need to reassess how we
design and use results from these experiments.”

Benjamin
Cook, of NASA’s Goddard Institute for Space Studies and Columbia
University, New York, worked with Wolkovich to create the massive new
archive of long-term, natural phenology observations to gauge the
accuracy of the phenological predictions based on these plant warming
experiments. The archive includes data from 1,558 species of wild plants
on four continents. The historical records showed that leafing and
flowering will advance, on average, five to six days per degree
Celsius—a finding that was consistent across species and datasets. These
data show that estimates based on data from warming experiments are
underpredicting advances in flowering by eight and a half times and
advances in leafing by four times. The authors expect the data archive
to be an important benchmark in future phenology studies.

“These
results are important because we rely heavily on warming experiments to
predict what will happen to ecosystems in the future,” said Cook, who
helped bring together a research team including support from the
National Center on Ecological Analysis and Synthesis to build the
archive of real-world observations. “With these long-term observational
records you may be able to pick up a shift in a plant community over a
few years that you wouldn’t be able to observe in an experiment.”

Cherry blossoms in Washington, DC have been blooming earlier in recent decades, including a very early bloom this spring in March (pictured). Credit: Elizabeth Wolkovich, Biodiversity Research Centre, University of British Columbia-Vancouver

The
study of phenology, the timing of annual plant events such as the first
flowering and leafing out of spring, provides one of the most
consistent and visible responses to climate change. Long-term historical
records, some stretching back decades and even centuries, show many
species are now flowering and leafing out earlier, in step with rising
temperatures. Because these records aren’t available everywhere and
predicted future warming is often outside the range of historical
records, ecologists often use controlled experiments that create warmer
conditions in small plots to estimate how different species will respond
to expected temperature increases.

The
timing of plants’ flowering and leafing out in spring is not only a
basic, natural indicator of the state of the climate. Predicting plant
responses to climate change has important consequences for human water
supply, pollination of crops and overall ecosystem health.

Wolkovich,
Cook and colleagues suggest a number of potential reasons the estimates
based on experimental data have underpredicted the plant response to
higher temperatures. There could be additional effects of climate change
not mirrored in the controlled experiments, or from the fact that the
methods used to create warmth in the studies could be creating
counteracting effects such as drying out soils or reducing the amount of
sunlight reaching the plants.

“Continuing
efforts to improve the design of warming experiments while maintaining
and extending long-term historical monitoring will be critical to
pinpointing the reasons for the differences, and will yield a more
accurate picture of future plant communities and ecosystems with
continuing climate change,” Wolkovich said.

Source: NASA