Climate Extremes Impacting Plants, Crops

Oak Ridge National Laboratory – US Department of Energy
News Release
March 5, 2008

Killer freeze of ’07 illustrates paradoxes of warming climate

OAK RIDGE, Tenn., March 5, 2008 – A destructive
spring freeze that chilled the eastern United
States almost a year ago illustrates the threat a
warming climate poses to plants and crops,
according to a paper just published in the
journal BioScience. The study was led by a team
from the Department of Energy’s Oak Ridge
National Laboratory.

The “Easter freeze” of April 5-9, 2007, blew in
on an ill wind. Plants had been sending out young
and tender sprouts two to three weeks earlier
than normal during an unusually warm March. Plant
ecologists, as well as farmers and gardeners,
took note of the particularly harsh turn of the
weather in early April.

“The warm weather was as much a culprit for the
damage as the cold,” said lead author Lianhong Gu
of ORNL’s Environmental Sciences Division.

“We see the paradox in that mild winters and
warm, early springs make the plants particularly
vulnerable to late-season frosts,” Gu said. Gu’s
team observed satellite images and field data to
establish the extent of the 2007 spring freeze.
They also assessed the long-term and short-term
effects on the terrestrial carbon cycle with
respect to plant activity in normal years.
Short-term effects were “profound,” Gu said.

“In the period just after the freeze we saw a
large reduction in the fraction of absorbed
photosynthetically active radiation, which is a
sensitive indicator of plant growth,” he said.
“We also noted that the regrowth in the following
weeks and months did not result in the levels of
plant development in previous years.”

Gu’s team hypothesized that the freeze could have
long-term effects on forest carbon uptake because
of damage the cold did to the prematurely
developed plant tissues, which could affect
future growing seasons. The associated carbon and
nutrient losses may affect growth in future years.

Beyond devastated horticultural crops, the study
noted that some species suffered more than
others. Yellow poplar trees were “surprisingly”
slow to put out flushes of new leaves, and white
oaks were particularly hard hit with freeze
damage to both new leaves and flowers. On the
other hand, trees located along shorelines–where
the water stores heat–and underneath dense
canopies seemed to be protected from the cold.

Compounding the stress on plants, as noted in the
International Panel on Climate Change’s fourth
assessment report, is the prospect of prolonged
droughts, which also occurred in the region last
summer.

“This freeze should not be viewed as an isolated
event; rather, it represents a realistic climate
change scenario that has long concerned plant
ecologists,” Gu said.

Authors on the paper include Gu, Paul Hanson,
Dale Kaiser, Mac Post and Bai Yang of ORNL;
Ramakrishna Nemani of NASA’s Ames Research
Center; Stephen G. Pallardy of the University of
Missouri at Columbia, and Tilden Meyers of the
National Oceanic and Atmospheric Administration’s
Air Resources Laboratory in Oak Ridge. The DOE
Office of Science’s Biological and Environmental
Research program funded the studies.

ORNL is managed by UT-Battelle for the Department of Energy.
Oak Ridge National Laboratory
P.O. Box 2008, Oak Ridge, TN 37831
U.S. Department of Energy

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