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Sediment samples suggest how plants would fare in hotter, drier future
Jim Barlow,
Life Sciences Editor
217-333-5802; jebarlow@illinois.edu
1/13/04
CHAMPAIGN, Ill. —
Sediment samples dating back thousands of years and taken from under
the deep water of West Olaf Lake in Minnesota have revealed an unexpected
climate indicator that can be factored into future projections.
In the Jan. 13 issue of the Proceedings of the National Academy of Sciences,
scientists at the University of Illinois at Urbana-Champaign report
that native C4 plants did not fare well during prolonged periods of
severe drought that occurred in the middle Holocene (4,000 to 8,000
years ago).
C4 plants, so designated because of their biochemical pathway of photosynthesis,
are generally expected to do well in warmer, drier climates driven by
rising levels of carbon dioxide. Elevated carbon dioxide concentrations
alone should favor C3 plants, which use another photosynthesis pathway.
While the middle Holocene had much lower levels of carbon dioxide, the
general climate conditions of that time provide a good model for study,
said Feng Sheng Hu, a professor in the plant
biology and geology departments
at Illinois.
The sediment from West Olaf Lake, which contains residue of plant life,
indicates that weedy C3 plants such as Ambrosia (ragweed) adapted well
during severe-drought episodes because of their ability to exploit very
limited amounts of water available during the growing season, said David
M. Nelson, lead author of the paper and a doctoral student in ecology
and evolutionary biology working with Hu.
The findings suggest that even C4 plants could face disastrous consequences
during long periods of drought, despite the fact that they use water
more efficiently than C3 plants, Nelson said. Barren areas unsuitable
for agriculture may be much more extensive in the Midwest under warmer,
drier conditions predicted for the future, he said.
“Previous studies of past grassland change have been hampered
by the fact that pollen grains of grasses cannot be separated into species,
making it difficult to understand climate adaptations of C3 and C4 plants
during the middle Holocene,” Hu said. “This study offers
new details about grassland responses to long periods of severe drought.”
The researchers analyzed and compared sediment from West Olaf Lake with
samples from Steel Lake, about 75 miles northeast in Hubbard County.
Today West Olaf Lake is along the border of the Great Plains and the
more hilly deciduous forest of west central Minnesota. Steel Lake is
in more geographically diverse terrain that features a dense coniferous
forest that was less susceptible to long-term drought.
The middle Holocene C3 and C4 estimates of the two lakes were based
on an analysis of carbon isotopes in charcoal particles produced by
fires and well preserved in the stratified layers of sediment. Because
of the presence of aragonite, a carbonate mineral, at West Olaf Lake,
climate data were extracted by using X-ray diffraction. Climate conditions
at Steel Lake came from oxygen-18 isotope levels.
“These analyses gave a picture of precipitation and aridity over
time,” Nelson said. “At West Olaf Lake, during the most
severe, long droughts in the early years of the middle Holocene, C4
plants were low in abundance. Only as temperatures cooled and moisture
availability rose later in the middle Holocene did C4 plants increase
in abundance.”
The West Olaf Lake area was rich in weeds such as Ambrosia during the
Holocene’s drier middle years. During the period’s early
years, severe droughts limited plant productivity, reducing the accumulation
of flammable fuels. During the milder, wetter later years of the period,
rising C4 plant productivity coincided with an increase of fires.
At Steel Lake, C4 plants were abundant in the middle Holocene. Researchers
did not see the inverse relationship between C4 plants and drought,
which were not as severe.
Other contributing authors on the paper were Jian Tian, a doctoral student
in geology at Illinois, Ivanka Stefanova of the University of Minnesota
and Thomas A. Brown of the Lawrence Livermore National Laboratory in
California.
