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 a 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.
The study was funded by a Packard Fellowship in Science and Engineering and by a National Science Foundation grant to Hu. Carbon dating was done under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in California.
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.