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Will a warmer climate make plants more prone to spring freezes?

glacier graphic
(Photo by Carlye Calvin.)

An analysis of an April 2007 freeze that devastated crops and forests across the mid-South warns of the risk of more such events as climate change unfolds and the potential variability of springtime temperatures goes up.

According to the authors of the study, published in the March 2008 issue of BioScience, “Plant ecologists have long been concerned with a seemingly paradoxical scenario in the relationship between plant growth and climate change: warming may actually increase the risk of plant frost damage.” The study was led by Lianhong Gu (Oak Ridge National Laboratory).

Temperatures dipped well below freezing from Missouri to the mid-Atlantic during the period of 5–9 April after a long stretch of unusually warm weather. At the Missouri Ozark AmeriFlux site, nighttime readings stayed above 15°C (59°F) for a full week before the freeze, then dipped as low as –7°C (19°F). The early warmth pushed plant growth two weeks ahead of schedule in many areas, intensifying the effects of the subsequent freeze.

More than $100 million in damage was reported in North Carolina alone, and the effects on many crops and trees persisted through the summer—a sharp contrast to 1996, when a consistently cold spring delayed greening but had less impact on the year’s total plant growth.

Unusual damage patterns resulted from the asymmetric nature of the 2007 freeze. For example, the tops of forest canopies were denuded, stimulating growth closer to ground level, and trees near lakes and rivers were partially protected by the relative warmth emanating from those waters. The authors noted the role of the freeze’s damage in reducing regional carbon uptake: “All signs have so far indicated that the 2007 spring freeze had at least a short-term, profound effect on the terrestrial carbon cycle in much of the central and eastern United States.”

Along with generating climatic uncertainty, greenhouse gases may also set up plants for freeze damage in another way. According to the study, “There is a mounting consensus that, for many plant species, growth under elevated CO2 can reduce their resistance and tolerance to freezing temperatures.” A drop in stomatal conductance induced by the extra CO2 appears to reduce plants’ ability to acclimate to lower temperatures.

Oak Ridge National Laboratory
University of Missouri in Columbia

Six-year assessment finds pollution plaguing national parks

glacier graphic
(Photo courtesy U.S. National Park Service.)

Dozens of toxic compounds have invaded some of America’s most-loved national parks, designed to be sanctuaries from the industrialized world. Over a six-year period, the Western Airborne Contaminants Assessment Project (WACAP)—the largest such study to date outside of developed areas—sampled snow, water, vegetation, lake sediment, and fish at 20 national parks in Alaska, California, Colorado, Montana, New Mexico, Oregon, Texas, Washington, and Wyoming. The results were released on 26 February.

Of the 100 contaminants tested for, the study found evidence of 70, including long-banned compounds such as DDT (dichloro-diphenyl-trichloroethane) as well as still-legal pollutants such as mercury and PCBs (polychlorinated biphenyls). The focus of the study was on semi-volatile organic compounds and heavy metals.

Along with studying the parks’ ecosystems, researchers traced prevailing winds to determine likely sources of pollutants. Many long-lived pollutants appear to have reached the parks from as far away as Asia, especially for mountainous parks, whose high terrain intercepts globe-circling jet streams. However, even pesticides with short atmospheric lifetimes affected some parks located near high-intensity agricultural areas.

Results from the project add considerably to the state of the science on contaminant transport and subsequent biological and ecological effects on remote ecosystems in the western United States. “These well-documented and carefully analyzed data will provide a basis for evaluating future changes in the status of these ecosystems,” says WACAP scientific director Dixon Landers (U.S. Environmental Protection Agency).

The core study area included eight national park areas: Glacier, Mount Rainier, Olympic, Rocky Mountain, Sequoia and Kings Canyon, Denali, Gates of the Arctic, and Noatak. The study was funded primarily by the National Park Service (NPS). Principal investigators included scientists from NPS, EPA, the U.S. Geological Survey, the Forest Service, and the universities of Oregon State and Washington.

U.S. National Park Service
U.S. Forest Service
U.S. Geological Survey
Oregon State University
U.S. Environmental Protection Agency
University of Washington

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