 By JOAN LOWY Scripps Howard News Service April 15, 2005
Changes are mostly subtle, but thanks to global warming, blooming and leafing dates are getting earlier and the range of some plants and trees is changing. In most places in the Northern Hemisphere, the average time for the first blooming and budding starts seven days earlier, said Mark D. Schwartz, a geography professor at the University of Wisconsin-Milwaukee. "However, you're still getting a tremendous amount of variation from one year to the next," Schwartz said. "It's a bit premature to say you can rely on being able to put seeds in earlier." In the Northeast and Midwest, lilacs are blooming four days earlier, apple trees nine days earlier and grape vines six days earlier, said David Wolfe, a professor of plant ecology at Cornell University and co-author with Schwartz of a study on early blooming dates. It is scheduled for publication soon in the International Journal of Biometeorology. The main reason is warming temperatures. While the planet has warmed an average of 1 degree F over the past century, the warming has been greater in some northern latitudes and during the winter months. In the Northeastern United States in the winter, the warming has been about 3 degrees F, Wolfe said. "If you believe in thermometers, then you have to conclude it's warming," Wolfe said. "And clearly the living world is responding." As the changes become more pronounced, they are likely to affect gardening all over the world, but especially in colder regions. Bulbs will come up earlier, grass will green earlier and periods of high temperatures in the summers may become more frequent and more intense. "For cooler regions of the country, the obvious potential good news is that a longer growing season may give you the option of crops you couldn't grow before," Wolfe said. In the Northeast, that might include watermelons, tomatoes and some types of flowers, he said. In the Southeast and places where it's already warm, "there's not too much good news for gardeners," Wolfe said. "The gardener could be facing invasive insects, weeds and diseases they've never seen before or become even more aggressive." Studies show that weedy vines like kudzu, honeysuckle and poison ivy could increase by as much as 70 percent with higher concentrations of carbon dioxide, the main greenhouse gas that's driving global warming. Among the plants expected to do particularly well in an environment rich in carbon dioxide is ragweed. "There's a whole synchrony to how the living world works out there," Wolfe said. "No plant or animal or insect is operating on its own." If some plants, animals or insects are more responsive to climate change and others less responsive, "this could throw off synchrony between plants and pollinators, for example," he said. Some scientists had speculated that increased carbon dioxide, which acts like a fertilizer for plants, would compensate for the increased stress that plants may suffer from higher temperatures. But when Mary Peet, a professor of horticulture science at North Carolina State University, tested that assumption on tomatoes, she found that higher temperatures dramatically decreased pollen dispersal. That turns out to be true of almost all plants that set fruit or seeds. When the average daily temperature reaches 84 degrees - that's the average of daytime and nighttime temperatures - "essentially there is no pollen production," Peet said. "This is in tomatoes, but many species are quite sensitive to heat stress, including beans like soybeans and snap beans." "The important thing for gardeners to know is that not only is there a warming trend going on, but that we've documented that these living things have shifted their behavior," Wolfe said. If you think your flowers and trees have been blooming earlier and are behaving differently, Wolfe said, "It's not just your imagination - it probably really is happening."
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