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Global Warming—Tracking the Effects of Climate Change on Plants
Plants & Gardens News Volume 18, Number 3 | Fall/Winter 2003
by Niall Dunne
Here's a familiar news headline: Greenhouse gases released into the atmosphere by human activity (such as fossil fuel burning) are trapping heat and causing air and ocean temperatures to rise. Weather-station records indicate that the earth's surface-air temperature has warmed by an average of 1°F over the last 100 years—far too rapidly to be attributable to normal climate variability. Experts are predicting another 2.5°F to 10.4°F rise by the end of this century unless emissions are reduced.
And the consequences of such a dramatic jump in temperature? These have a familiar ring as well: elevated sea levels threatening coastal regions; increased severity of extreme weather events such as hurricanes, flooding, and drought; higher incidence of heat stress and respiratory illness among people in urban areas; and widespread migration of infectious diseases.
What we don't hear much about is the effects that global warming will have on the structure of plant and animal communities. Scientists have only recently gone beyond speculating about this issue to collecting, collating, and examining real data on it. Gardeners are even getting in on the action by participating in large-scale research projects such as the University of Alberta's PlantWatch, which is recording the effects that warming is having on the flowering of plants (see "Fun With Phenology").
Precocious Petals
As avid observers of nature, most of us gardeners are probably aware at some level that changes are under way in the clockwork of many of our favorite plants. Spring seems to be arriving a little earlier for a lot of them these days. (In fact, one report says that between 1959 and 1993 the entire biological spring in North America shifted forward six days.)
To us, the most obvious changes are phenological ones such as early leafing and flowering. (Phenology is the study of the cycling of biological events.) And we're not just hallucinating. The data is already in. For example, in central Alberta (western Canada), the quaking aspen (Populus tremuloides), an important commercial pulpwood tree, has shown a 26-day shift to earlier bloom over the past century.
A phenological study of springtime events at a site in southern Wisconsin, begun in 1936 by environmental icon Aldo Leopold, found that spring began on average 0.12 days earlier per year. One of the 55 species observed in the study was forest phlox (Phlox divaricata); from the years 1936 to 1999, the phlox's bloom time shifted 14 days forward.
A recent Cornell University study of the phenology of spring wildflowers at a site in the Hudson Highlands of southeastern New York found significantly earlier bloom (averaging 19.8 days over a period of 50 years) in 6 of the 15 species examined.
But wait a minute. Is this being caused by global warming? Didn't I read somewhere that flowering in plants is determined less by temperature than by the relative length of day and night—the photoperiod? Light, or lack of it, is certainly the determining factor in the flowering of many temperate annuals and perennials. However, most spring-flowering temperate woody species, as well as some perennial herbs, flower in response to accumulated heat.
Okay, so our plants are flowering earlier than they should be. Yippee! More time to enjoy beautiful blooms in the garden! This is true on a superficial level. But when considered from an ecological perspective, early flowering (along with early budbreak, first leafing, etc.) is more a cause for concern than celebration.
Mangos Take Manhattan
On the face of it, a rise in temperature and rainfall—not to mention higher concentrations of carbon dioxide fueling photosynthesis—seems like a gardener's blessing. Imagine growing juicy ripe mangos and tropical orchids outdoors in Zone 7! And indeed, in the near term, agriculture, forestry, and horticulture could benefit from global warming in terms of increased production and a relaxation of hardiness constraints. But these initial benefits may easily be offset or even reversed by a gradual intensification of new problems related to everything from pollination to pest control.
For natural ecosystems, significant warming could have far-reaching, perhaps disastrous, effects. (Some researchers are identifying climate change as one of the major challenges—along with habitat destruction and invasive species—to plant and animal conservation in the 21st century.) Scientists say the warning signs are already here. According to recent research, many wild plants and animals are currently showing significant responses to rapid climate change.
A January 2003 paper in the journal Nature analyzed 143 separate studies on how global warming might be affecting the physiology, behavior, and evolution of wild species. Coauthored by scientists from NASA and Stanford University, among others, the paper concludes that consistent temperature-related shifts have already taken place in organisms ranging from mollusks to mammals and from grasses to trees.
What kind of shifts are we talking about? The authors identify four types. We know about one already: shifts in phenology triggered by temperature-related cues; these events include flowering and leafing, but also migration and egg laying, and in most cases they are taking place earlier than normal. Another is shifts in density and range (as one might expect, ranges are shifting poleward or upward in elevation as species move into new areas previously incompatible with their metabolisms). The other two are changes in morphology or behavior and changes at the genetic level.
But if a significant number of organisms are shifting, is there really a problem? Won't they all just pick up, move a little closer to Santa Claus, but essentially retain their ecological integrity? Not if the history of our planet teaches us anything, say the paper's authors. During rapid climatic changes in the past, species showed differential movement. In other words, they reacted independently, rather than in unison, to the environmental stress. "Such differential movement could result," say the authors, "in a disruption of the connectedness among species in current ecosystems."
Canaries in a Coal Mine
A paper in the October 2001 issue of the journal Science echoes this opinion. Discussing the threat that early flowering and fruiting of plants could present to long-distance migratory birds, the authors say that "climate warming may lead to a decoupling of species interactions—for example, between plants and their pollinators or between birds and their plant and insect food supplies."
Given the fine-scale synchrony of nature's many interactions, associations, and coadaptations, such a breakdown seems plausible. But the authors of the Science paper don't offer much in the way of evidence—presumably because much more research is needed.
They do offer an example of the plight of long-distance western European migratory birds that overwinter south of the Sahel region of Africa. These species have been suffering a decline in recent years, and the authors think that climate change may be partially to blame. If temperate plants in Europe are blooming and fruiting earlier than normal, they say, this could be benefiting year-round resident birds by allowing more of them to survive the winter and exploit the spring habitat. When the migrants eventually arrive, there may not be enough food and shelter left to go around.
The next step in the argument is obvious: Unless "decoupled" species can adapt to their newly warmed environment, they face the possibility of extinction.
Niall Dunne is the associate editor of Plants & Gardens News