On Average, Plants Flowering a Week Sooner
By Curt Suplee
The reason, according to a new study from Smithsonian Institution scientists, is "global warming"--or at least warmer winter and spring nights.
The finding surprised the researchers, who had expected that rising minimum temperatures in the Washington area during the December-to-May season might have advanced the date of first flowering by at most a day or two since 1970, when record-keeping for the project began.
Instead, they found that 89 species representing dozens of different kinds of plants had begun to bloom much earlier over the past three decades. The smallest advance was three days; the longest was 46 days for false strawberry, a familiar fixture in local yards. Dogwoods are now flowering seven days earlier. Columbine and bluebells have moved up by 17 days, and Jack-in-the-pulpit is blooming 11 days sooner.
"I was quite excited" to hear about the findings, said Ghillean T. Prance, former director of the Royal Botanic Gardens at Kew, which is located southwest of London, England. "It's nearly identical to the kinds of dates we're getting."
Precocious flowering eventually may have substantial effects on local vegetation, the Smithsonian scientists said yesterday. The trend "will more likely favor species that are preadapted to take advantage of a longer growing season," said botanist Stanwyn G. Shetler of the National Museum of Natural History, a co-author of the paper that has been submitted to a research journal. Similarly, plants that require a long, cold winter (such as sugar maples) could "simply disappear" from the region.
"Among the earliest to flower are the introduced [non-native] plants such as false strawberry, chickweed and dandelions. They seem to be able to exploit the slightest amount of warmth," Shetler said. "One subtle and serious effect is that invading species may get the upper hand."
In addition, early blooming may extend the annual suffering for those allergic to pollen, said W. John Kress, chairman of the museum's Department of Botany, and could result in "a prolonged flowering period," although that has not been studied. It also raises the question of whether bees and other pollinating creatures will be able to keep the same timetable.
Further, "the flowering patterns may not coincide with precipitation patterns," producing a situation in which plants are ready to bloom before there is adequate soil moisture, said co-author Mones S. Abu-Asab, now at the National Cancer Institute.
In 1970, Shetler and colleagues began a program to record the earliest date at which various plants flowered. Over the years, more than 125 people conducted the observations at several sites in the District--including the C&O canal, Rock Creek Park and the National Arboretum--as well as Arlington, Silver Spring, Beltsville and other locations. The archive now contains data on more than 600 species.
The scientists then selected 100 species for which the longest records were available, and compared their flowering dates to three variables in local climate during the blossoming season: average minimum temperature, average precipitation and amount of precipitation per month. The analysis showed no relationship between precipitation and date of first flowering. But the correlation with temperature was striking.
Between 1970 and 1999, the average minimum temperature in the flowering season (December to May) has increased about 0.4 degrees Fahrenheit in downtown Washington and 2.2 degrees at College Park, Md.
Of the 100 species studied, 11 have been flowering somewhat later--from half a day to 10 days. "We tried to find any common pattern among those, but we failed to find any," Abu-Asab said. The most delayed species was the Japanese honeysuckle, a notorious bane of local gardeners.
Other recent research has shown that the growing season is lengthening in parts of the Northern Hemisphere. By one estimate, it has increased by 11 days in the 20th century, during which average global temperatures have risen about 1.2 degrees F.
The Smithsonian researchers attribute early flowering to global warming, although their study only involves the Washington area over a period of 30 years, which most scientists would regard as too short an interval to document long-term global warming.
"It might not necessarily be global warming," said Vernon E. Kousky, a research meteorologist at the National Weather Service's Climate Prediction Center. "It's a safe bet that it could be the 'urban heat island' effect," whereby the accumulation of heat-holding asphalt and concrete raises the local average temperature, particularly the minimum temperatures.
Either way, he said, "maybe we should move up the date of the Cherry Blossom Festival."
Prance said that the similar English results make the global case more plausible. "It suggests that this is not an exception, or a matter of local climate variation," Prance said. "The fact that it's on both sides of the Atlantic makes it much more significant."
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