NEW YORK -- A tiny, cloverlike plant with heart-shaped leaflets caught Steve Brill's attention as he scanned the ground of a Brooklyn park.
"We have really messed up our climate if this plant, which dies in November, is alive now," Brill announced as he introduced the plant, yellow wood sorrel, to the group following him.
Brill leads foraging tours for edible plants in the New York area, and his first tour of the 2012 season, in Prospect Park, yielded some surprises brought by the unusually mild winter. The lemony-flavored sorrel, for instance, had shown up at least a month earlier than normal.
Sunday (March 4) marked the first tour of his 30th season. Brill said he has noticed a gradual shift in the annual cycle over the years, with many plants showing up about three weeks earlier than they once did, and then lasting much longer. This year is unprecedented -- some plants never even died off for the winter, he said. [Gallery: Signs of Early Spring in Brooklyn]
Changes in timing
Scientific evidence for similar shifts in timing among all kinds of plants and animals is abundant. For example, studies indicate lilacs in North America are leafing out and flowering earlier; in Japan, gingko trees are getting their first leaves earlier and losing them later; bee species in the northeastern North America are emerging earlier, keeping pace with the flowers upon which they feed; British butterflies are also showing up sooner; and birds appear to be shifting the timing of their migrations.
One study even looked at National Park attendance to find evidence of a similar shift in seasonal timing -- called phenology -- for humans.
"There is a study coming out every week showing changes are occurring," said Jake Weltzin, the executive director of the USA National Phenology Network and an ecologist with U.S. Geological Survey, which recruits volunteers to monitor seasonal changes in plants and animals.
Attributing these changes directly to global climate change is more difficult, but researchers are beginning to do just that; they're finding evidence that shifts in climate are directly linked to changes in the timing of biological events, Weltzin said.
Changes in the timing of events such as spring blooms, insect emergence and bird migrations have consequences.
First of all, not all species respond in the same way; some are better able to adapt than others. This means mismatches can occur, if, say, bees and the flowering plants they pollinate don't respond at the same rate. Mismatches like this can affect the prospects of the species involved.
"It is going to rig the game for certain species and the ones that are successful, it is going to change the individuals within them," said Mark Schwartz, a distinguished professor of geography and climate at the University of Wisconsin-Milwaukee. "It is an evolutionary process."
The fingerprint of climate change
Schwartz studies how plants respond to changes in seasons and climate, drawing upon observations of plants made by volunteers. With records of the first appearance of their leaves and flowers going back to 1956, lilacs -- all of which are genetically identical to minimize variation among them -- have the longest record.
Using observations from the lilacs, as well as cloned honeysuckle, Schwartz has built models to fill in the gaps in the data to predict how temperature might affect the arrival of spring leaves and blooms.
Using this technique, Schwartz and colleagues have shown that first leaf and flower dates crept ahead by around one day per decade between 1955 and 2002 across most Northern Hemisphere temperate regions. Other studies that have assessed data on many species have also found temperature-related shifts in spring.
His spring plant models have also been used to look at how natural patterns, such as cycles in atmospheric pressure and ocean temperature, play into earlier springs. [What's Causing Early Spring?]
"The argument seems compelling from what I have seen, that we are in a longer-term trend toward things being quite different," Schwartz said.
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