Delay of bloom blamed on climate change

Flowers that fail to open early in spring may be taking cue from previous autumn

Oddball spring flowers that seem unaffected by climate change — or have gotten it backwards — could be taking some cues from unusual autumns the year before.

PULLED BOTH WAYS Old man’s beard may bloom as if it couldn’t care less about the early-spring climate change that’s advancing many flowering times. Yet a new analysis finds that Clematis vitalba is sensitive after all but gets conflicting signals from spring and autumn. Hechtonichus/Wikimedia Commons

Like their early flowering peers, plants in a new study show that many of these on-time and late bloomers also have had their timing shifted by a warming globe — but the paradoxical effects have just been more difficult to observe.

Climate change is bringing warmer and earlier springtimes to meadows, mountain slopes and many other ecosystems worldwide, with many a bud bursting into bloom days or weeks earlier than in decades past. Yet a minority of plants continue to bloom at their regular time, and some even appear to delay their blooms, says Benjamin Cook of the NASA Goddard Institute for Space Studies in New York City. While some have suggested explanations for these contrarian plants’ out-of-step timing, the effect has been tricky to analyze.

Looking at plant bloom times in terms of both spring and fall temperature quirks, however, shows that some species are sensitive to signals from unusual fall temperatures, Cook and his colleagues report online May 21 in Proceedings of the National Academy of Sciences. Warmer fall temperatures may mask or delay any tendencies to bloom earlier in the spring.

Biologists have known that certain plants need a good chill over the winter before they thrill to the warming of spring and break out of dormancy to bloom. If weather doesn’t cool down as soon as it used to in the fall, then the chilling doesn’t start on time and there’s a delay in the spring response. Requirements for this cool-down time-out, called vernalization, are better known for crops and garden beauties than for most wild species.

To try to quantify how climate change tweaks vernalization for a wide range of species, Cook and his colleagues turned to a 47-year record (1954 through 2000) of the first bloom dates for 384 species growing near the same site around Chinnor, England. And to broaden the range, the researchers looked at 31 years of first blooms from 106 species around Washington, D.C.

Statistical methods found that most of the plants — 72 percent in England and 73 percent in the United States — responded only to spring warming and bloomed earlier, but showed no discernible influence of warming fall temperatures.

For many of the other plants though, adding in a delay caused by a warm fall made the springtime pattern make sense at last. For example, Clematis vitalba, a relative of the vines common in gardens, at first glance doesn’t appear affected by climate change. Yet the analysis showed a tendency to advance bloom 5.1 days in spring, which a delaying pull from the previous fall cancels.

In contrast Chinnor records show the ground-covering bugleweed (Ajuga reptans) blooming almost six days later than it used to. Warming springs are urging the bloom time forward, the new analysis shows, but not strongly enough to compensate for a big delay from autumn.

Although vernalization is an old concept, “What is new here is that they believe you can be predictive,” says ecologist Alastair Fitter of the University of York in England. He and his father had collected the Chinnor data, but he wasn’t part of the new analysis.

Understanding what’s going on with the species that don’t seem to have gotten the memo on climate change could have practical value for conservationists, he says. Decisions about management could turn out quite differently if biologists figure out that plants involved are responding to fall as well as spring climate glitches.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

More Stories from Science News on Life