Plant studies miss the full effect of climate change

Climate change is affecting plants' seasonal activities more strongly than biological experiments suggest. The finding suggests that such studies may have to be reworked to get a better picture of the effects of global warming.

"This is huge," says Benjamin Cook, a climate scientist at NASA's Goddard Institute for Space Studies in New York, and a member of the team behind the study. "We are relying heavily on these experiments to predict what will happen 100 years from now."

Shifts in seasonal patterns of growth and activity are some of the most obvious ecological responses to climate change. For instance, plants may unfurl their leaves or burst into flower earlier in the spring. The study of such changes, known as phenology, is important because big trouble could be in store if species that rely on one another ? such as flowering plants and their pollinators ? get out of sync in a warming world.

To predict what the future holds, ecologists are artificially warming selected plots in natural ecosystems using infrared lamps, soil-heating cables or open-top enclosures that act like greenhouses. But researchers led by Elizabeth Wolkovich of the University of California, San Diego, have found that such experiments aren't a reliable guide to the future.

Day per degree

Wolkovich gathered results from 36 warming experiments and compared them with 14 long-term studies of plant responses to actual environmental warming. In total, she collected information on over 1500 plant species across four continents. Her team looked at the timing of flowering and leaf growth in the spring, expressing the results as a change in days per degree Celsius of warming.

The warming experiments dramatically underestimated responses to climate change, says Wolkovich. They indicated that flowering and leafing would advance on average by about one day per degree of warming, but long-term observations show that responses to climate warming so far have been four to eight times as large.

Some experiments have warmed plots beyond the range recorded in long-term observations, which might suggest that plants' response to warming begins to plateau at higher temperatures. However, Wolkovich says that the discrepancy cannot be explained this way.

When Wolkovich and her colleagues considered only species represented both in experiments and in observational studies, the difference was even more pronounced. For these species the experiments suggested, surprisingly, that plants would flower later on average under warmer conditions. Observational studies have shown the opposite: flowering times have advanced as the world has warmed.

Why warming experiments should be getting it wrong is unclear. Richard Primack of Boston University says the results suggest that warming experiments do not deliver as much heat as researchers think ? some might be dissipated by wind, for instance. Another possibility, notes Wolkovich, is that warming experiments may inadvertently dry soil, which is likely to delay leaf growth and flowering.

Walden data

Primack has studied changes in plant seasonality at Concord, Massachusetts, using records that were started in the 19th century by the philosopher, poet and naturalist Henry David Thoreau. Now Primack is relating the Concord observations to results from the Boston-Area Climate Experiment run by Jeff Dukes of Purdue University in West Lafayette, Indiana, and the University of Massachusetts, Boston. Preliminary results echo Wolkovich's findings, Primack says.

Changes in seasonal patterns of plant growth and development are not the only likely consequences of climate change. Warming experiments are also being used to investigate predicted changes in total ecosystem productivity and the cycling of carbon, water and nutrients.

Primack suggests that researchers may have to deploy more temperature sensors in their experimental plots to determine how much of the heat they are delivering is being dissipated. "A lot of the results coming out of warming experiments may need to be recalibrated," he warns.

Wolkovich presented the findings at the American Geophysical Union meeting in San Francisco last week.

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