"Until now scientists assumed that as the climate warms, growing seasons will become longer and forests will be able to absorb more carbon dioxide from the atmosphere," Ram Oren of Duke University, US, and Swedish University of Agricultural Sciences told environmentalresearchweb. "Our findings mean that lengthening growing season with global warming will not increase photosynthesis because day-length will not change."

According to Oren, in the best case, respiration will not increase with global warming and forests will take up about as much carbon dioxide as they do now. In the worst case, however, tree respiration will rise with temperature and the net amount forests will absorb from the atmosphere may actually go down. "We have some reasons to assume the first is the likely outcome – not as good as we hoped until now, but better than the worst scenario," he added.

Oren and colleagues looked at changes in leaf photosynthetic capacity for 23 types of tree, using direct measurements for 11 species as well as data recorded by other scientists. They found that photosynthetic capacity peaked just after the summer solstice and started to decline before air temperatures maxed out.

The team investigated the effects of their results on a coupled vegetation–climate model by decreasing photosynthetic capacity after the longest day; normally models link the rate of photosynthesis to temperature. Using day-length decreased global net primary production by 3.4%, from 58.7 Pg C to 56.7 Pg C.

"As for the modelling, the seasonal pattern of photosynthesis is typically pegged to temperature, so roughly, the higher the temperature the higher is photosynthesis," said Oren. "Modellers have realized for sometime now that this produces too high values late in the season, yet only now they are given a demonstrated mechanism to deal with it."

According to Danielle Way of Duke University and University of Western Ontario, the findings indicate that some of the techniques used to monitor forest growing season length may not be giving the whole story. "Measurements from satellites can tell us how green leaves are, and we know from these studies that leaves are developing earlier in the spring and staying green longer in the autumn," she said. In the past 50 years, warming has lengthened temperate growing seasons by 3.6 days per decade.

But the study shows that leaves can remain green in the late summer and autumn while photosynthetic capacity drops off, Way said. "So although leaves may stay green in a warmer climate, it doesn't necessarily mean that trees will take up as much carbon dioxide from the atmosphere as we assumed."

Oren says that modellers can begin incorporating the mechanism into their models almost immediately, evaluating their results against data. "A number of us may collaborate on such work with modellers," he said. "The experimentalists would need to assess how broad is the response. Is it as obvious in conifers as in broadleaved species? In evergreens as in deciduous species? However, taking such research forward depend on funding, the availability of which differs among investigators."

The team reported its results in PNAS.