“These values are substantial when considering the amount of global atmospheric carbon,” said William L. Bauerle, an ecophysiologist at Colorado State and the study’s lead author. “As carbon levels rise and temperatures warm, trees can only do so much to protect the planet from the effects of climate change.”

Tree leaves, responding to shorter days, simply do not fix the planet’s fast-rising carbon dioxide levels to the extent that scientists earlier assumed, the study shows. This is true even when warm temperatures delay signs of aging in tree leaves.

The new findings will help refine global models of atmospheric carbon cycling and predictions about the impact of climate change, said Bauerle, who also holds a graduate faculty appointment at Duke’s Nicholas School of the Environment.

The new peer-reviewed study was published today (May 14) in the Proceedings of the National Academy of Sciences.  Ram Oren and Danielle Way of the Nicholas School were co-authors.

“The net amount of carbon dioxide forests absorb from the atmosphere is the balance between what they take in photosynthesis and what they release in respiration. 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. Our findings mean that any lengthening of the growing season with global warming will not increase photosynthesis because day-length will not change,” said Oren, Nicholas Professor of Earth System Science.

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

The result: Net primary production – the amount of carbon dioxide the Earth’s vegetation absorbs during photosynthesis, minus that released in plant respiration – drops from 58.7 petagrams of carbon per year to 56.7 petagrams of carbon per year, according to the PNAS study. That’s a downward correction of 3.4 percent. (1 petagram equals 1 billion metric tons.)

Likewise, new insights about the impact of photoperiod on tree leaves will improve estimates of current carbon uptake based on vegetation greenness sensed with space-based technologies, the authors write.

“In this study, we demonstrate how to scale from the leaf to the global level,” said Bauerle, a faculty member in CSU’s Department of Horticulture and Landscape Architecture.

The research team’s findings are based on measurements of tree leaf photosynthesis rates over five growing seasons.

The scientists used portable steady-state gas exchange systems to monitor photosynthetic activity in the leaves of 11 tree species, including red maple, green ash, honey locust, white oak and birch. The team, representing research institutions in the United States, Canada and Sweden, also used previously published data from 12 other species.

Their leaf-level measurements controlled for factors including light, temperature and humidity to analyze photosynthetic response. The researchers found that photosynthetic activity begins to decline many weeks before the leaves of deciduous trees change color and drop to the ground during fall.

“Even in the early fall, tree leaves are lush and green, but our study found that their physiological activity is much less than we’d expect based on appearance. Because of that, we have been overestimating the amount of carbon they are fixing,” Bauerle said.

"The findings indicate that some of the techniques that we use to monitor forest growing season length by trees may not be giving us the whole story,” said Way. “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. That longer growing season should mean that trees are absorbing more carbon from the atmosphere than they used to, helping buffer anthropogenic carbon dioxide emissions. But our work shows that leaves can remain green in the late summer and autumn, while photosynthetic capacity drops off, since photosynthesis is strongly controlled by day length. 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."