On the first day of summer – the longest day of the year – tree leaves are lush and green, luminous in the June sunlight.
Yet just a day after the summer solstice, length of daylight begins to incrementally decrease; tree leaves begin to shut down, and the activity of photosynthesis declines. As the season progresses, this drop in photosynthetic activity means trees absorb less carbon dioxide than they had on the longest day of the year and ultimately sequester far less carbon on a global scale than earlier thought, a team of scientists has found.
Photoperiod, rather than temperature, is a primary driver of leaf activity, according to research findings published May 14 in the Proceedings of the National Academy of Sciences. In fact, photosynthetic activity wanes long before autumn’s chill, the study shows.
Correctly accounting for the impact of photoperiod on tree leaf activity adjusts global estimates of carbon sequestration downward by more than 3 percent, according to the study, titled “Photoperiodic regulation of the seasonal pattern of photosynthetic capacity and the implications for carbon cycling.”
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 study in PNAS. That’s a downward correction of 3.4 percent (1 petagram equals 1 billion metric tons).
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 nine researchers write. That is true even when warm temperatures delay signs of aging in tree leaves.
“These values are substantial when considering the amount of global atmospheric carbon,” said William L. Bauerle, an ecophysiologist at Colorado State University 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.”
Ram Oren, Nicholas Professor of Earth System Science at Duke University’s Nicholas School of the Environment, said the research discoveries are significant in the context of climate change. Until now, Oren said, scientists assumed that as the climate warms, growing seasons would lengthen and forests would absorb more carbon dioxide from the atmosphere.
“Our findings mean that lengthening growing seasons with global warming will not increase photosynthesis, because day length will not change,” Oren said.
Scientists know from earlier studies that leaves generally are developing earlier in the spring and staying green longer in the autumn, noted Danielle Way, a collaborator who studies global change ecology at Duke University.
“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,” Way said. “Leaves may stay green in a warmer climate, but that doesn’t mean trees will take up as much carbon dioxide from the atmosphere as we assumed.”
The findings will help refine global models of atmospheric carbon cycling and predictions about the impact of climate change, according to the study.
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 an additional 12 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.