PS 92-57 - Seasonal patterns of photosynthetic capacity: Photoperiodic control and its carbon cycling implications

Friday, August 10, 2012
Exhibit Hall, Oregon Convention Center
William L. Bauerle1, Ram Oren2, Danielle A. Way3, Song S. Qian4, Paul C. Stoy5, Peter E. Thornton6, Joseph D. Bowden1, Forrest M. Hoffman7 and Robert F. Reynolds8, (1)Horticulture and Landscape Architecture, Colorado State University, Fort Collins, CO, (2)Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, (3)Department of Biology, University of Western Ontario, London, ON, Canada, (4)Environmental Sciences, The University of Toledo, Toledo, OH, (5)Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, (6)Environmental Sciences Division & Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, (7)Computational Earth Sciences Group, Climate Change Science Institute (CCSI), Oak Ridge National Laboratory, Oak Ridge, TN, (8)Agricultural, Forest, and Environmental Sciences, Clemson University, Clemson, SC
Background/Question/Methods

While temperature is an important driver of seasonal changes in photosynthetic physiology, photoperiod also regulates leaf activity. Climate change will extend growing seasons if temperature cues predominate, but photoperiod-controlled species will show limited responsiveness to warming. We show that photoperiod explains more seasonal variation in photosynthetic activity across 23 tree species than temperature. Although leaves remain green, photosynthetic capacity peaks just after summer solstice and declines with decreasing photoperiod, before air temperatures peak.  

Results/Conclusions

In support of these findings, saplings grown at constant temperature, but exposed to an extended photoperiod maintained high photosynthetic capacity, while photosynthetic activity declined in saplings experiencing a naturally shortening photoperiod; leaves remained equally green in both treatments. Incorporating a photoperiodic correction of photosynthetic physiology into a global-scale terrestrial carbon cycle model significantly improves predictions of seasonal atmospheric CO2 cycling, demonstrating the benefit of such a function in coupled climate system models. Accounting for photoperiod-induced seasonality in photosynthetic parameters reduces modeled global gross primary production ~4 PgC y-1, resulting in a ~2 PgC y-1 decrease of net primary production. Such a correction is also needed in models estimating current carbon uptake based on remotely-sensed greenness. Photoperiod-associated declines in photosynthetic capacity could limit autumn carbon gain in forests, even if warming delays leaf senescence. Assessments of late season carbon sequestration under a changing climate should focus on potential adverse impacts of warming via increased ecosystem respiration.