The Science

Amazon forest ecosystems are observed by satellites to green-up and by flux towers to increase in photosynthetic uptake during the dry season, but the mechanisms for this at the tree and leaf scale have been much debated. Here scientists from Brown University and the University of Arizona tested how leaf age–dependent physiology and leaf demography combine to affect photosynthetic capacity of a central Amazon forest during the dry season in a field-based study independent of remote sensing or eddy covariance methods. They found the first direct field evidence that branch-scale photosynthetic capacity increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity derived from flux towers.

The Impact

This new study is the first to directly show the mechanistic basis for the much-debated Amazon forest dry season green-up phenomenon. It highlights the role of endogenous phenological rhythms—not just seasonal variation in climate drivers—as a key mechanism regulating the seasonality of photosynthesis. This is important because in most Earth system models (ESMs), the seasonality of tropical evergreen ecosystems is driven by climatic seasonality, not biological phenology, and many of these models do not yet correctly simulate this pattern. This study thus strongly supports the incorporation of leaf phenology into ESMs as a means to represent the best understanding of the key processes regulating photosynthesis.

Summary

The research team conducted demographic surveys of leaf age composition and measured age-dependence of leaf physiology in broadleaf canopy trees of abundant species at a central eastern Amazon site. Using a novel leaf-to-branch scaling approach, they used these data to independently test the much-debated hypothesis—arising from satellite and tower-based observations—that leaf phenology could explain the forest-scale pattern of dry season photosynthesis. They found that photosynthetic capacity, as indicated by parameters of biochemical limitations on photosynthesis [V_cmax, J_max, and triose-phosphate utilization (TPU)], was higher in recently matured leaves than in either young or old leaves, and stomatal conductance was higher for recently matured leaves than for old leaves. Most tree branches had several different leaf-age categories simultaneously present, and the number of recently mature leaves on branches of the focal trees increased as the dry season progressed (before October 15 versus after October 15), as old leaves were exchanged for young leaves that then matured. Together, these findings suggest that aggregated whole-branch V_cmax increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity observed from flux towers.

Principal Investigator

Loren Albert
Brown University
loren_albert@brown.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This project received U.S. Department of Energy (DOE) support through GOAmazon (award DE-SC0008383). It was also supported by the U.S. National Science Foundation (NSF; award OISE-0730305 to S. Saleska), the Philecology Foundation through University of Arizona Biosphere 2 and a Marshall Foundation of Arizona dissertation fellowship to L.P. Albert. J. Wu was supported in part by the Next-Generation Ecosystem Experiment (NGEE)–Tropics project of the Office of Biological and Environmental Research within the DOE Office of Science.

References