Photosynthetic Capacity of Branches Increases During the Dry Season in a Central Amazon Forest
First direct evidence from individual trees that new leaf growth and development cause overall forest green-up
Amazon forest ecosystems are observed by satellites to green-up and by 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 we 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. We 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.
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, 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 earth system models as a means to represent our best understanding of the key processes regulating photosynthesis.
We 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, we used this 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. We found that photosynthetic capacity, as indicated by parameters of biochemical limitations on photosynthesis (Vcmax, Jmax, and TPU), was higher in recently matured leaves than either young or old leaves, and stomatal conductance was higher for recently matured leaves than old leaves. Most tree branches had several different leaf age categories simultaneously present, and the number of recently mature leaves on branches of our 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 Vcmax increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity observed from flux towers.
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University of Arizona
This project received U.S. DOE support through GoAmazon, award DE-SC0008383. It was also supported by the US 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 DOE’s Office of Biological and Environmental Research.
Albert, L.P., J. Wu, N Prohaska, P.B. de Camargo, T.E. Huxman, E.S. Tribuzy, V.Y. Ivanov, R.S. Oliveira, S. Garcia, M.N. Smith, RC Oliviera, Jr., N. Restrepo-Coupe, R. da Silva, S.C. Stark, G.A. Martins, D.V. Penha, S.R. Saleska. (2018) Age-dependent leaf physiology and consequences for crown-scale carbon uptake during the dry season in an Amazon evergreen forest. New Phytologist. DOI: 10.1111/nph.15056