CAMARGO, P.

COSME, R.

HUXMAN, T.

PROHASKA, N.

SALESKA, S.

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1) Our results show that photosynthetic capacity (e.g. Vcmax) is higher for leaves that matured during the most recent dry season than for older leaves from previous periods of growth.  2) For many trees, leaf demography shifted during the dry season such that recently matured leaves replaced old leaves.   For instance, leaf demography of an Erisma uncinatum, the most abundant canopy tree species at our site, had significantly more recently matured leaves, and significantly fewer old leaves, during surveys late in the dry season (after mid-October) than early in the dry season (prior to mid-September).  3) Together, shifts in leaf demography and the effects of leaf age on leaf function could increase GPP during the dry season at the KM67 site.  Thus, leaf phenology may be a critical driver of GPP seasonality in moist tropical forests of the equatorial Amazon.4) Most current vegetation models do not include vegetative phenology for broadleaf evergreen forests. Incorporating leaf age and vegetation into the models may improve match between vegetation models and observed eddy flux data (3,4).  This study reveals patterns of leaf-age dependent physiology and vegetation phenology that can help with future model parameterization.Leaf demography and physiology of the Tapajós National Forest: could phenology cause a forest-level increase in gross primary productivity during the dry season? Loren Albert1*, Jin Wu1, Neill Prohaska1, Plinio Barbosa de Camargo2, Raimundo Cosme Jr.3, Travis E Huxman4 and Scott R Saleska1(1)University of Arizona, Tucson, AZ, United States, (2)University of Sao Paulo, Sao Paulo, United States, (3)EMBRAPA Brazilian Agricultural Research Corportation, Campinas, Brazil, (4)University of California, Irvine, Irvine, CA, United States Tropical forests such as the forests of the Amazon basin are a significant component of the earth’s carbon budget (1), yet how these forests respond to seasonal changes in weather, and the extent to which tree biology synchronizes with seasonal cycles, are poorly understood.           For evergreen forests in equatorial Amazon that experience dry seasons, eddy covariance observations and remote sensing assessments suggest a late-dry season increase in gross primary productivity (GPP) (2).However, most global vegetation models project a dry-season decrease in GPP (Fig. 1) (3).1. Tropical forests have seasons too: wet/dry seasonality in Amazonia  * Five emergent trees and one understory tree. * Measured photosynthesis (A/Ci curves) of young, mature and old   leaves with LI-6400. * Sun and shade leaves * Measurements performed in situ or with cut branches.References6. Discussion(1) Pan, Y. et al. A Large and Persistent Carbon Sink in the World's Forests. Science 333, 988–993 (2011)(2) Restrepo-Coupe, N. et al. What drives the seasonality of photosynthesis across the Amazon basin? A cross-site analysis of eddy flux tower measurements from the Brasil flux network. Agricultural and Forest Meteorology (2013).(3) Figure reference: Restrepo-Coupe, N., et al. in prep.(4) Doughty, C. E. & Goulden, M. L. Seasonal patterns of tropical forest leaf area index and CO2 exchange. J. Geophys. Res. 113, G00B06 (2008).(5) Chavana-Bryant et al. Ontogenic trait shifts in Amazonian canopy trees (in review)(6) Chavana-Bryant et al. Leaf phenology of Amazonian canopy trees as revealed by spectral and physiochemical measurements (in preparation)(7) Kim, Y. et al. Seasonal carbon dynamics and water fluxes in an Amazon rainforest. Global Change Biology 18, 1322–1334 (2012).(8) De Weirdt, M. et al. Seasonal leaf dynamics for tropical evergreen forests in a process-based global ecosystem model. Geosci. Model Dev. 5, 1091–1108 (2012).2. Methods: Leaf Physiology4. Physiology & Leaf Demography ResultsStomatal conductance for mature leaves is higher than for young or old leaves.  Box plots of the stomatal conductance (mean conduct-anceby leaf age for three trees), show that conduct-ance is dependent upon leaf age.Vcmax is also leaf-age dependent.Box plots of maximum carboxylat-ion rate (Vcmax) by leaf age (mean Vcmax by leaf age for four trees) suggest that mature leaves have the highest Vcmax.For many trees, the number of mature leaves increased as the dry season progressed.The example on the right showshow the proportion of young, mature and old leaves changed across time (data compiled from 2012, 2013 and 2014).  However,there was variation between trees. * Six emergent trees and fourteen understory    trees. * Collected one meter branches multiple times    throughout the dry season (following 5, 6). * Sun and shade microenvironments within the    tree  * Counted young, mature and old leaves on    sampled branches.3. Methods: Leaf DemographyIncreased)gross)primary)produc1vity)from)early)to)late)dry)seasonChanges)in)environmentChanges)in)vegeta1on)func1onChanges)in)leaf)demography?Changes)in)leaf)area)index?Changes)in)photosynthesis?ID: B11G-0108 255075100young mature oldLeaf AgeVcmax (..mol/(m² s))Mean Vcmax by leaf age for sun leaves of four TreesGPP#(gC#m(2#d(1)#ED#######IBIS#CLM####JULES#100150200young mature oldLeaf.age.simpleConductanceMean Conductance by leaf age from three treesObservedGPP (gC m-2 d-1)          The observedseasonality of GPP couldbe due to seasonal changes in vegetation function associated with phenology (4), but phenology and leaf physiology of tropical evergreen forests is not well-characterized at siteswith simultaneous eddy covariance observations.  We measured leaf physiology for leaves of different ages, and surveyed leaf demography, at KM 67 in the Tapajôs National Forest, Brazil.Young M ture  OldMonth Model-data mismatchHypothesized drivers of late dry season GPP increaseConductance(mmol m-2 s-1)Stomatal conductance by leaf age      CO2Young M ture OldVcmax(µmol m-2 s-1)  Vcmax by leaf age    YoungOldCredit: envirofoto.com5. Physiology & Leaf Demography ResultsMonthLeaf ages over timeProportionMonthLeaf age-weighted Vcmax(µmol m-2 s-1)Branch-level VcmaxEstimating Vcmax at thebranch level reveals that shifts in leaf demography plus the effects of leaf age on leaf function could combine to increase GPP during the dry season at the KM67 site.            Vcmax estimated at the branch level is simply the proportion ofleaves from each leaf age category multiplied by mean Vcmax for that leaf age category.  The example above shows estimated branch-level Vcmax for Erisma uncinatum.* Contact information: lalbert@email.arizona.edu

Leaf demography and physiology of the Tapajós National Forest: could phenology cause a forest-level increase in gross primary productivity during the dry season?

http://www.alice.cnptia.embrapa.br/alice/bitstream/doc/1003891/5/LORENAGU2014.pdf

Leaf demography and physiology of the Tapajós National Forest: could phenology cause a forest-level increase in gross primary productivity during the dry season?

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