In Ollinger et al. (1), we reported that mass-based concentrations of nitrogen in forest canopies (%N) are positively associated with whole-canopy photosynthetic capacity and canopy shortwave albedo in temperate and boreal forests, the latter result stemming from a positive correlation between %N and canopy near infrared (NIR) reflectance. This finding is intriguing because a functional link between %N and NIR reflectance could indicate an influence of nitrogen cycling on surface energy exchange, and could provide a means for estimating %N using broad-band satellite sensors

Frolking, Steve

Hollinger, David Y

Lepine, Lucie

Ollinger, Scott V.

Reich, Peter B

Richardson, Andrew D.

English

PubMed

University of New Hampshire

Nitrogen cycling, forest canopy reflectance, and emergent properties of ecosystems

Scott V. Ollinger

Peter B. Reich

Steve Frolking

Lucie C. Lepine

David Y. Hollinger

Andrew D. Richardson

Crossref

Proceedings of the National Academy of Sciences

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositoryEarth Sciences Scholarship Earth Sciences7-2-2013Nitrogen cycling, forest canopy reflectance, andemergent properties of ecosystemsScott V. OllingerUniversity of New Hampshire - Main Campus, scott.ollinger@unh.eduPeter B. ReichUniversity of Minnesota - Twin CitiesSteve FrolkingUniversity of New Hampshire - Main Campus, steve.frolking@unh.eduLucie LepineUniversity of New Hampshire - Main CampusDavid Y. HollingerUSDA Forest ServiceSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/earthsci_facpubThis Article is brought to you for free and open access by the Earth Sciences at University of New Hampshire Scholars' Repository. It has been acceptedfor inclusion in Earth Sciences Scholarship by an authorized administrator of University of New Hampshire Scholars' Repository. For moreinformation, please contact nicole.hentz@unh.edu.Recommended CitationS. V. Ollinger, P. B. Reich, S. Frolking, L. C. Lepine, D. Y. Hollinger, and A. D. Richardson, "Nitrogen cycling, forest canopy reflectance,and emergent properties of ecosystems," Proceedings of the National Academy of Sciences, vol. 110, no. 27, pp. E2437–E2437, May2013.AuthorsScott V. Ollinger, Peter B. Reich, Steve Frolking, Lucie Lepine, David Y. Hollinger, and Andrew D. RichardsonThis article is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/earthsci_facpub/265LETTERNitrogen cycling, forest canopy reﬂectance,and emergent properties of ecosystemsIn Ollinger et al. (1), we reported that mass-based concentrations of nitrogen in forestcanopies (%N) are positively associated withwhole-canopy photosynthetic capacity andcanopy shortwave albedo in temperate andboreal forests, the latter result stemming froma positive correlation between %N and can-opy near infrared (NIR) reﬂectance. Thisﬁnding is intriguing because a functional linkbetween %N and NIR reﬂectance could in-dicate an inﬂuence of nitrogen cycling onsurface energy exchange, and could providea means for estimating %N using broad-bandsatellite sensors.Recently, Knyazikhin et al. (2) dismissedour ﬁndings as counterintuitive and criticizedsubsequent studies (3, 4) for not consideringphysical mechanisms through which plantsinteract with light. Using a subset of datafrom ref. 1, Knyazikhin et al. (2) concludedthat the %N-NIR relationship resulted froma spurious correlation between %N andstructural properties that inﬂuence NIR scat-tering and are attributable to differences be-tween conifer and broadleaf species. Theauthors reasoned that the lack of a direct bio-chemical mechanism means that NIR reﬂec-tance contains no useful information aboutcanopy nitrogen, and that there can be nolink between nitrogen, albedo, and climate.We argue that, quite to the contrary, the setof complex linkages between leaf, canopy,tree, and ecosystem properties that lead torepeatable correlations between mean %Nand NIR reﬂectance represents a usefuldiagnostic tool, as well as an emergentproperty of ecosystems that has adaptiveevolutionary origins.We commend Knyazikhin et al. (2) forexamining physical mechanisms inﬂuencingthe %N-NIR relationship. However, theirarguments rely on an assumption that auseful link between nitrogen and reﬂec-tance requires a direct, biochemical mecha-nism. Such a mechanism would indeed becounterintuitive because nitrogen-containingcompounds absorb, rather than reﬂect, andtypically inﬂuence narrow spectral featuresrather than broad spectral regions. Instead,our primary hypotheses involved functionalassociations between %N and structuraltraits known to inﬂuence NIR scatteringand reﬂectance. Our early ideas focusedon anatomical leaf traits and were basedon the fact that high rates of photosynthe-sis require both high %N and internal leafstructures that permit rapid CO2 diffusionto chloroplasts. Our subsequent measure-ments failed to ﬁnd a correlation betweenleaf %N and enhanced NIR scattering, andinstead pointed to structural traits at thestem or canopy scale (3, 5). However, in allcases, our focus has been on functional asso-ciations between %N and plant structuresrather than on direct effects of nitrogen itself.As Knyazikhin et al. (2) note, the abun-dance of broadleaf and conifer speciesstrongly affects reﬂectance patterns inmixed-species stands. However, composi-tion alone cannot explain all variability inNIR reﬂectance. In our full dataset, whichcontains a greater number of pure broad-leaf and pure conifer stands than the dataconsidered by Knyazikhin et al. (2), the%N-NIR relationship was signiﬁcant withinas well as across forest types, and was alsosigniﬁcant across multiple biomes whennonforest vegetation types were included.Even where mixed stands dominate, quan-tifying subpixel variation in forest compo-sition is far from trivial and the ability toestimate whole-canopy %N has many use-ful applications, even where species com-position is an important driver.By arguing that the %N-NIR relationshipis spurious, Knyazikhin et al. (2) effectivelyassume that a highly signiﬁcant relationshipthat emerged from a large number of carefulmeasurements was a coincidence. A morelikely explanation stems from the fact thatnitrogen availability is a primary constrainton carbon assimilation and, thereby, has astrong inﬂuence on multiple plant and can-opy traits, including leaf morphology, leafdistribution, leaf orientation, proportionalallocation to leaves versus wood, stemgeometry, lateral branching, canopy height,and forest community composition (nitro-gen availability is widely known to inﬂu-ence the abundance of broadleaf versusconifer species). The literature containscountless examples of these associations(3), and an inﬂuence of altered nitrogencycling on NIR reﬂectance via any of themcannot be discounted.Identifying physical drivers of canopy NIRalbedo is necessary, and modeling photonscattering in a canopy (2) can play a usefulrole. However, these physical mechanismsoperate within a set of ecologically drivenlinkages between leaf, canopy, tree, and eco-system properties, the prediction of whichwould represent a major step in understand-ing the role of nitrogen in the Earth system.Scott V. Ollingera,1, Peter B. Reichb,c, SteveFrolkinga, Lucie C. Lepinea, David Y.Hollingerd, and Andrew D. RichardsoneaEarth Systems Research Center, Institute forthe Study of Earth, Oceans, and Space, andDepartment of Natural Resources and theEnvironment, University of New Hampshire,Durham, NH 03824; bDepartment of ForestResources, University of Minnesota, St. Paul,MN 55108; cHawkesbury Institute for theEnvironment, University of Western Sydney,Penrith, NSW2753, Australia; dUS Departmentof Agriculture Forest Service, Northern ResearchStation, Durham, NH 03824; and eDepartmentof Organismic and Evolutionary Biology,Harvard University, Cambridge, MA 021381 Ollinger SV, et al. (2008) Canopy nitrogen, carbon assimilation,and albedo in temperate and boreal forests: Functional relationsand potential climate feedbacks. Proc Natl Acad Sci USA 105(49):19336–19341.2 Knyazikhin Y, et al. (2013) Hyperspectral remote sensingof foliar nitrogen content. Proc Natl Acad Sci USA 110(3):E185–E192.3 Ollinger SV (2011) Sources of variability in canopy reﬂectanceand the convergent properties of plants. New Phytol 189(2):375–394.4 Hollinger DY, et al. (2010) Albedo estimates for land surfacemodels and support for a new paradigm based on foliage nitrogenconcentration. Glob Change Biol 16(2):696–710.5 Wicklein HF, et al. (2012) Variation in foliar nitrogen and albedoin response to nitrogen fertilization and elevated CO2. Oecologia169(4):915–925.Author contributions: S.V.O., P.B.R., S.F., L.C.L., D.Y.H., and A.D.R.wrote the paper.The authors declare no conﬂict of interest.1To whom correspondence should be addressed. E-mail: scott.ollinger@unh.edu.www.pnas.org/cgi/doi/10.1073/pnas.1304176110 PNAS | July 2, 2013 | vol. 110 | no. 27 | E2437LETTER

Reich, Peter B. (R16861)

Lepine, Lucie C.

Hollinger, David Y.

Western Sydney ResearchDirect

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Nitrogen cycling, forest canopy reflectance, and emergent properties of ecosystems

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