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Harvard Forest Symposium Abstract 2009

  • Title: Exploring Linkages Between Canopy Nitrogen, Carbon Assimilation and Albedo in U.S. and Canadian Forests
  • Primary Author: Scott Ollinger (University of New Hampshire - Main Campus)
  • Additional Authors: Steve Frolking (University of New Hampshire - Main Campus); David Hollinger (USDA Forest Service); Mary Martin (University of New Hampshire - Main Campus); Lucie Plourde (University of New Hampshire - Main Campus); Andrew Richardson (Northern Arizona University)
  • Abstract:





    The availability of nitrogen represents a key constraint on carbon cycling in terrestrial ecosystems and it is in this capacity that the role of nitrogen in the Earth's climate system has been considered. Despite this, few studies have included continuous variation in plant N status as a driver of broad-scale carbon cycle analyses. This is partly due to uncertainties in how leaf-level physiological relationships scale to whole ecosystems and because methods for regional to continental detection of plant N concentrations have yet to be developed. In recent work, we have shown that that ecosystem CO2 uptake capacity in temperate and boreal forests scales directly with whole-canopy nitrogen concentrations, mirroring a leaf-level trend that has been observed for woody plants worldwide. We further show that both CO2 uptake capacity and canopy nitrogen concentration are strongly and positively correlated with shortwave surface albedo (Ollinger et al. 2008). These results suggest that nitrogen plays an additional, and previously overlooked, role in the climate system via its influence on vegetation reflectivity and shortwave surface energy exchange.





    Although these findings have important implications for ecosystem-climate interactions, the specific mechanisms driving the observed linkages remain unclear. For example, although canopy %N and albedo are significantly correlated, other canopy traits such as LAI and canopy structure could underlie the observed trends. We have begun using data from HF and other sites within the AmeriFlux network to explore the basis for these relationships by examining linkages among structure, chemistry and reflectance at the leaf to canopy scales. Canopy structure has been characterized using lidar remote sensing as a means of mapping the 3-dimensional surface geometry of upper canopy surfaces (a property known as rugosity). Thus far, neither rugosity nor LAI have shown any relationship with nitrogen or albedo, but the nitrogen-albedo relationship remains intact. At the leaf level, spectral reflectance measurements of fresh foliage from a variety of deciduous and evergreen species show a significant relationship between mean full-spectrum reflectance (functionally similar to albedo) and nitrogen (R2 = 0.54). Moreover, a stronger relationship was observed between full-spectrum reflectance and leaf mass per unit area (LMA). This result is consistent with reports from research conducted decades ago that the cellular structure of foliage is a key property in regulating absorption of light reaching the Earth's surface (e.g. Gates et al. 1965, Knipling 1970).





    Reference:





    Gates, D.M., Keegan, H.J., Schleter, J.C., Weidner, V.R. 1965. Spectral properties of plants, Appl. Optics 4, 11-20.





    Knipling, E.B. 1970. Physical and physiological basis for the reflectance of visible and near-infrared radiation from vegetation. Rem. Sens. Env. 1, 155-159.





    Ollinger, S.V., A.D. Richardson, M.E. Martin, D.Y. Hollinger, S.E. Frolking, P.B. Reich, L.C. Plourde, G.G. Katul, J.W. Munger, R. Oren, M-L. Smith, K.T. Paw U, P.V. Bolstad, B.D. Cook, M.C. Day, T.A. Martin, R.K. Monson, H.P. Schmid. 2008. Canopy nitrogen, carbon assimilation and albedo in temperate and boreal forests: Functional relations and potential climate feedbacks. Proceedings of the National Academy of Sciences. 105(49): 1933519340.


  • Research Category: Forest-Atmosphere Exchange, Regional Studies

  • Figures:
  • D:DocsAbstractsHF Abstract_Ollinger_2009_Fig1.pdf
    D:DocsAbstractsHF Abstract_Ollinger_2009_Fig2.pdf
    D:DocsAbstractsHF Abstract_Ollinger_2009_Fig3.pdf