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Harvard Forest Symposium Abstract 2010

• Title: Variation in foliar nitrogen and albedo in response to N fertilization
• Primary Author: Haley Wicklein (University of New Hampshire - Main Campus)
• Additional Authors: Megan Bartlett (Harvard University); Michelle Day (University of New Hampshire - Main Campus); David Hollinger (USDA Forest Service); Lucie Lepine (University of New Hampshire - Main Campus); Mary Martin (University of New Hampshire - Main Campus); Scott Ollinger (University of New Hampshire - Main Campus); Andrew Richardson (Northern Arizona University)
• Abstract:

  Recently, it has been demonstrated that foliar nitrogen (N) and photosynthetic capacity are significantly and positively correlated with shortwave surface albedo (the fraction of incident solar radiation that is reflected by a surface) over a broad range of forests and plant functional types (Ollinger et al. 2008, Hollinger et al. 2009). However, the mechanism(s) driving the N- albedo relationship remain elusive and few studies have examined how increased N levels affect biophysical factors such as albedo. There are many sources of variation in albedo, ranging from chemical and structural properties at the leaf level, to micro-scale structural changes in the canopy (i.e. leaf clumping, leaf inclination angle) to macro-scale differences in the canopy or landscape (i.e. canopy volume, leaf area index, spatial vegetation patterns). This study investigated the hypothesis that optical changes at the leaf level due to (or covarying with) changes in N integrate to whole canopy albedo. If the albedo-N relationship is driven by changes at the leaf level, we would expect to see higher albedo in plots with high N fertilization treatments than those receiving solely ambient N deposition.
  
  
  
  The chronic N amendment site at Harvard Forest in Petersham, MA includes three treatments: high N (fertilized with 150 kg N ha-1 yr-1), low N (50 kg N ha-1 yr-1), and ambient deposition (around 8 kg N ha-1 yr-1). In each treatment plot we measured seven black oak (Quercus velutina) and five red maple (Acer rubrum) individuals. Leaves were collected from two to three canopy heights from trees in each treatment plot. For each tree height we measured reflectance and transmittance spectra for stacks of 1, 2, 4, and 8 leaves, both abaxial and adaxial sides. We also measured % N, % lignin, water content and equivalent water thickness (EWT), and leaf mass per unit area (LMA) of the leaves. Analyses were carried out separately for each height class (upper, mid, lower), however all height classes exhibited the same patterns. Although foliar %N was significantly higher in the high N treatment than the low N or control treatments, there were no significant differences in leaf level shortwave albedo, transmittance, or absorption (single leaves, adaxial side, over the 500 to 2225 nm range) between treatments for either red maples or black oaks. Similarly there was no difference in NIR (700 to 1300 nm) reflectance or transmittance between treatments, although visible (500 to 700 nm) reflectance was significantly higher in the high N treatment for both species, likely due to an increase in pigment concentration. There was also no significant relationship between LMA, water content, EWT, or lignin concentration and leaf-level albedo for either the red maples or black oaks. Our hypothesis that the canopy level relationship between foliar %N and albedo is caused by variability in leaf level reflectance properties was not substantiated. Other mechanisms that may be responsible for the N-albedo relationship observed over whole ecosystems include differences in leaf angle and orientation, leaf clumping along stems and branches, and branch and/or canopy structure.
  
  
  
  References:
  
  
  
  Hollinger, D.Y., S.V. Ollinger, A.D. Richardson, T.P. Meyers, D.B. Dail, M.E. Martin, N.A. Scott, T.J. Arkebauer, D.D. Baldocchi, K.L. Clark, P.S. Curtis, K.J. Davis, A.R. Desai, D. Dragoni, M.L. Goulden, L. Gu, G.G. Katul, S.G. Pallardy, K.T. Paw U, H.P. Schmid, P.C. Stoy, A.E. Suyker, and S.B. Verma. 2009. Albedo estimates for land surface models and support for a new paradigm based on foliage nitrogen concentration. Global Change Biology 16: 696-710.
  
  
  
  Ollinger, S.V., A.D. Richardson, M.E. Martin, D.Y. Hollinger, S. 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, and H.P. Schmid. 2008. Canopy nitrogen, carbon assimilation and albedo in temperate and boreal forests: functional relations and potential climate feedbacks. Proc. National Acad. Sciences 105: 19335–19340.

• Research Category: Large Experiments and Permanent Plot Studies