Variation in foliar nitrogen and albedo in response to nitrogen fertilization and elevated CO2
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Foliar nitrogen has been shown to be positively correlated with midsummer canopy albedo and canopy near infrared (NIR) reflectance over a broad range of plant functional types (e.g., forests, grasslands, and agricultural lands). To date, the mechanism(s) driving the nitrogen–albedo relationship have not been established, and it is unknown whether factors affecting nitrogen availability will also influence albedo. To address these questions, we examined variation in foliar nitrogen in relation to leaf spectral properties, leaf mass per unit area, and leaf water content for three deciduous species subjected to either nitrogen (Harvard Forest, MA, and Oak Ridge, TN) or CO2 fertilization (Oak Ridge, TN). At Oak Ridge, we also obtained canopy reflectance data from the airborne visible/infrared imaging spectrometer (AVIRIS) to examine whether canopy-level spectral responses were consistent with leaf-level results. At the leaf level, results showed no differences in reflectance or transmittance between CO2 or nitrogen treatments, despite significant changes in foliar nitrogen. Contrary to our expectations, there was a significant, but negative, relationship between foliar nitrogen and leaf albedo, a relationship that held for both full spectrum leaf albedo as well as leaf albedo in the NIR region alone. In contrast, remote sensing data indicated an increase in canopy NIR reflectance with nitrogen fertilization. Collectively, these results suggest that altered nitrogen availability can affect canopy albedo, albeit by mechanisms that involve canopy-level processes rather than changes in leaf-level reflectance.
KeywordsAlbedo Nitrogen Leaf structure Nitrogen fertilization Free air CO2 enrichment
We thank G. James Collatz for helpful comments on a draft of this manuscript, Rob Braswell for providing the SAIL-2 model code, and Richard Norby, Colleen Iversen, and Jeffery Warren for support at ORNL. We are indebted to Michael Eastwood, ER-2 pilots Denis Steel, Tim Williams, and the rest of the AVIRIS team for aircraft data acquisition. This work was funded by a grant from the North American Carbon Program (NACP) NASA’s Terrestrial Ecology and Carbon Cycle Science Programs and a graduate fellowship provided by the Research and Discover program. The ORNL FACE experiment was supported by the US Department of Energy, Office of Science, Biological and Environmental Research Program. A.D.R. and M.K.B. acknowledge support, through the Harvard Forest REU program, from the National Science Foundation (Grant DBI-04-52254).
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