Canopy nitrogen, carbon assimilation, and albedo in temperate and boreal forests: Functional relations and potential climate feedbacks

Abstract

The availability of nitrogen represents a key constraint on carbon cycling in terrestrial ecosystems, and it is largely in this capacity that the role of N 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 because of 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. Here, we show that ecosystem CO 2 uptake capacity in temperate and boreal forests scales directly with whole-canopy N concentrations, mirroring a leaf-level trend that has been observed for woody plants worldwide. We further show that both CO 2 uptake capacity and canopy N concentration are strongly and positively correlated with shortwave surface albedo. These results suggest that N plays an additional, and overlooked, role in the climate system via its influence on vegetation reflectivity and shortwave surface energy exchange. We also demonstrate that much of the spatial variation in canopy N can be detected by using broad-band satellite sensors, offering a means through which these findings can be applied toward improved application of coupled carbon cycle-climate models.nitrogen cycle ͉ climate change ͉ foliar nitrogen ͉ ecosystem-climate feedback ͉ remote sensing T errestrial ecosystems influence the Earth's climate through regulation of mass and energy exchange with the atmosphere. In recent years, much of the focus has been directed toward factors affecting the Ϸ1-to 2-Pg carbon (C) sink that is believed to exist in the terrestrial biosphere (1). Candidate mechanisms include forest regrowth after agricultural abandonment (2) and growth enhancement from nitrogen (N) deposition and/or elevated CO 2 (3, 4). In each case, interactions between C and N play a central role. Observations of N limitations to productivity are widespread (5), and responses to experimental CO 2 fertilization are often restricted by N availability (6-8). Although evidence of an N deposition effect on net C exchange has only recently been documented (4), the importance of N as a regulator of C assimilation is well established through the widely observed relationship between leaf-level photosynthetic capacity (A max ) and foliar N concentrations (9-11). Despite this understanding, few analyses have included continuous variation in plant N status as a driver of C cycle processes at regional to global scales. There are at least 2 reasons for this. First, because evidence for the photosynthesis-foliar N relationship comes predominantly from leaf-level observations, there is uncertainty about whether similar trends occur over whole-plant canopies or whether canopy-level factors such as leaf area index (LAI) become dominant at this scale. Second, although many ecosystem models simulate plant and soil N dynamics, there are no widely available methods for obtaining mapped foliar N estimates over ...

Albedo estimates for land surface models and support for a new paradigm based on foliage nitrogen concentration

“…When leaf area index is high and below a nitrogen concentration of $ 2.5%, both Ollinger et al (2008) and this dataset show a strong linear relationship with similar slopes; here, a foliage nitrogen change from 1% to 2% resulted in an albedo increase of 0.067, in Ollinger et al (2008) who limited their analysis to forests, the same change in nitrogen increased albedo by 0.05. Based on albedo data from several soybean fields (Fig.…”

“…Previously (Ollinger et al, 2008), we hypothesized that the nitrogen-albedo correlation results from how leaf structure covaries with leaf function. This involves two well-known associations between form and function; that between leaf nitrogen and photosynthetic capacity (e.g.…”

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Predicting macronutrient concentrations from loblolly pine leaf reflectance across local and regional scales

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The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes

“…Surface albedo varies strongly in space and time depending on seasonal trends, soil-vegetation contrasts, canopy chemistry and structure (Ollinger et al, 2008). If the albedo of the soil surface is significantly different from above-canopy albedo, for example, any changes in vegetation cover that may expose the soil can be important (Berbet & Costa, 2003).…”