Canopy Nitrogen, Carbon Assimilation, and Albedo in Temperate and Boreal Forests: Functional Relations and Potential Climate Feedbacks. Ollinger, S. V., Richardson, A. D., Martin, M. E., Hollinger, D. Y., Frolking, S. E., Reich, P. B., Plourde, L. C., Katul, G. G., Munger, J. W., Oren, R., Smith, M. L., Paw, Bolstad, P. V., Cook, B. D., Day, M. C., Martin, T. A., Monson, R. K., & Schmid, H. P. Proceedings of the National Academy of Sciences, 105(49):19336–19341, December, 2008.
doi  abstract   bibtex   
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 CO2 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 CO2 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. [Excerpt: Discussion] The strong linkages among canopy %N, shortwave albedo, and CAmax raise the possibility of an unrecognized feedback in the Earth's climate system involving the N cycle as a factor influencing surface energy exchange, in addition to its known effects on C assimilation. As an example, warming trends in boreal and arctic regions result in warmer soils, accelerated decomposition, and increased nutrient mineralization (22). Expected consequences include an increase in the N nutrition of plant canopies (23) or a shift in species composition to those requiring higher N supplies and having higher N concentrations in foliage. Although the effects of changes in major plant functional groups have been included in coupled land surface-climate models (24), those associated with changes in N cycling or shifts in species composition within biomes have not. Based on our findings, these specific changes (i.e., increased N supply or altered composition, both resulting in higher canopy %N) should increase gross C assimilation [and presumably net C sequestration (4, 25)] and surface albedo, both representing negative feedbacks to warming. Similar effects can be expected from anthropogenic N deposition and widespread use of N fertilizers, which also serve to increase canopy %N (26). Conversely, leaf N concentrations in temperate forests are inversely related to mean annual temperature (27) and are driven down by rising CO2 (6$\Downarrow$-8), suggesting that temperature and CO2 increases could lead to reduced canopy albedo and a positive feedback to warming. Additional scenarios can be envisioned, but the overall implication is that alterations to plant N availability have important and unrecognized impacts on the climate system that warrant serious consideration (see SI Text for details on surface heating effects over the observed range of %N and albedo). This is particularly relevant in light of the degree to which human activities have altered the N cycle globally (28). [\n] It would be premature to draw management implications from these results or to conclude that N fertilization, increased N pollutant emissions, or shifts from low-N to high-N tree species would help offset climate warming induced by greenhouse gas emissions. We also cannot assume that changes in C assimilation will always lead to a change in net C sequestration, given the potential for synergistic changes in ecosystem respiration. Moreover, interrelations among canopy %N, CAmax, and albedo need to be evaluated in additional ecosystem types, particularly in tropical forests, agricultural systems, and ecosystems with low LAI or sparse vegetation cover, and assessing the full outcome of competing feedbacks will require new analyses using coupled climate-vegetation models. Finally, our analysis did not address N-induced changes in transpiration or potential consequences for cloud formation, which could have important effects on the net change in energy budgets (15) but were beyond the scope of our study. Despite these uncertainties, the observation of strong N-C-albedo linkages in forests add a new dimension to our understanding of the role played by ecosystems within the climate system and warrant consideration in climate projection efforts given the extent to which all of these variables are being altered by human activities. [\n] [...]
@article{ollingerCanopyNitrogenCarbon2008,
  title = {Canopy Nitrogen, Carbon Assimilation, and Albedo in Temperate and Boreal Forests: Functional Relations and Potential Climate Feedbacks},
  author = {Ollinger, S. V. and Richardson, A. D. and Martin, M. E. and Hollinger, D. Y. and Frolking, S. E. and Reich, P. B. and Plourde, L. C. and Katul, G. G. and Munger, J. W. and Oren, R. and Smith, M. L. and {Paw} and Bolstad, P. V. and Cook, B. D. and Day, M. C. and Martin, T. A. and Monson, R. K. and Schmid, H. P.},
  year = {2008},
  month = dec,
  volume = {105},
  pages = {19336--19341},
  issn = {1091-6490},
  doi = {10.1073/pnas.0810021105},
  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 CO2 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 CO2 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.

[Excerpt: Discussion]

The strong linkages among canopy \%N, shortwave albedo, and CAmax raise the possibility of an unrecognized feedback in the Earth's climate system involving the N cycle as a factor influencing surface energy exchange, in addition to its known effects on C assimilation. As an example, warming trends in boreal and arctic regions result in warmer soils, accelerated decomposition, and increased nutrient mineralization (22). Expected consequences include an increase in the N nutrition of plant canopies (23) or a shift in species composition to those requiring higher N supplies and having higher N concentrations in foliage. Although the effects of changes in major plant functional groups have been included in coupled land surface-climate models (24), those associated with changes in N cycling or shifts in species composition within biomes have not. Based on our findings, these specific changes (i.e., increased N supply or altered composition, both resulting in higher canopy \%N) should increase gross C assimilation [and presumably net C sequestration (4, 25)] and surface albedo, both representing negative feedbacks to warming. Similar effects can be expected from anthropogenic N deposition and widespread use of N fertilizers, which also serve to increase canopy \%N (26). Conversely, leaf N concentrations in temperate forests are inversely related to mean annual temperature (27) and are driven down by rising CO2 (6{$\Downarrow$}-8), suggesting that temperature and CO2 increases could lead to reduced canopy albedo and a positive feedback to warming. Additional scenarios can be envisioned, but the overall implication is that alterations to plant N availability have important and unrecognized impacts on the climate system that warrant serious consideration (see SI Text for details on surface heating effects over the observed range of \%N and albedo). This is particularly relevant in light of the degree to which human activities have altered the N cycle globally (28).

[\textbackslash n] It would be premature to draw management implications from these results or to conclude that N fertilization, increased N pollutant emissions, or shifts from low-N to high-N tree species would help offset climate warming induced by greenhouse gas emissions. We also cannot assume that changes in C assimilation will always lead to a change in net C sequestration, given the potential for synergistic changes in ecosystem respiration. Moreover, interrelations among canopy \%N, CAmax, and albedo need to be evaluated in additional ecosystem types, particularly in tropical forests, agricultural systems, and ecosystems with low LAI or sparse vegetation cover, and assessing the full outcome of competing feedbacks will require new analyses using coupled climate-vegetation models. Finally, our analysis did not address N-induced changes in transpiration or potential consequences for cloud formation, which could have important effects on the net change in energy budgets (15) but were beyond the scope of our study. Despite these uncertainties, the observation of strong N-C-albedo linkages in forests add a new dimension to our understanding of the role played by ecosystems within the climate system and warrant consideration in climate projection efforts given the extent to which all of these variables are being altered by human activities. 

[\textbackslash n] [...]},
  journal = {Proceedings of the National Academy of Sciences},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-3789965,~to-add-doi-URL,albedo,boreal-forests,climate,climate-change,feedback,forest-resources,nitrogen,temperate-forests},
  lccn = {INRMM-MiD:c-3789965},
  number = {49}
}
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