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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 C[O.sub.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 C[O.sub.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

We used the eddy-correlation technique to investigate the exchange of CO2 between an undisturbed old-growth forest and the atmosphere at a remote Southern Hemisphere site on 15 d between 1989 and 1990. Our goal was to determine how environmental factors regulate ecosystem CO2 exchange, and to test whether present knowledge of leaf-level processes was sufficient to understand ecosystem-level exchange. On clear summer days the maximum rate of net ecosystem CO2 uptake exceeded 15 microns mol center dot m to the -2 center dot s to the -1, about an order of magnitude greater than the maximum values observed on sunny days in the winter. Mean nighttime respiration rates varied between approximately equal to -2 and -7 microns mol center dot m to the -2 center dot s to the -1. Nighttime CO2 efflux rate roughly doubled with a 10 degrees C increase in temperature. Daytime variation in net ecosystem CO2 exchange rate was primarily associated with changes in total photosynthetically active photon flux density (PPFD). Air temperature, saturation deficit, and the diffuse PPFD were of lesser, but still significant, influence. These results are in broad agreement with expectations based on the biochemistry of leaf gas exchange and penetration of radiation through a canopy. However, at night, the short-term exchange of CO2 between the forest and the atmosphere appeared to be regulated principally by atmospheric transport processes. There was a positive linear relationship between nocturnal CO2 exchange rate and downward sensible heat flux density. This new result has implications for the development of models for diurnal ecosystem CO2 exchange. The daytime light-use efficiency of the ecosystem (CO2 uptake/incident PPFD) was between 1.6 and 7.1 mmol/mol on clear days in the summer but decreased to 0.8 mmol/mol after frosts on clear winter days. Ecosystem CO2 uptake was enhanced by diffuse PPFD, a result of potentially global significance given recent increases in Northern Hemisphere haze.

The chaotic state of the atmosphere transcends the leaf-air interface to affect plant processes such as water use and transport. The intensity and frequency of atmospheric disturbances and plant-atmosphere coupling affect stomatal response and plant function. Atmospheric humidity-free transpirations affect exchange between forest vegetation and the atmosphere.