Canopy Nitrogen, Carbon Assimilation and the Albedo of North American Forest Ecosystems

Terrestrial ecosystems can influence the Earth's climate through exchanges of carbon with the atmosphere and through their influence on land surface albedo. We present evidence from an ongoing NACP study suggesting that these two mechanisms are related and that linkages between them are moderated by the nitrogen status of plant canopies. Across a diverse series of forested AmeriFlux sites, ecosystem photosynthetic capacity was positively correlated with canopy N concentrations as estimated from air and space-borne imaging spectroscopy. Although canopy N detection has traditionally focused on narrow-band spectral features, we also found strong correlations with simpler and widely available reflectance properties and with albedo estimates from the MODIS satellite sensor. Our findings indicate that high nitrogen ecosystems have more reflective canopies, absorb less radiant energy and fix more carbon than their low N counterparts. If these patterns can be shown to exist more broadly, our results suggest a previously unrecognized feedback in the Earth's climate system involving the nitrogen cycle as a moderator of carbon cycling and surface energy exchange under changing environmental conditions.