Ollinger et al. (1) report that albedo depends on canopy nitrogen. A critic may dismiss this as correlation-does-notequal- causation because the underlying mechanisms linking albedo with nitrogen are unclear. Albedo is controlled by processes other than nitrogen, for instance, canopy water content (2, 3). Moist dark soil has a lower albedo than dry bright soil (4), so a drier canopy should change the albedo without changing the canopy nitrogen. Similarly, within the spatial resolution of a MODIS pixel, a tree may fall, revealing bare soil and thus changing the albedo of the pixel (5). Would the canopy nitrogen change appropriately? Ollinger et al. (1) excluded patterns in photosynthetically active radiation wavelengths, thus making it difficult to separate albedo from the canopy versus other objects (essentially the basis of vegetation indices). The albedo of the surface would increase with snowfall (not predicted by canopy nitrogen), though this effect can easily be excluded.
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