For everything there is a season, including Amazonian tropical forests

Seasonality of productivity in tropical forests gives insight into the ecological question of resource limitation in these important high-biomass, climatically sensitive habitats. Diverse evidence from ecological studies, eddy-flux towers, and satellites had accumulated by the mid-2000s suggesting that many tropical forests are more light- than water-limited, and hence "green-up" during higher-sunlight annual dry seasons. Recent work, however, argues that the satellite-based evidence (from MODIS) of dry-season green-up in Amazon forests is an artifact of seasonal variations in sun-sensor geometry. Here we review three lines of evidence to address this new controversy about Amazon forest seasonality: first, we show that even after correcting for sun-sensor geometry artifacts, remotely sensed MODIS data significantly rejects the null hypothesis of no seasonal change in canopy greenness; second we use a re-analysis of eddy flux measurements at four towers across the equatorial Amazon to show that dry season increases in canopy-scale photosynthetic capacity are robust, independent of seasonal variations of climatic drivers. Finally, tower-mounted cameras at two of the eddy flux sites provide new independent evidence for "green-up" by showing that crown-scale leaf-flushing events are concentrated in dry seasons. This work shows that remote sensing observations remain consistent with those from ground-based towers in support of the conclusion that many Amazon forests green-up with sunlight in the dry season.