For everything there is a season, including Amazonian tropical forests

Monday, 15 December 2014
Scott R Saleska1, Jin Wu1, Bruce W Nelson2, Julia V Tavares3, Loren Albert1, Neill Prohaska1, Kaiyu Guan4, Rodrigo da Silva5, Alessandro C De Araujo6, Antonio Donato Nobre2, Natalia Restrepo-Coupe7 and Alfredo R Huete8, (1)University of Arizona, Tucson, AZ, United States, (2)INPA National Institute of Amazonian Research, Manaus, Brazil, (3)National Institute for Amazon Research (INPA), Manaus, AM, Brazil, (4)Stanford University, Stanford, CA, United States, (5)Universidade Federal do Oeste do Pará, Santarem, Brazil, (6)EMBRAPA Brazilian Agricultural Research Corporation- EMBRAPA, Embrapa Amazonia Oriental, Belem, Brazil, (7)University of Technology Sydney, Ultimo, Australia, (8)University of Technology Sydney, Plant Functional Biology and Climate Change, Ultimo, Australia
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.