Seasonal Changes in Leaf Area of Amazon Forests from Leaf Flushing and Abscission

Author: Arindam Samanta
Date: 
December 8, 2011
Type: 
Poster presentation
Venue: 
AGU Fall Meeting 2011
Citation: 

Arindam Samanta, Y. Knyazilhin, L. Xu, R. Dickinson, R. Fu, M.H. Costa, S. Ganguly, S.S. Saatchi, R.R. Nemani, R. Myneni (2011) Seasonal Changes in Leaf Area of Amazon Forests from Leaf Flushing and Abscission. AGU Fall Meeting 2011.

A large increase in near-infrared (NIR) reflectance of Amazon forests during the light-rich dry season and a corresponding decrease during the light-poor wet season has been observed in satellite measurements. This has been variously interpreted as seasonal changes in leaf area resulting from net leaf flushing in the dry season and net leaf abscission in the wet season, enhanced photosynthetic activity during the dry season from flushing new leaves and as change in leaf scattering and absorption properties between younger and older leaves covered with epiphylls. Reconciling these divergent views using theory and observations is the goal of this article. The observed changes in NIR reflectance of Amazon forests could be due to similar, but small, changes in NIR leaf albedo (reflectance plus transmittance) only, from exchanging older leaves with newer ones, with total leaf area unchanged. However, this argument ignores accumulating evidence from ground-based studies of higher leaf area in the dry season relative to the wet season, seasonal changes in litterfall and does not satisfactorily explain why NIR reflectance of these forests decreases in the wet season. A more convincing explanation for the observed increase in NIR reflectance during the dry season and decrease during the wet season is one that invokes changes in both leaf area and leaf optical properties. Such an argument is consistent with known phonological behavior of tropical forests, ground-based reports of seasonal changes in leaf area, litterfall, leaf optical properties and fluxes of evapotranspiration, and thus, reconciles the various seemingly divergent views.