Flush foliar na estação seca em floresta de terra firme na Amazônia central via webcam

Abstract

There is an ongoing debate about the response of Amazon forests to extreme and to normal seasonal drought. Satellite observed Amazon green-up during the 2005 drought remains controversial, with green-up attributed by some to cloud/aerosol contamination. Satellite detection of dry-season green-up in years of normal rainfall is also confounded in the southern Amazon by decreasing solar zenith angle. To address these concerns by direct assessment of changes in the forest canopy, a high-resolution (1028x720p) 3-band video camera was mounted 54 m above the forest floor on an eddy covariance tower 60 km north of Manaus. Camera aim was south, perpendicular to the solar transit, with an oblique downward view. Automatic color balance was turned off. For 134 days from the middle of the dry season to mid rainy season (23 Sept, 2010 to 03 Feb, 2011) one frame was captured every 10s. From these, a single frame was chosen per day, obtained under diffuse solar illumination near local noon with minimal variation (< +-10%) in overall reflected scene brightness. Auto-exposure of the camera compensated this minor variation. Changes in greenness and woodiness were examined using three approaches: (1) visual classification of transition types for all trees that underwent marked full-crown changes during the dry season; (2) relative green channel brightness averaged over the entire frame; and (3) unstandardized Principal Component Analysis of the entire frame to remove spatial variations in illumination (PC1) and estimate area of exposed bark (threshold of PC2). Of 13 trees that underwent full-crown transitions in the first month (dry season), 11 were experiencing some stage of leaf flush . Relative brightness of the green channel over the entire frame showed that a flush of new leaves was already taking place at the beginning of the study and continued at least into October, when leaf maturation caused changes in leaf colors, limiting the usefulness of this indicator. The area of exposed bark decreased steeply during the mid-dry season leaf flush and further suggested that leafing-out continued at a slower pace into the rainy season. Due to spectral stability of exposed bark over time, this is a more reliable indicator of leaf phenology than the leaves themselves. The results of this study contribute an understanding of what controls seasonality of vegetation greenness in Amazonia and its photosynthesis response.