Seasonal Behaviour of Vegetation of the Amazon Basin

Introduction The Amazon basin represents a major component of regional and global carbon and water cycles. Understanding seasonal and spatial variations of tropical forest structure and functioning in Amazonia is important for understanding and predicting the fate of Amazon forests under climate change. Seasonality in productivity of tropical forests is more subtle than that observed in temperate zones that are dominated by an active growing and a dormant season. Field-based studies of tree phenology in the humid tropics show flowering and flushing of new leaves along with increases in leaf area during the dry season when solar radiation availability is larger relative to the more cloudy wet season. Eddy covariance flux tower measurements show enhanced photosynthesis and larger rates of evapotranspiration in the dry season. Tower measurements are crucial for understanding the mechanisms of functional phenology in tropical forests. Satellite-based remote sensing data also show forest greening at the basin-scale during the dry season, thereby greatly extending site-based studies. Earth system models have generally represented Amazon forests as water-limited or with year-round constant leaf area and thus predict dry season declines in productivity. However, some models show enhanced productivity in the dry season. There are many challenges in field, satellite and model assessments of tropical forest phenology, with field methods time consuming and limited to narrow spatial and temporal scales, satellite data subject to cloud contamination and optical artefacts and models subject to uncertainties. The paradigms of light-limitation or water limitation in tropical forests remain controversial, yet increasingly there is a consensus that basin-wide predictions of modelled carbon and water fluxes can be constrained by incorporation of remote sensing data and local flux measurements. Biogeography of the Amazon Basin Knowledge of spatial and seasonal variations in Amazon tropical forests resulting from meteorological drivers, soils, topography, and disturbance (fire, logging, deforestation) is important for understanding their ecological function and future fate with climate change. The Amazon Basin encompasses an area of 7.5 × 10 6 km 2 and is covered mostly of dense tropical evergreen broadleaf rainforests from wet equatorial forests to tropical dry forests. These primary forests are mostly closed canopies with high LAI of 5–7, and are approximately 40 m tall (Goulden et al . 2004).