Abstract ID: 481

Characterizing Seasonal Patterns of Leaf Phenology in Central Amazon Várzea Forest with the MODIS Enhanced Vegetation Index

Evasion of carbon dioxide and methane from Amazonian floodplain environments is an important component of the regional carbon budget. Seasonally flooded forests, covering approximately 70% of the mainstem Amazon floodplain (várzea), supply large amounts of carbon through leaf-fall linked to the annual flood cycle. Knowledge of the timing of leaf-fall and green-up of várzea forests is necessary for biogeochemical modeling, but previously has been limited to a handful of field sites. This study explored whether time series of Enhanced Vegetation Index (EVI) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) could be used to track seasonal changes in greenness of várzea forests. MODIS 16-day composite EVI, NDVI (Normalized Difference Vegetation Index), and VI Quality Assurance (VI-QA) values for 2000-2005 were extracted for 21 closed-canopy forest sites along the Solimões-Amazon floodplain west of Manaus. VI values were filtered to exclude dates with VI-QA values greater than 3. For each polygon, a cubic smoothing spline was fit to the median for all valid pixels on each date, and the resulting time series was examined in conjunction with river stage levels for the Manacapuru gauge. All sites showed a regular seasonal variation in EVI, ranging from a mean low for all sites of 0.41 to a mean high of 0.61. The amplitude of variability in NDVI was about 50% that of EVI. Minimum EVI, corresponding to minimum leaf area, occurred in late May about one month preceding maximum river stage, and EVI peaked in mid-October, about 40 days before lowest river levels. The phase of the EVI curve was thus opposite to that of river stage and offset 30-40 days in advance. These results are in general agreement with published field observations of leaf phenology at várzea stands near Manaus and the Purus confluence. We conclude that MODIS EVI is an appropriate surrogate for seasonal changes in greenness of várzea forests. However, since the seasonal EVI patterns noted here are similar to those reported for terra firme forest and attributed to seasonal variations in solar irradiance, future work will seek to clarify the relationship to flooding by extending the analysis to sites along rivers with annual flood waves out of phase with the solar irradiance cycle.

Session:  Biogeochemistry - Floodplain ecosystem processes.

Presentation Type:  Oral