We used in situ and satellite measurements to investigate the seasonal patterns of leaf area index (LAI) and gross ecosystem CO2 exchange (GEE) by an evergreen tropical forest. The forest experienced a dry season from June through November. The rates of light-saturated CO2 uptake (GEE) were comparatively high from December through March and low from May through July. In situ measurements showed that LAI varied seasonally, with a minimum from May through September. Leaf production and leaf abscission were reduced from December through April. Leaf abscission increased in May, which reduced LAI. High rates of leaf abscission and production occurred from July through September associated with leaf turnover. Leaf abscission decreased abruptly in October, while production continued, which rapidly increased LAI. Leaf phenology was not directly correlated with changes in soil water. The seasonal cycle of in situ LAI differed markedly from the seasonal cycles of in situ normalized difference vegetation index (NDVI) and the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD15 LAI product. We hypothesize that the NDVI and MOD15 seasonality at the site is driven partly by seasonal changes in leaf age and leaf reflectance. We developed three simple models to investigate the causes of GEE seasonality. The first two models showed that the seasonal changes in LAI alone, and the effects of leaf age on leaf-level photosynthesis alone, could not account for the observed GEE seasonality. The third model showed that the combined effect of seasonal changes in LAI and seasonal changes in leaf age and leaf photosynthesis was sufficient to account for the observed GEE seasonality.