Abstract
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.
Highlights
[2] Many tropical forests, including those in the eastern Amazon basin, experience extended dry seasons
We focused on two questions: (1) How does leaf area index (LAI) vary seasonally at km-83? (2) What controls the seasonal changes in whole-forest photosynthesis at km-83? We addressed the first question by developing a new data set of the leaf phenology at km-83
We addressed the second question with a series of simple models that distinguished between the possibility that seasonal change in LAI is the main driver of seasonal change in CO2 uptake, and the alternative possibility that leaf aging is the main driver of seasonal changes in CO2 uptake
Summary
[2] Many tropical forests, including those in the eastern Amazon basin, experience extended dry seasons. Researchers [Malhi et al, 1998] and models [Williams et al, 1998] indicated that evergreen tropical forest photosynthesis declines during the dry season with drought stress. More recent studies within the Large-Scale BiosphereAtmosphere Experiment in Amazonia (LBA-ECO) have found that evergreen tropical forests often avoid drought stress [Saleska et al, 2003], and that the photosynthetic capacity of some forests increases before the end of the dry season. Goulden et al [2004] reported that the light-saturated rate of canopy photosynthesis at the LBA-ECO Tapajos km-83 evergreen forest, which experiences a dry season from $July through $December, was $30% greater from October to April than from May to September. We addressed the second question with a series of simple models that distinguished between the possibility that seasonal change in LAI is the main driver of seasonal change in CO2 uptake, and the alternative possibility that leaf aging is the main driver of seasonal changes in CO2 uptake
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