Chlorophyll Fluorescence Data Reveals Climate-Related Photosynthesis Seasonality in Amazonian Forests
Amazonia is the world largest tropical forest, playing a key role in the global carbon cycle. Thus, understanding climate controls of photosynthetic activity in this region is critical. The establishment of the relationship between photosynthetic activity and climate has been controversial when based on conventional remote sensing-derived indices. Here, we use nine years of solar-induced chlorophyll fluorescence (ChlF) data from the Global Ozone Monitoring Experiment (GOME-2) sensor, as a
... sensor, as a direct proxy for photosynthesis, to assess the seasonal response of photosynthetic activity to solar radiation and precipitation in Amazonia. Our results suggest that 76% of photosynthesis seasonality in Amazonia is explained by seasonal variations of solar radiation. However, 13% of these forests are limited by precipitation. The combination of both radiation and precipitation drives photosynthesis in the remaining 11% of the area. Photosynthesis tends to rise only after radiation increases in 61% of the forests. Furthermore, photosynthesis peaks in the wet season in about 58% of the Amazon forest. We found that a threshold of ≈1943 mm per year can be defined as a limit for precipitation phenological dependence. With the potential increase in the frequency and intensity of extreme droughts, forests that have the photosynthetic process currently associated with radiation seasonality may shift towards a more water-limited system. chlorophyll content and photosynthetic activity [8, 9] , coherently tracks seasonal changes in tower-based photosynthesis measurements and canopy phenology           . Results based on the analysis of EVI show that the seasonality of photosynthesis in Amazonia is strongly linked to temporal fluctuations of radiation and precipitation [15, 16, 19, 20] . More specifically, satellite observations demonstrate that photosynthetic activity tends to raise following radiation increases during the dry season across Amazonia            . This process is explained by the occurrence of new leaf flushing triggered by increased radiation availability during the dry season [7, 10, 12, 18] . New leaves are more light-use efficient than old senescent leaves. This process occurs where forests are not limited by water, because their water demands can be supplied during the dry season by the redistributed subsurface water of the wet season  . However, in southern Amazonia, where water availability constrains leaf phenology, this process is not observed, with photosynthesis declining during the dry season in these areas [19, 20] . Although EVI is commonly applied as a proxy for the photosynthetic capacity of vegetation in Amazonia [10, 16, 19, 21] , the biophysical interpretation of this index has been controversial  . Previous studies have questioned the observed greening during the dry season in Amazonia, suggesting that the observed patterns can be related to artifacts in the EVI data [22, 23] or changes in forest structure  . However, the recent use of MODIS products, fully corrected for removing atmospheric and radiometric artifacts  , provide solid evidence about the climate controls of phenology and photosynthesis seasonality [15, 16, 19, 20] . Remote sensing data, therefore, provide potentially powerful observations of whole system patterns that can be used to mechanistically test forest-climate interactions.