Abstract
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 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.
Highlights
The vulnerability of Amazonian forests to climate change depends on the physiological responses of plants to changes in the climate system
The primary aim of this paper was to analyze the dependence of photosynthesis seasonality on climate in Amazonia, using a new remote sensing dataset, chlorophyll fluorescence (ChlF) from Global Ozone Monitoring Experiment-2 (GOME-2), which is independent of vegetation indices commonly used to study photosynthetic activity
In this study we aim to answer three research questions: (i) How does photosynthesis vary as a function of climate seasonality? (ii) What is the association between climate-drivers of photosynthesis seasonality and the spatial gradient of mean annual precipitation? (iii) When is the peak of maximum photosynthesis activity in Amazonia? To elucidate these questions, monthly ChlF data from the GOME-2 sensor [41] was analyzed in conjunction with remote sensing based precipitation data, derived from the Tropical Rainfall Measuring Mission (TRMM) [45] and surface incident shortwave radiation, derived from the Global Land Data Assimilation System (GLDAS) [46]
Summary
The vulnerability of Amazonian forests to climate change depends on the physiological responses of plants to changes in the climate system. Photosynthesis is the primary process linking the climate system with the biosphere [4]. Quantifying photosynthesis across Amazonia, the world largest tropical forest, is fundamentally important for the global carbon cycle. This biome contributes up to 14% of all carbon fixed by photosynthesis in the terrestrial biosphere [5]. Current formulations of Earth System Models (ESM) are unable to correctly capture the direction of seasonal changes in Amazonian photosynthesis as observed by flux tower data [6,7], which shows, contrarily to models, a steady increase in photosynthesis starting in the dry season
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