OCO-2 Solar-Induced Chlorophyll Fluorescence Variability across Ecoregions of the Amazon Basin and the Extreme Drought Effects of El Niño (2015–2016)

Abstract

Amazonian ecosystems are major biodiversity hotspots and carbon sinks that may lose species to extinction and become carbon sources due to extreme dry or warm conditions. We investigated the seasonal patterns of high-resolution solar-induced chlorophyll fluorescence (SIF) measured by the satellite Orbiting Carbon Observatory-2 (OCO-2) across the Amazonian ecoregions to assess the area´s phenology and extreme drought vulnerability. SIF is an indicator of the photosynthetic activity of chlorophyll molecules and is assumed to be directly related to gross primary production (GPP). We analyzed SIF variability in the Amazon basin during the period between September 2014 and December 2018. In particular, we focused on the SIF drought response under the extreme drought period during the strong El Niño in 2015–2016, as well as the 6-month drought peak period. During the drought´s peak months, the SIF decreased and increased with different intensities across the ecoregions of the Amazonian moist broadleaf forest (MBF) biome. Under a high temperature, a high vapor pressure deficit, and extreme drought conditions, the SIF presented differences from −31.1% to +17.6%. Such chlorophyll activity variations have been observed in plant-level measurements of active fluorescence in plants undergoing physiological responses to water or heat stress. Thus, it is plausible that the SIF variations in the ecoregions’ ecosystems occurred as a result of water and heat stress, and arguably because of drought-driven vegetation mortality and collateral effects in their species composition and community structures. The SIF responses to drought at the ecoregional scale indicate that there are different levels of resilience to drought across MBF ecosystems that the currently used climate- and biome-region scales do not capture. Finally, we identified monthly SIF values of 32 ecoregions, including non-MBF biomes, which may give the first insights into the photosynthetic activity dynamics of Amazonian ecoregions.

Capturing the Impact of the 2018 European Drought and Heat across Different Vegetation Types Using OCO-2 Solar-Induced Fluorescence

“…Physiological alterations include change in leaf biochemistry via reductions in enzymatic activity [84] or a reduction in mesophyll/chlorophyll, and stomatal conductance [85], resulting in lower CO 2 uptake by the plants and thus reduced photosynthesis (i.e., lower SIF values). The structural changes include foliar changes like leaf wilting or rolling (changes in leaf inclination), thus reducing effective leaf area index (LAI), which in turn causes a reduction in fPAR and energy available for photosynthesis and fluorescence emission [69,86]. Furthermore, plants adopt different mechanisms to minimize water and heat stress, such as chloroplast avoidance movement, during which plants move their chloroplast from the cell surface to side walls of cell [87] so as to minimize APAR [88] and thus SIF.…”

Unveiling spatial and temporal heterogeneity of a tropical forest canopy using high-resolution NIRv, FCVI, and NIRvrad from UAS observations

“…This semi-deciduous moist tropical forest receives approximately 2640 mm mean annual precipitation and has a mean temperature of 26 o C with a dry season from approximately January through April (Detto et al, 2018). There is high species diversity, with approximately 500 tree species, approximately 60 species per ha, and about 6.3% of trees at >30cm diameter at breast height (dbh) (Bohlman and O'brien, 2006;Condit et al, 2000).…”

“…One reason why vegetation indicators and LiDAR captured variability at spatial scales larger than the most common tree crown sizes on BCI is that canopy heights tend to be more uniform on BCI compared to other tropical forests, possibly due to wind (Bohlman and O'brien, 2006). For example, Dipterocarpus dominated South-East Asian forests have emergent trees that can reach up to 60 m in height.…”

“…The aforementioned complexity of measuring SIF and subsequent interpretation limit the number of studies using SIF in comparison to RIs (Hao et al, 2021;Zhang et al, 2020;Liangyun Liu, 2016;Van Wittenberghe et al, 2015;Baldocchi et al, 2020;Wang et al, 2020;Wu et al, 2020;Badgley et al, 2019). An additional barrier in tropical regions is that SIF data in tropical forests is even more scarce than in other biomes and, likely due to data scarcity and combined complexities of the SIF measurements and heterogeneity of tropical forest canopies, relationships to plant function in tropical forests are even more poorly understood than other biomes (Merrick et al, 2019;Wang et al, 2019;Yang et al, 2018a;Liu et al, 2017;Castro et al, 2020;Köhler et al, 2017).…”

Capturing the Impact of 2018 European Drought and Heat Using OCO-2 Solar-Induced Fluorescence

“…lower SIF values). The structural changes include foliar changes like leaf wilting or rolling (changes in leaf inclination), thus reducing effective leaf area index (LAI), which in turn causes a reduction in fPAR and energy available for photosynthesis and fluorescence emission [88,89]. Furthermore, plants adopt different mechanism to minimize water and heat stress, like chloroplast avoidance movement during which plants move their chloroplast from the cell surface to side walls of cell [90] so as to minimize APAR [91] and thus SIF.…”