Spatiotemporal Patterns and Phenology of Tropical Vegetation Solar-Induced Chlorophyll Fluorescence across Brazilian Biomes Using Satellite Observations

Abstract

Solar-induced fluorescence (SIF) has been empirically linked to gross primary productivity (GPP) in multiple ecosystems and is thus a promising tool to address the current uncertainties in carbon fluxes at ecosystem to continental scales. However, studies utilizing satellite-measured SIF in South America have concentrated on the Amazonian tropical forest, while SIF in other regions and vegetation classes remain uninvestigated. We examined three years of Orbiting Carbon Observatory-2 (OCO-2) SIF data for vegetation classes within and across the six Brazilian biomes (Amazon, Atlantic Forest, Caatinga, Cerrado, Pampa, and Pantanal) to answer the following: (1) how does satellite-measured SIF differ? (2) What is the relationship (strength and direction) of satellite-measured SIF with canopy temperature (T can ), air temperature (T air ), and vapor pressure deficit (VPD)? (3) How does the phenology of satellite-measured SIF (duration and amplitude of seasonal integrated SIF) compare? Our analysis shows that OCO-2 captures a significantly higher mean SIF with lower variability in the Amazon and lower mean SIF with higher variability in the Caatinga compared to other biomes. OCO-2 also distinguishes the mean SIF of vegetation types within biomes, showing that evergreen broadleaf (EBF) mean SIF is significantly higher than other vegetation classes (deciduous broadleaf (DBF), grassland (GRA), savannas (SAV), and woody savannas (WSAV)) in all biomes. We show that the strengths and directions of correlations of OCO-2 mean SIF to T can , T air , and VPD largely cluster by biome: negative in the Caatinga and Cerrado, positive in the Pampa, and no correlations were found in the Pantanal, while results were mixed for the Amazon and Atlantic Forest. We found mean SIF most strongly correlated with VPD in most vegetation classes in most biomes, followed by T can . Seasonality from time series analysis reveals that OCO-2 SIF measurements capture important differences in the seasonal timing of SIF for different classes, details masked when only examining mean SIF differences. We found that OCO-2 captured the highest base integrated SIF and lowest seasonal pulse integrated SIF in the Amazon for all vegetation classes, indicating continuous photosynthetic activity in the Amazon exceeds other biomes, but with small seasonal increases. Surprisingly, Pantanal EBF SIF had the highest total integrated SIF of all classes in all biomes due to a large seasonal pulse. Additionally, the length of seasons only accounts for about 30% of variability in total integrated SIF; thus, integrated SIF is likely captures differences in photosynthetic activity separate from structural differences. Our results show that satellite measurements of SIF can distinguish important functioning and phenological differences in vegetation classes and thus has the potential to improve our understanding of productivity and seasonality in the tropics.

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

“…SIF-fPAR temporal mismatches were found in 2015 [13], where the land surface temperature (proxy for PAR) and nadir BRDF (bidirectional reflectance distribution function) adjusted reflectance (NBAR) in the near-infrared band of the MODIS satellite (an fPAR proxy) showed a temporal mismatch with GOSAT SIF across the climate zones in Amazonia. We agree with the implications of previous findings on the seasonality of SIF in the Amazon [13], which showed that there is leaf phenological diversity [63] that cannot be captured at the scale of KG-climate zones [7,29] and biomes [30,44]. Thus, it may be valid to aggregate SIF time-series of retrievals over combined classes of ERs to capture SIF dynamics and magnitude differences.…”

“…The phenological variations in Figure 2B have distinctive yearly minimums and maximums which allow one to coarsely characterize each ER-SIF´s signature. Hence, our research findings confirm the great potential of OCO-2 in capturing the inter-biome temporal variability in the SIFs of different South American biome vegetation types [30].…”

“…The subtle intra-biome differences in monthly SIF 757 patterns across the MBF-ER (Figure 2A,B) suggest that there is an SIF phenological diversity that is finer in spatial resolution than the currently applied resolution of climate zones for assessing SIF (such as [7,29]). These phenological differences are evident between the ERs of different biomes and have been analyzed beyond the extent of the Amazon basin in Brazil [30]. The KG-climate zones combine the SIF values of areas from different biomes.…”

“…In 2009, the first global satellite SIF measurements were obtained from the GOSAT mission instruments [28]; since then, SIF has been employed to study drought´s impacts on vegetation [7,29] and to assess the diversity in leaf phenology [13,30] across Amazonia. GOSAT-SIF data reveal a productivity reduction of 15% based on the GPP in Amazonia during the 2010 extreme drought event [29].…”

“…These biomes are a set of ERs that meet climatic, biological, and ecological criteria ( Table 1). Although recognized at coarse global biophysical scales [11,30,44], the SIF variability across more specific vegetation types (e.g., the ER from [42]) remains unresolved. Hence, in this study, we analyze the high-resolution OCO2-SIF response across different ERs of the Amazon basin to investigate SIF´s seasonal phenology patterns.…”