Abstract
Although quantifying the massive exchange of carbon that takes place over the Amazon Basin remains a challenge, progress is being made as the remote sensing community moves from using traditional, reflectance-based vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), to the more functional Photochemical Reflectance Index (PRI). This new index, together with satellite-derived estimates of canopy light interception and Sun-Induced Fluorescence (SIF), provide improved estimates of Gross Primary Production (GPP). This paper traces the development of these new approaches, compares the results of their analyses from multiple years of data acquired across the Amazon Basin and suggests further improvements in instrument design, data acquisition and processing. We demonstrated that our estimates of PRI are in generally good agreement with eddy-flux tower measurements of photosynthetic light use efficiency (ε) at four sites in the Amazon Basin: r2 values ranged from 0.37 to 0.51 for northern flux sites and to 0.78 for southern flux sites. This is a significant advance over previous approaches seeking to establish a link between global-scale photosynthetic activity and remotely-sensed data. When combined with measurements of Sun-Induced Fluorescence (SIF), PRI provides realistic estimates of seasonal variation in photosynthesis over the Amazon that relate well to the wet and dry seasons. We anticipate that our findings will steer the development of improved approaches to estimate photosynthetic activity over the tropics.
Highlights
Tropical forests affect the global climate through their massive exchange of carbon, water and heat
Photochemical Reflectance Index (PRI) values, averaged for the period 2000 to 2012, were generally higher when acquired in the forward direction (Figure 5) compared with those acquired from the backscatter direction (Figure 6)
PRIf values were lowest in the northern Amazonia during the months of February, March and
Summary
Tropical forests affect the global climate through their massive exchange of carbon, water and heat. Under optimal conditions, absorbed radiation is utilized by the plant to split water (photochemical photosynthesis (ε): quenching) and provide electrons for the photosynthetic fixation of CO2. Supply of radiation in excess,absorbed the xanthophyll pigment is converted (photochemical quenching) and provide electrons for the photosynthetic fixation of CO2. Rapidly via intermediate antheraxanthin to zeaxanthin, and this reaction is reversed when radiation is in situations where plants receive more sunlight than they can use Opportunities foroffuture experimental, they have the potential of providing improved estimates of GPP at increasingly refined
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