Affecting Factors and Recent Improvements of the Photochemical Reflectance Index (PRI) for Remotely Sensing Foliar, Canopy and Ecosystemic Radiation-Use Efficiencies

Chao Zhang,Martín Garbulsky,Josep Peñuelas,Iolanda Filella

doi:10.3390/rs8090677

Abstract

Accurately assessing terrestrial gross primary productivity (GPP) is crucial for characterizing the climate-carbon cycle. Remotely sensing the photochemical reflectance index (PRI) across vegetation functional types and spatiotemporal scales has received increasing attention for monitoring photosynthetic performance and simulating GPP over the last two decades. The factors confounding PRI variation, especially on long timescales, however, require the improvement of PRI understanding to generalize its use for estimating carbon uptake. In this review, we summarize the most recent publications that have reported the factors affecting PRI variation across diurnal and seasonal scales at foliar, canopy and ecosystemic levels; synthesize the reported correlations between PRI and ecophysiological variables, particularly with radiation-use efficiency (RUE) and net carbon uptake; and analyze the improvements in PRI implementation. Long-term variation of PRI could be attributed to changes in the size of constitutive pigment pools instead of xanthophyll de-epoxidation, which controls the facultative short-term changes in PRI. Structural changes at canopy and ecosystemic levels can also affect PRI variation. Our review of the scientific literature on PRI suggests that PRI is a good proxy of photosynthetic efficiency at different spatial and temporal scales. Correcting PRI by decreasing the influence of physical or physiological factors on PRI greatly strengthens the relationships between PRI and RUE and GPP. Combining PRI with solar-induced fluorescence (SIF) and optical indices for green biomass offers additional prospects.

Highlights

Terrestrial gross carbon uptake, expressed as gross primary productivity (GPP), and its response to climatic changes play a key role in projections of future carbon cycles and climate [1,2]
We describe the principal suggestions for improving the estimation of Radiation-use efficiency (RUE) and carbon uptake using photochemical reflectance index (PRI)
Damm et al [108] demonstrated the sensitivity of canopy reflectance and vegetation indices derived from spatial and spectral high-resolution data to varying irradiances, and reported that unknown direct/diffuse irradiance caused by complex interactions of surface irradiance and reflectance anisotropy accounted for up to 32% of the uncertainty of PRI for crops

Summary

Introduction

Terrestrial gross carbon uptake, expressed as gross primary productivity (GPP), and its response to climatic changes play a key role in projections of future carbon cycles and climate [1,2]. Observations of vegetation primary productivity in situ, and empirical [6] or process-based [1,7] models have been successfully used to estimate the global distribution of GPP These measurements, rarely provide high-quality data and contain errors originating from the uncertainties of field work, which impede a comprehensive understanding of the global terrestrial carbon cycle [2,7]. The quantification of GPP variation by remote-sensing techniques is generally based on a model of radiation-use efficiency (RUE) [12,13]. This model mainly considers the absorbed photosynthetically active radiation (APAR) and the actual photochemical efficiency [12,13]. APAR has been extensively analyzed and is usually derived from vegetation indices of greenness such as the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI) [10,14,15,16,17]

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