Photochemical Reflectance Index (PRI) captures the ecohydrological sensitivity of a semi-arid mixed conifer forest



The Photochemical Reflectance Index (PRI) corresponds to the de-epoxidation state of the xanthophyll cycle and is one of the few pigment-based vegetation indices sensitive to rapid plant physiological responses. As such, new remotely-sensed PRI products present opportunities to study diurnal and seasonal processes in evergreen conifer forests, where complex vegetation dynamics are not well reflected by the small annual changes in chlorophyll content or leaf structure. Because PRI is tied explicitly to short and long term changes in xanthophyll pigments which are responsible for regulatig stress, this study characterized PRI in a semi-arid, sub-alpine mixed conifer forest, in order to assess its potential as a proxy for water stress by extension of its association with photoprotection. To determine the sensitivity of PRI to seasonal changes in ecohydrological variability and gross primary productivity, canopy spectral measurements were combined with eddy covariance flux and sap flow methods. Seasonally, there was a significant relationship between PRI and sap flow velocity (R2=0.56), and multiple linear regression analysis demonstrated the PRI response to dynamic water and energy limitations in this system. Although PRI was an effective indicator of stomatal response to ecohydrological constraints on a seasonal time scale, top-of-canopy leaf-level gas exchange, chlorophyll fluorescence, and hyperspectral reflectance measurements suggest that diurnal PRI saturates under conditions of severe light stress. This research indicates that remotely-sensed PRI has potential to fill spatial and temporal gaps in the ability to distinguish how water availability influences carbon dynamics of forested ecosystems.
Date made availableJan 2 2020

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