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

Authors: YANG, J.C. - University Of Arizona; MAGNEY, T.S. - California Institute Of Technology; YAN, D. - University Of Arizona; Knowles, John; SMITH, W.K. - University Of Arizona; Scott, Russell - Russ; BARRON-GAFFORD, G.A. - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2020
Publication Date: 10/27/2020
Citation: Yang, J., Magney, T., Yan, D., Knowles, J.F., Smith, W., Scott, R.L., Barron-Gafford, G. 2020. The Photochemical Reflectance Index (PRI) captures the ecohydrologic sensitivity of a semi-arid mixed conifer forest. Journal of Geophysical Research-Biogeosciences. 125(11), Article 005624. https://doi.org/10.1029/2019JG005624.
DOI: https://doi.org/10.1029/2019JG005624

Interpretive Summary: The exchange of carbon and water between forests and the atmosphere via photosynthesis is the primary mechanism by which carbon dioxide is removed from Earth’s atmosphere. As a result, changes in this relationship due to drought or other environmental stress have the potential to influence the future trajectory of Earth’s climate with broad socio-economic implications. For decades, satellites have been able to measure the "greenness" of vegetation, but these methods do not probe deeply enough beneath the surface to provide information about plant function and photosynthesis in forests that remain green year-round. In contrast, a newly developed alternative remote sensing method, termed the photochemical reflectance index, or PRI, is able to detect rapid changes in leaf pigments that are responsible for regulating plant stress that can be detrimental to growth. Here, we used a tall tower to collect PRI data from a semi-arid evergreen conifer forest in Arizona, USA to show that variations in PRI corresponded to the way in which plants regulated water loss as environmental conditions changed throughout the course of a typical growing season. The PRI was thus an effective indicator of the photosynthetic response to environmental conditions in this warm, dry ecosystem that may mimic the increasingly stressful conditions that are forecast for montane forest vegetation in the future. These results demonstrate that PRI has the potential to advance scientific understanding of how water availability influences forest growth and subsequent carbon dioxide removal from the atmosphere.

Technical Abstract: 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 regulating 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 ecohydrologic variability and associated changes in 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 ecohydrologic 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 and moisture 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.