Solar-induced Chlorophyll Fluorescence (SIF) and the Photochemical Reflectance Index (PRI) are proxies of plant physiological processes including photosynthesis and the xanthophyll cycle. Sub-hourly measurements of SIF and PRI from near-surface remote sensing provide real-time monitoring of plant physiological response to changing environmental conditions. However, we have very limited data on how these proxies perform in dryland ecosystems. In this study, we present the response of SIF and PRI to vapor pressure deficit and root zone soil moisture at diurnal to seasonal scales from June through November of 2018 at arid grassland (US-WKG) and semi-arid evergreen conifer forest (US-MtB) eddy covariance (EC) sites in southeastern Arizona. For comparison, we also include measurements of Normalized Difference Vegetation Index (NDVI) and EC-derived gross primary productivity (GPP). To better understand seasonal drought dynamics, we further divided the months of interest into pre-monsoon, monsoon, and post-monsoon periods, and we examined the correlation between environmental and response variables at both diurnal and seasonal time scales for each period. We have previously found that large-scale and intermittent satellite measurements of greenness and SIF are closely correlated with grassland productivity. In evergreen forests, however, greenness and productivity are more decoupled. We expect the results of this work to provide new insights into the potential for remote sensing-based, real-time monitoring of the plant physiological response to water stress.
Yan, D., Scott, R.L., Biederman, J.A., Barron-Gafford, G., Yang, X., Moore, D.J.P., Dannenberg, M.P., Wang, X., Knowles, J.F., Yang, J., Smith, W.K. (2018): Quantifying plant physiological response to water stress with high-frequency, near-surface observations of chlorophyll fluorescence and photochemical reflectivity. Abstract B31N-2694 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.