The Photochemical Reflectance Index (PRI) Captures the Ecohydrologic Sensitivity of a Semiarid Mixed Conifer Forest

Abstract

At the seasonal time scale, daily photochemical reflectance index (PRI) measurements track changes in photoprotective pigment pools as plants respond to seasonally variable environmental conditions. As such, remotely sensed PRI products present opportunities to study seasonal processes in evergreen conifer forests, where complex vegetation dynamics are difficult to capture due to small annual changes in chlorophyll content or leaf structure. Because PRI is tied explicitly to short-and long-term changes in photoprotective pigments that are responsible for regulating stress, we hypothesize that PRI by extension could serve as a proxy for stomatal response to seasonally changing hydroclimate, assuming plant functional responses to stress covary in space and time. To test this, we characterized PRI in a semiarid, montane mixed conifer forest in the Madrean sky islands of Arizona, USA, during the monsoon growing season subject to precipitation pulse dynamics. To determine the sensitivity of PRI to ecohydrologic variability and associated changes in gross primary productivity (GPP), canopy spectral measurements were coupled with eddy covariance CO 2 flux and sap flow measurements. Seasonally, there was a significant relationship between PRI and sap flow velocity (R 2 = 0.56), and multiple linear regression analysis demonstrated a PRI response to dynamic water and energy limitations in this system. We conclude that PRI has potential to serve as a proxy for forest functional response to seasonal ecohydrologic forcing. The coordination between photoprotective pigments and seasonal stomatal regulation demonstrated here could aid characterization of vegetation response to future changes in hydroclimate at increasing spatial scales. Plain Language Summary In order to understand how ecosystems interact with climate and how these relationships may change under future conditions, it is necessary to study how carbon and water move between plants and the atmosphere and how environmental stress, such as changing water availability, impacts these processes. For decades, satellites used in remote sensing studies have been able to measure how "green" an ecosystem is, but for forests that remain visibly green year-round even under stressful conditions, these greenness indices may not probe deeply enough beneath the surface to provide a clear picture of plant function. An alternative reflectance-based index, termed the photochemical reflectance index, or PRI, is able to detect seasonal changes in leaf pigments that are responsible for regulating plant stress. Here, we use tower-based PRI data from a semiarid conifer forest to show that variations in PRI correspond to the way in which plants regulate their water loss as moisture and temperature conditions change throughout the growing season. This result demonstrates that PRI is an effective indicator of how ecosystems respond to stressful environmental conditions, and therefore has the potential to advance our ability to distinguish how water availability influences forest productivity.

Heatwave breaks down the linearity between sun‐induced fluorescence and gross primary production

“…in the afternoon of the hotter days) the response of NPQ saturated, indicating that the temporal kinetics of stomata and the light reactions can decouple during heatwave events (Meinzer et al ., 2017). The apparent link between NPQ and stomatal conductance, also suggested in previous research (Medrano et al ., 2002; van der Tol et al ., 2009) and exploited through RS with the PRI index (Suárez et al ., 2008; Yang et al ., 2020), needs to be further investigated. In particular, a process‐based understanding of the relationship between stomatal conductance, NPQ and SIF needs to be established to correctly use SIF as a proxy for transpiration.…”

Drone‐based physiological index reveals long‐term acclimation and drought stress responses in trees

“…Among the carotenoid‐sensitive indices, PRI was most successful in capturing changes in photochemical efficiency over short timescales and was thus selected over CCI for this study. Its potential as a proxy for monitoring drought stress has been amply investigated for Mediterranean vegetation, where summer drought represents the primary environmental constraint (Goerner, Reichstein, & Rambal, 2009 ; Hernández‐Clemente, Navarro‐Cerrillo, Suárez, Morales, & Zarco‐Tejada, 2011 ; Marino et al, 2014 ; Peguero‐Pina, Morales, Flexas, Gil‐Pelegrín, & Moya, 2008 ; Peñuelas, Munné‐Bosch, Llusià, & Filella, 2004 ; Stylinski, Gamon, & Oechel, 2002 ; Yang et al, 2020 ; Zarco‐Tejada, González‐Dugo, & Berni, 2012 ; Zhang, Preece, Filella, Farré‐Armengol, & Peñuelas, 2017 ). Among these are early studies by Zarco‐Tejada and colleagues, which demonstrate the ability of airborne PRI to track seasonal and diurnal changes in stem water potential, stomatal conductance and canopy to air temperature differences in Spanish tree orchard ecosystems under different irrigation regimes (e.g., Suárez et al, 2008 ; Suárez et al, 2010 ; Zarco‐Tejada et al, 2012 ; Zarco‐Tejada et al, 2013 ).…”

Is foliar spectrum predictive of belowground bacterial diversity? A case study in a peach orchard

“…Previous studies reported similar results of foliar spectral traits in assessing plant attributes (productivity) in other semi-arid ecosystems. Vegetation indexes, such as NDVI, GI, and OSAVI, reflected information of leaf area index and chlorophyll content, and were often used in inversion of vegetation coverage and growth status (Li et al, 2009;Ferna et al, 2018;Yang et al, 2020). NDVI and OSAVI estimated green biomass and vegetative coverage in grass ecosystem (Ferna et al, 2018), while PRI estimates instant light use efficiency at leaf scale, which was used to track changes of photoprotective pigment pools and capture eco-hydrologic sensitivity of a mixed conifer forest (Yang et al, 2020).…”

“…Vegetation indexes, such as NDVI, GI, and OSAVI, reflected information of leaf area index and chlorophyll content, and were often used in inversion of vegetation coverage and growth status (Li et al, 2009;Ferna et al, 2018;Yang et al, 2020). NDVI and OSAVI estimated green biomass and vegetative coverage in grass ecosystem (Ferna et al, 2018), while PRI estimates instant light use efficiency at leaf scale, which was used to track changes of photoprotective pigment pools and capture eco-hydrologic sensitivity of a mixed conifer forest (Yang et al, 2020). Daily PRI measurements could be used to track changes of photoprotective pigment pools as plants respond to seasonal environment change.…”