Potential of Photochemical reflectance index for indicating photochemistry and light use efficiency in leaves of European beech and Norway spruce trees

Hyperspectral reflectance is becoming more frequently used for measuring the functions and productivity of ecosystems. The purpose of this study was to re-evaluate the potential of the photochemical reflectance index (PRI) for evaluating physiological status of plants. This is needed because the reasons for variation in PRI and its relationships to physiological traits remain poorly understood. We examined the relationships between PRI and photosynthetic parameters in evergreen Norway spruce and deciduous European beech grown in controlled conditions during several consecutive periods of 10-12 days between which the irradiance and air temperature were changed stepwise. These regime changes induced significant changes in foliar biochemistry and physiology. The responses of PRI corresponded particularly to alterations in the actual quantum yield of photosystem II photochemistry (ΦPSII). Acclimation responses of both species led to loss of PRI sensitivity to light use efficiency (LUE). The procedure of measuring PRI at multiple irradiance-temperature conditions has been designed also for testing accuracy of ∆PRI in estimating LUE. A correction mechanism of subtracting daily measured PRI from early morning PRI has been performed to account for differences in photosynthetic pigments between irradiance-temperature regimes. Introducing ∆PRI, which provided a better estimate of non-photochemical quenching (NPQ) compared to PRI, also improved the accuracy of LUE estimation. Furthermore, ∆PRI was able to detect the effect of drought, which is poorly observable from PRI

Potential of Photochemical reflectance index for indicating photochemistry and light use efficiency in leaves of European beech and Norway spruce trees

Hyperspectral reflectance is becoming more frequently used for measuring the functions and productivity of ecosystems. The purpose of this study was to re-evaluate the potential of the photochemical reflectance index (PRI) for evaluating physiological status of plants. This is needed because the reasons for variation in PRI and its relationships to physiological traits remain poorly understood. We examined the relationships between PRI and photosynthetic parameters in evergreen Norway spruce and deciduous European beech grown in controlled conditions during several consecutive periods of 10-12 days between which the irradiance and air temperature were changed stepwise. These regime changes induced significant changes in foliar biochemistry and physiology. The responses of PRI corresponded particularly to alterations in the actual quantum yield of photosystem II photochemistry (ΦPSII). Acclimation responses of both species led to loss of PRI sensitivity to light use efficiency (LUE). The procedure of measuring PRI at multiple irradiance-temperature conditions has been designed also for testing accuracy of ∆PRI in estimating LUE. A correction mechanism of subtracting daily measured PRI from early morning PRI has been performed to account for differences in photosynthetic pigments between irradiance-temperature regimes. Introducing ∆PRI, which provided a better estimate of non-photochemical quenching (NPQ) compared to PRI, also improved the accuracy of LUE estimation. Furthermore, ∆PRI was able to detect the effect of drought, which is poorly observable from PRI

Diurnal and Seasonal Proximally Sensed Photochemical Reflectance Index (PRI) in a High-Stress Semi-Arid Mixed Conifer Forest

A lack of accurate, reliable data on coupled carbon and water fluxes for Earth’s expansive ecosystems remains a major barrier to a complete understanding of the terrestrial carbon cycle. The remotely sensed Photochemical Reflectance Index (PRI) measures deepoxidation of the xanthophyll cycle at wavelength 531nm and is one of the few pigment-based vegetation indices sensitive to rapid plant physiological responses. PRI presents new opportunities to study ecosystems on a diurnal time scale, as well as seasonal processes in evergreen systems where complex vegetation dynamics are not reflected by small annual changes in chlorophyll content or leaf structure. However, systematic PRI acquisition in conjunction with leaf and ecosystem flux measurements are needed in natural, diverse ecosystems. The growing field of proximal remote sensing affords the opportunity to bridge leaf, canopy and ecosystem scales, for a physiological inspection of whole ecosystem dynamics. The Southwest U.S. provides a natural setting for examining the influence of environmental drivers on the productivity of drought-sensitive forests, as well as for evaluating our ability to track these relationships using optical methods. We studied PRI in a semi-arid, sub-alpine mixed conifer forest, in order to assess its ability to serve as a proxy for dynamic photoprotection. We combined canopy spectral measurements with eddy covariance flux and sap flow methods to determine the sensitivity of PRI to seasonal changes in gross primary productivity (GPP) and the ecohydrological variability of a high stress environment. In addition, we combined top-of-canopy leaf-level gas exchange, chlorophyll fluorescence, and hyperspectral measurements to determine the sensitivity of PRI to diurnal changes in needle photosynthetic function, and confirm the extent to which canopy diurnal patterns reflect leaf physiology. At the canopy scale we found that the relationship between PRI and GPP was inconsistent over the course of the monsoon season, shifting from a negative relationship in July and August (R2=.62), to a positive relationship in September (R2=.48). Multiple linear regression with soil moisture and air temperature showed that PRI responded to dynamic water and energy limitations of this system (R2=.41). We report for the first time a relationship between seasonal PRI and sap flow in a natural forest (R2=.55). These results suggest that on a seasonal scale PRI is an effective indicator of photosynthetic response to ecohydrological constraints. On a diurnal scale we found that PRI remained constant throughout the day at both leaf and canopy scales, and we suggest that saturated light conditions drive retention of xanthophylls in a de-epoxidized state. We contribute evidence that remotely sensed PRI has potential to fill a major gap in our ability to distinguish how water availability influences forest productivity and associated carbon dynamics

Utilisation des propriétés spectrales pour détecter le stress dans les peuplements nordiques d'épinettes noires

Dans la forêt boréale, l’augmentation de la fréquence et de la superficie d’îlots de pessières à lichens sur le territoire québécois a déjà été observée et pourrait résulter en une migration vers le sud de la limite nordique des pessières à mousses. Ce phénomène survient après des échecs de régénération, qui ont lieu lorsque le milieu est préalablement fragilisé lorsqu’une nouvelle perturbation affecte le peuplement. Avec la possibilité de détecter ce stress en analysant les propriétés spectrales de la végétation, les zones perturbées pourraient alors être identifiées. L’objectif principal de la présente étude est d’établir des liens entre d’une part, les informations extraites des signatures spectrales et d’autre part, les indices de végétation et les différents types de stress affectant les écosystèmes boréaux. Cela permettra de savoir s’il est possible d’identifier les pessières à mousses à risque de subir un accident de régénération en étudiant les propriétés spectrales de la végétation comme indicateur de stress. Pour répondre à cet objectif, des sites d’échantillonnage ont été positionnées aux 51e, 52e et 53e parallèles le long de la route de la baie James. Les placettes ont été regroupées en paires afin de faire des tests appariés et ainsi comparer les deux types de peuplements. Sur le terrain, les signatures spectrales ont été prises sur les feuilles aléatoirement prélevées sur cinq épinettes noires. Ces mesures ont été prises tout au long de la saison de croissance (3 campagnes d’échantillonnage). Quatre indices de végétation (NDVI, NDWI, PRI et SIPI) ont été extraits des signatures spectrales, et la pente moyenne du red-edge a été calculée. Les résultats obtenus ont permis de déterminer que certaines des pessières à mousses ont des valeurs très proches de celles des pessières à lichens, qui sont considérées comme des écosystèmes stressés. À partir de ces résultats, il est possible de supposer que le stress peut également être identifié à l’échelle du paysage (sur les images satellitaires) et ainsi permettre un suivi et une gestion après les feux et les épidémies afin de limiter les pertes de ce précieux écosystème.In the boreal forest of northern Québec, the size and quantity of lichen woodlands patches is increasing, and taking over the spruce-moss forest territory. The phenomenon has been observed, and scientists now believe that the northern limit of the spruce-moss forest will slowly move south. This shift of ecosystem happens when the forest stand is already fragilized, and a perturbation occurs. Vegetation’s spectral properties can be used as a tool to assess and identify disturbed forest areas The main objective of this study is to establish relations between data extracted from spectral signatures, vegetation indexes and different types of stress that could affect boreal ecosystems. The identification spruce-moss woodlands prone to regeneration failure could be achieved with the study of spectral properties as stress indicators. In order to achieve this objective, sites from 3 latitudes (51, 52 and 53) have been sampled on James Bay Road. Plots have been regrouped in pairs for subsequent pairwise statistical tests to compare results from both forest stand types. Spectral signatures have been measured on 5 randomly chosen black spruces. These measurements were taken throughout the growing season (3 sampling campaigns). Four vegetation indexes have been extracted from spectral signatures (NDVI, NDWI, PRI and SIPI), and the mean slope of the red-edge area have been calculated. Results have shown that some of the spruce-moss stands have had very similar values to those from the lichen woodlands, that are considered as stressed ecosystems. From these results, it is possible to assume that stressed ecosystems can be detected at landscape level (on satellite images). Monitoring vegetation stress can help improve forest management after forest fires and insect’s epidemics to prevent the loss of this beautiful ecosystem