Diurnal Changes in Leaf Photochemical Reflectance Index in Two Evergreen Forest Canopies

Abstract

The spectral properties of plant leaves relate to the state of their photosynthetic apparatus and the surrounding environment. An example is the well known photosynthetic downregulation, active on the time scale from minutes to hours, caused by reversible changes in the xanthophyll cycle pigments. These changes affect leaf spectral absorption and are frequently quantified using the photochemical reflectance index (PRI). This index can be used to remotely monitor the photosynthetic status of vegetation, and allows for a global satellite-based measurement of photosynthesis. Such earth observation satellites in near-polar orbits usually cover the same geographical location at the same local solar time at regular intervals. To facilitate the interpretation of these instantaneous remote PRI measurements and upscale them temporally, we measured the daily course of leaf PRI in two evergreen biomes-a European boreal forest and an Amazon rainforest. The daily course of PRI was different for the two locations: At the Amazonian forest, the PRI of Manilkara elata leaves was correlated with the average photosynthetic photon flux density (PPFD) (R 2 = 0.59, p < 0.01) of the 40 minutes preceding the leaf measurement. In the boreal location, the variations in Pinus sylvestris needle PRI were only weakly (R 2 = 0.27, p < 0.05) correlated with mean PPFD of the preceding two hours; for Betula pendula, the correlation was insignificant (p > 0.5) regardless of the averaging period. The measured daily PRI curves were specific to species and/or environmental conditions. Hence, for a proper

Remote sensing tracks daily radial wood growth of evergreen needleleaf trees

“…Furthermore, it will be critical to examine the suitability of PRI to track intra‐annual radial tree growth dynamics in other ecosystems dominated by different evergreen needleleaf tree species. This is an important consideration given that the sensitivities of PRI to changing environmental conditions have been shown to be species specific (Gitelson et al., 2017; Mõttus et al., 2019). Our findings also beg the question of whether there are tenable relationships between the magnitude of PRI change and radial growth dynamics.…”

“…Rugged, in situ radiometers (Garrity, Vierling, & Bickford, 2010; Pontailler & Genty, 1996) enable tree‐level PRI measurements that—unlike tower‐, air‐, or space‐based measurements—minimize the confounding effects of phenological changes in understory broadleaf and graminoid species that often contribute to the canopy reflectance signal (Rautiainen et al., 2012). Tree‐mounted radiometers can collect PRI datasets across a range of weather conditions, minimizing bias towards fair weather conditions often prevalent in datasets collected with handheld radiometers (Mõttus et al., 2019) or from satellites (due to cloud cover; Eitel et al., 2019). Such automated PRI time series data are therefore appropriate for studying continuous measurement of changes in stem radius from automated point dendrometers mounted on the same tree.…”

“…Tree-mounted radiometers can collect PRI datasets across a range of weather conditions, minimizing bias towards fair weather conditions often prevalent in datasets collected with handheld radiometers (Mõttus et al, 2019) or from satellites (due to cloud cover; Eitel et al, 2019). Such automated PRI time series data are therefore appropriate for studying continuous measurement of changes in stem radius from automated point dendrometers mounted on the same tree.…”

Smart Indoor Plantation System Using Soil Moisture Sensor and Light Dependent Resistor Sensor

“…Photoconductivity is the ability of a photoresistor to lower its resistance as the intensity of the light it receives rises [33], [37], [41]. As well as lightsensitive detector circuits, photoresistors may be used in circuits that are triggered by either light or dark [42]. High-resistance semiconductors are used to make photoresistor devices.…”

Early Diagnosis of Vegetation Health From High-Resolution Hyperspectral and Thermal Imagery: Lessons Learned From Empirical Relationships and Radiative Transfer Modelling

“…To correct for this and detect photoprotection, the unstressed (i.e. predawn) PRI value needs to be subtracted from leaf measurements, which can be further elaborated to deconvolving daily and seasonal changes [79]. Unfortunately, this task requires multiple successful image acquisitions in a single day, which may not always be feasible.…”