## 1 Introduction  
The light use efficiency (LUE) model [1; 2] has provided an avenue for quantifying the terrestrial carbon cycle using remote sensing data [3; 4]. The LUE model can be conceptualized as being driven by structural and pigment pool changes in vegetation and, secondly, by the physiological status described by the vegetation's photosynthetic process [5]. The structural component is quantified by the amount of photosynthetically active radiation absorbed by the canopy (APAR); while the physiological component is described as the efficiency by which the absorbed light, or light use efficiency (LUE), is used to fix carbon during a specific period. Much work has been done in displaying a relationship between optical remote sensing and the APAR term, although it is not without error and complexity [6; 7; 8; 9; 10; 11]. However, as LUE variability can be driven by multiple individual and combined factors, the connection between efficiency and remote sensing signals have been harder to characterize, leading to considerable uncertainty in many ecosystems [12; 13; 14]. Some of the physiological effects on light use efficiency include changing climatic conditions, temperature, water availability, phenology, vegetation functional type, and vegetation species [15; 16; 17; 18; 19; 20; 21; 22]. These complex interactions confound the parametrization of in LUE in global productivity models, and, as such, efforts continue towards the better accounting of efficiency changes that would, in turn, lead to more accurate CO\({}_{2}\) flux estimations. Developments in proximal and spaceborne remote sensing have led to advances in estimating vegetation LUE at the leaf level, stand level, and whole ecosystem resolutions [14; 23; 24; 25]. One of the main proposed tools to do so is through the use of the photochemical reflectance index (PRI). First developed in leaf level studies, this vegetation index has been shown to be a proxy of xanthophyll pigment activity and closely linked to photosystem II (PSII) photochemical efficiency [26; 23; 27]. During periods of light saturation and excess energy PSII centers remain in a reduced state, leaving them susceptible to photoinhibitory damage [28]. Non-photochemical quenching (NPQ) performed through the chemical transformation of carotenoid pigments that compose the xanthophyll cycle prevent damage to photosynthetic centers. More specifically, NPQ of excess light is done through the de-epoxidation of violaxanthin to the photoprotective pigment zeaxanthin, with antheraxanthin acting as a transitional pigment [28; 29; 30]. De-epoxidation provides a sink for excess energy and a method of light regulation that can be measured optically via the PRI [27; 31; 32]. PRI tracks short-term decreases in reflectance at 531 nm that occur in response to increased zeaxanthin concentrations and shrinking chloroplast (associated with increased thylakoid pH) resulting from xanthophyll de-epoxidation [23; 27]. The index uses reflectance at 570 nm (insensitive to NPQ) as a reference band. Testing of PRI over seasonal timescales for foliar, canopy and ecosystem level scales has shown PRI to be a good indicator of physiological efficiency across different vegetation types and scales, however, with numerous temporal and spatial challenges [24]. Radiative transfer modeling studies have identified a number of confounding factors including leaf area distribution, soil type, and view and illumination angles affect the interpretation of PRI [33; 34]. Additionally, in situ studies have shown a strong effect of chlorophyll/carotenoid ratios on seasonal PRI [24]. These changes in pigment pool sizes can account for a significant proportion of seasonal PRI (constitutive) changes, compared to those driven by xanthophyll cycle activity (facultative changes), and lead to misrepresentations of the PRI-photosynthetic relationship [19; 35; 36; 37; 38]. As such, seasonal studies are needed to characterize the effects of vegetation structure changes and identify the facultative and constitutive components in PRI [39].