Photosynthetic light use efficiency (LUE) determines the ability of a plant to assimilate atmospheric carbon dioxide to biomass and is known to be controlled by environmental conditions, light regimes and forest age. The photochemical reflectance index (PRI), derived from leaf or canopy remotely sensed spectra, has been shown to be an effective and accurate estimator of LUE. In this study, we propose a new LUE estimation method that separates the PRI into daily maximal PRI (PRI0) for indicating daily maximal light use efficiency (LUE_max) and ΔPRI, defined as the difference between PRI0 and instantaneous PRI, for estimating the diurnal physiological stress (fstress). We develop and apply the method across three temperate pine stands and a deciduous stand of different ages, in Southern Ontario, Canada. Half hourly canopy level spectra were acquired from a tower-based spectro-radiometer system (AMSPEC-III) over the growing season at the four stands. Results show that the PRI0 predicted well LUE_max (R² > 0.6, p < 0.05) in both coniferous and deciduous stands and was able to track seasonal changes in pigment pools sizes. The ΔPRI was sensitive to short-term meteorological conditions, specifically temperature, vapor pressure deficit (VPD), and light variations resulting in strong correlations (p < 0.05) with fstress and half hourly LUE. This new method significantly improves the estimation accuracy (R² increases from 0.1 to around 0.7) for PRI-based LUE estimation across all four stands of varying age and species composition and suggests that PRI-based LUE estimation has the ability to inform on both the effects of seasonal and diurnal change in photosynthetic efficiency under different meteorological conditions.

光合光利用效率（LUE）决定植物将大气中的二氧化碳吸收为生物质的能力，并且已知受环境条件，光照制度和森林年龄的控制。由叶或冠层遥感光谱得出的光化学反射指数（PRI）已被证明是LUE的有效和准确的估算器。在这项研究中，我们提出了一种新的LUE估计方法，该方法将PRI分为每日最大PRI（PRI0），以指示每日最大光利用效率（LUE _max）和ΔPRI，定义为PRI0和瞬时PRI之间的差，用于估计日生理压力（压力）。我们在加拿大安大略省南部的三个温带松树林和不同年龄的落叶林中开发和应用该方法。在四个站的生长季节中，从塔式分光辐射计系统（AMSPEC-III）获得了半小时的冠层水平光谱。结果表明，PRI0可以很好地预测 针叶和落叶林的LUE _max（R ² > 0.6，p <0.05），并且能够追踪色素库大小的季节性变化。ΔPRI对短期气象条件敏感，特别是温度，蒸气压赤字（VPD）和光线变化，导致与应力的相关性强（p <0.05）半小时的费用 这种新方法显着提高了在年龄和物种组成各不相同的所有四个林分中基于PRI的LUE估计的估计准确性（R ²从0.1增加到0.7左右），并表明基于PRI的LUE估计能够告知两个气象条件下季节和昼夜变化对光合效率的影响