Abstract
Light use efficiency (LUE) is a key indicator of vegetation photosynthesis, which provides important insights into how vegetation productivity responds to environmental conditions. The photochemical reflectance index (PRI) is based on reflectance at 531 and 570 nm, which reflects the xanthophyll cycle process of plants under different radiation conditions, and makes LUE related to plant optical characteristics. In this study, tower-based PRI and eddy covariance (EC) based LUEs were used to explore the ability of PRI to track LUE variations in a subtropical, evergreen mixed forest in South China. The results indicate that there is a stronger relationship between PRI and LUE, corrected by the bidirectional reflectance distribution function (BRDF), where R2 = 0.46 before correction and R2 = 0.60 after correction. Generally, PRI is able to capture diurnal and seasonal changes in LUE. Simultaneously, this study highlights a significant correlation between LUE and PRI, but there is also a large seasonal difference in its correlation. The correlation in winter was significantly stronger than summer. The strongest correlation is found in November (R2 = 0.91) and the weakest is in July (R2 = 0.34). Photosynthetically active radiation (PAR) had a strong influence on the LUE-PRI relationship, while vapor pressure deficit (VPD) and air temperature (Ta) had negative influences on the relationship between LUE and PRI. Terrestrial laser scanning is used to retrieve the vertical structure of forest crown. Our results show that the vegetation canopy structure (i.e., effective leaf area index, LAIe), extracted from terrestrial laser scanning (TLS) point data in subtropical mixed forests, had a weak influence on LUE. Our research suggests that environmental factors and vegetation canopy structures should be considered when using PRI to accurately estimate LUE.
Highlights
Gross Primary Production (GPP) represents the amount of organic carbon fixed by terrestrial plants through photosynthesis and is the driving force for multiple ecosystem functions [1,2,3]
The heterogeneity of the forest canopy structure resulted in directional spectroscopic observations of the vegetation canopy reflectivity
Using the bidirectional reflectance distribution function (BRDF) model correction, the spatial structure characteristics of vegetation can be more accurately observed through spectral observations [45], can eliminate the uncertainty brought by the sun and observation angles, and increase the comparability of different observations
Summary
Gross Primary Production (GPP) represents the amount of organic carbon fixed by terrestrial plants through photosynthesis and is the driving force for multiple ecosystem functions [1,2,3]. Different ecosystems have different photosynthetic carbon absorption capacities and respond differently to changing environmental conditions. Plant leaves adjust their physiological mechanisms to maintain a balance between daylight photosynthesis and light protection. Light use efficiency (LUE) is determined by environmental factors such as nutrients, moisture, and atmospheric temperature, resulting in differences in time and space in the photochemical reaction process. Some have reported that vegetation types, nutrients, temperature, soil moisture, and canopy structure complicate the interpretation of LUE [13,14,15,16,17,18]. Canopy LUE varies with the radiation received by canopy, while PRI shows similar properties due to its sensitivity to the fraction of shaded or sunlit leaves [20]
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