Abstract
Commercially available autonomous photochemical reflectance index (PRI) sensors are a new development in the remote sensing field that offer novel opportunities for a deeper exploration of vegetation physiology dynamics. In this study, we evaluated the reliability of autonomous PRI sensors (SRS-PRI) developed by METER Group Inc. as proxies of light use efficiency (LUE) in an aspen (Populus tremuloides) forest stand. Before comparisons between PRI and LUE measurements were made, the optical SRS-PRI sensor pairs required calibrations to resolve diurnal and seasonal patterns properly. An offline diurnal calibration procedure was shown to account for variable sky conditions and diurnal illumination changes affecting sensor response. Eddy covariance measurements provided seasonal gross primary productivity (GPP) measures as well as apparent canopy quantum yield dynamics (α). LUE was derived from the ratio of GPP to absorbed photosynthetically active radiation (APAR). Corrected PRI values were derived after diurnal and midday cross-calibration of the sensor’s 532 nm and 570 nm fore-optics, and closely related to both LUE (R2 = 0.62, p < 0.05) and α (R2 = 0.72, p < 0.05). A LUE model derived from corrected PRI values showed good correlation to measured GPP (R2 = 0.77, p < 0.05), with an accuracy comparable to results obtained from an α driven LUE model (R2 = 0.79, p < 0.05). The automated PRI sensors proved to be suitable proxies of light use efficiency. The onset of continuous PRI sensors signifies new opportunities for explicitly examining the cause of changing PRI, LUE, and productivity over time and space. As such, this technology represents great value for the flux, remote sensing and modeling community.
Highlights
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 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
The objectives of this paper are to (1) determine if automated SRS-photochemical reflectance index (PRI) values can be used as a proxy of canopy light use efficiency and apparent canopy quantum yield over an aspen forest and (2) to explore how continuous PRI measurements can be leveraged to better inform ecosystem scale LUE models
Summary
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 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. 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 Sensors 2018, 18, 3302; doi:10.3390/s18103302 www.mdpi.com/journal/sensors
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