Photochemical reflectance index (PRI) has been a promising method to estimate vegetation photosynthetic efficiency. However, the capability of canopy PRI in detecting environmental stress degrees and its underlying mechanism remains unclear, due to the strong confounding effects between soil background and canopy characteristics. This study aimed to explain how the canopy PRI responds to soil moisture and canopy characteristics under soil drying with their relative contributions and influencing pathways. A set of maize field experimental data with various drought treatments was applied to distinguish the roles played by soil moisture and canopy characteristics in determining the variations of seasonal canopy PRI. The results showed that PRI has the capability to capture canopy stress across growing season during soil drying. The mean canopy PRI across the entire growth period closely correlated with and RUE across varying drought treatments. Besides, the fraction of absorbed photosynthetic available radiation (fAPAR), canopy water content (CWC), and canopy chlorophyll content (CCC) were more related with PRI than leaf area index. The available soil water content (ASWC) was not directly linked to PRI but shared a positive linear relationship with PRI after eliminating the effects of canopy characteristics (including LAI, CCC, and CWC). Moreover, fAPAR and LAI were the most important direct and indirect factors affecting canopy PRI, respectively. The effect of CWC on canopy PRI was indirectly achieved by LAI and fAPAR. The results suggested that the confounding effects of LAI, CCC, and CWC on canopy PRI can be well presented by fAPAR. These findings imply that accounting for the fAPAR contribution is necessary to improve the accuracy for estimating photosynthetic efficiency and then monitoring crop stress through remote sensing.

Remotely sensed solar-induced fluorescence (SIF) has emerged as a novel approach for terrestrial vegetation monitoring. The in situ continuous optical remote sensing tool in conjunction with concurrent eddy covariance (EC) flux measurements provides a new opportunity to advance terrestrial ecosystem science. Here we introduce a network of ground-based SIF observations at flux tower sites across the mainland China referred as ChinaSpec. Until now, it consists of 15 tower sites including 5 cropland sites, 4 grassland sites, 4 forest sites and 2 wetland sites. At each of these sites, an automated spectroscopy system was deployed to collect continuous super-high resolution spectra for high-frequency SIF retrievals in synergy with EC flux measurements. The goal of ChinaSpec is to provide ground SIF measurements and promote the collaborations between optical remote sensing and EC flux communities in China. We present here the details of instrument specifications, data collection and processing procedures, data sharing and utilization protocols, and future plans. Furthermore, we show the examples how ground SIF observations can be used to track vegetation photosynthesis from diurnal to seasonal scales, to assist in the validation of fluorescence models and satellite SIF products (e.g., from OCO-2, TanSat and TROPOMI) with the measurements from these sites since 2016. This network of SIF observations could improve our understanding of the controls on the biosphere-atmosphere carbon exchange and enable the improvement of carbon flux predictions. This SIF network will also help integrate ground SIF measurements with EC flux networks which will advance ecosystem and carbon cycle researches globally.

Remotely sensed solar-induced fluorescence (SIF) has emerged as a novel and powerful approach for terrestrial vegetation monitoring. Continuous measurements of SIF in synergy with concurrent eddy covariance (EC) flux measurements can provide a new opportunity to advance terrestrial ecosystem science. Here we introduce a network of ground-based continuous SIF observations at flux tower sites across the mainland China referred to as ChinaSpec. The network consists of sixteen tower sites including 6 cropland sites, 4 grassland sites, 4 forest sites and 2 wetland sites. An automated SIF system was deployed at each of these sites to collect continuous high resolution spectra for high-frequency SIF retrievals in synergy with EC flux measurements. The goal of ChinaSpec is to provide long-term ground-based SIF measurements and promote the collaborations between optical remote sensing and EC flux observation communities in China. We present here the details of instrument specifications, data collection and processing procedures, data sharing and utilization protocols, and future plans. Furthermore, we show the examples how ground-based SIF observations can be used to track vegetation photosynthesis from diurnal to seasonal scales, and to assist in the validation of fluorescence models and satellite SIF products (e.g., from OCO-2 and TROPOMI) with the measurements from these sites since 2016. This network of SIF observations could improve our understanding of the controls on the biosphere-atmosphere carbon exchange and enable the improvement of carbon flux predictions. It will also help integrate ground-based SIF measurements with EC flux networks which will advance ecosystem and carbon cycle researches globally.