Air pollution stress on solar-induced chlorophyll fluorescence (SIF) of aspen trees: Implications for disentangling the confounding effects of plant physiology and canopy structure on the relationship between the canopy-scale SIF and photosynthesis

Abstract:

Solar-induced chlorophyll fluorescence (SIF) has been intensively studied in recent years due to its potential to infer photosynthesis rate and indicate plant responses to environmental changes. One of the key mechanisms associated with the potential, the physiological linkage between SIF and canopy photosynthesis (measured as gross primary production, GPP) at the canopy scale and beyond, however, is usually confounded by canopy structure and remains unclear. Short-term mild stress on plants that would affect plant physiology but not canopy structure might help disentangle the confounding effects between the two primary factors on the relationship between SIF and GPP.

To test the hypothesis, we used the natural variations of ozone and aerosol pollution, which may suppress plant growth in different degrees, and investigated their effects on SIF and GPP. We deployed an automated field spectroscopy system to obtain continuous measurements of SIF and canopy reflectance over an aspen (Populus euramericana Neva.) canopy in the Experimental Station of the Institute of Botany, Chinese Academy of Sciences, Beijing, China, during the summer and fall seasons in 2019 and 2020. We divided the study period into two phases: Phase 1, the peak growth stage during which canopy structure (quantified by vegetation indices) were relatively stable and photosynthetically active radiation (PAR) fluctuated greatly but did not exhibit significant temporal trends; Phase 2, early senescence stage during which canopy structure indices began to decline and PAR decreased rapidly. Specifically, we hypothesized that (1) during Phase 1 canopy structure and plant physiology are decoupled to some extent. The air pollution stress plays important roles, in addition to PAR, in driving the SIF variations; (2) during Phase 2 canopy structure and plant physiology are coupled. PAR and canopy structure would become the primary drivers of SIF variations. Using random forest and structural equation model, we analyzed the variations in SIF, PAR, ozone concentration, and aerosol loading and their relationships in the two phases. Our preliminary results showed that in Phase 1, the variations in SIF and fluorescence yield (SIFy) could be largely attributed to PAR, ozone and aerosols, while in Phase 2, SIF and SIFy were predominantly affected by PAR variation. By conducting phase comparison, we will discuss SIF’s potential to indicate the stress effects on photosynthesis and how the physiological linkage between SIF and GPP could be affected by canopy structure.