Disturbance and climate change are altering both the phenology, and gross primary productivity of terrestrial vegetation. Photosynthetic phenology (here defined as the seasonality of photosynthetic activity), and the underlying controls, also varies dramatically across terrestrial ecosystems and vegetation functional types. Remote sensing can help monitor these seasonal patterns, but we know relatively little about how reflectance indices or fluorescence signals change with spatial scales or across contrasting vegetation functional types.
Using data from flux tower sites in agriculture, prairie and boreal locations in North America, we examined the relationships between solar-induced fluorescence and reflectance-based indices of photosynthetic phenology. We compare new metrics of pigment composition (e.g. the chlorophyll/carotenoid index, CCI) and traditional greenness indices (e.g. the normalized difference vegetation index, NDVI) to fluorescence measurement at multiple spatial scales and using several different instrument designs. We also evaluated the relative importance of absorbed photosynthetically active radiation (APAR) vs. photosynthetic downregulation in controlling the SIF signals. The premise was that combined reflectance and fluorescence-based measurements of photosynthetic function can provide better insight into seasonal controls over photosynthetic activity than single metrics.
Results/Conclusions
Both CCI and SIF followed underlying changes in absorbed radiation and pigment activity tied to photosynthetic phenology, but a number of factors (ranging from changing snow cover to the different temporal responses of CCI and SIF) can often obscure the relationship between these two metrics. Measurements from different instruments and spatial scales revealed slightly different relationships between SIF and GPP due to the varying foreoptics, angular sampling and retrieval methods. Different vegetation types (e.g. evergreens vs. annual crops) differed in the influence of seasonal downregulation vs. APAR over the optical signals detected. This work points towards the value of experimental campaigns involving multi-scale observations and coordinated, cross-ecosystem studies, and offers suggestions for how existing flux tower networks could integrate optical sensing for improving the links between photosynthesis, GPP assessment, and remote sensing.