Spatial covariation between solar-induced fluorescence and vegetation indices from Arctic-Boreal landscapes

Abstract

The Arctic-Boreal Zone (ABZ) is characterized by spatially heterogeneous vegetation composition and structure, leading to challenges for inferring patterns in vegetation productivity. A mechanistic understanding of the patterns and processes underlying spectral remote sensing observations is necessary to overcome these challenges. Solar-induced chlorophyll fluorescence (SIF), near-infrared reflectance of vegetation (NIRv), and chlorophyll/carotenoid index (CCI) show promise for tracking productivity and disentangling links to the activity and distribution of chlorophyll at coarse spatial scales (e.g. 0.5°), but their effectiveness for studying mixed landscapes characteristic of the ABZ remains unclear. Here, we use airborne observations collected during NASA’s Arctic-Boreal Vulnerability Experiment to examine the spatial covariation between SIF, NIRv, and CCI at a scale (30 m) commensurate with the best available landcover products across interior Alaska. Additionally, we compare relationships among SIF and vegetation indices from spaceborne observations (TROPOMI and MODIS) resampled to a 0.01° (∼1000 m) scale. We find that the strength of the SIF-NIRv linear relationship degrades when compared from the spaceborne to the airborne scale (R 2 = 0.50 vs. 0.26) as does the strength of the SIF-CCI linear relationship (R 2 = 0.30 vs. 0.18), though the degradation of SIF-CCI is less severe than that of SIF-NIRv. The relationship of SIF with either vegetation index is strongly dependent on landcover class at both airborne and spaceborne scales. We provide context for how further work could leverage SIF with reflectance indices measurable from a variety of platforms to improve mapping of vegetation dynamics in this ecoregion.

Solar-Induced Chlorophyll Fluorescence (SIF): Towards a Better Understanding of Vegetation Dynamics and Carbon Uptake in Arctic-Boreal Ecosystems

“…1). By examining the advantages and limitations of spaceborne, airborne, and tower-based instruments, their applications can be optimized for capturing the spatial distribution and temporal dynamics of SIF in the Arctic-Boreal region [66,91,98].…”

“…1). Therefore, the small pixels of CFIS SIF, together with optical reflectance measurements, can better resolve the spatial distribution of different land covers [91,102,141] and align with the footprints of EC GPP [142,143]. The existing collections of snapshots can also help validate the spatial representativeness of downscaled SIF products discussed in Section 2.1.1.…”

“…Additionally, regional studies on the Arctic-Boreal vegetation dynamics are imbalanced across continents, with a greater focus on North America than Eurasia as a result of imbalanced data availability, while Eurasia is also a significant contributor to the global carbon cycle [124]. Therefore, enhancing the international network of airborne and tower-based SIF measurements is the key to validating spaceborne measurements and investigating scaledependent vegetation dynamics throughout the entire Arctic-Boreal region [91,107].…”

“…Given these strengths, SIF is an effective tool in the Arctic-Boreal region. Several studies have presented this advantage of SIF over canopy greenness empirically [66,86,[88][89][90][91]. However, the full potential of SIF in evaluating vegetation dynamics and constraining uncertainties on carbon fluxes with spatial details is still underexplored.…”

Insights into Canopy Escape Ratio from Canopy Structures: Correlations Uncovered through Sentinel-2 and Field Observation

“…However, the relationship becomes non-linear due to the decoupling between SIF, stomatal responses, and the carbon reactions in photosynthesis at finer spatiotemporal scales [23]. Similarly, the linear correlation between NIRv and SIF diminishes at finer spatiotemporal scales, such as those observed from airborne platforms, unlike at coarser scales, with larger than 500 m, seen in satellite observations [24]. This difference occurs because physiological limits on photosynthesis exert a greater influence than canopy structure variations at these finer resolutions.…”