Investigating remotely sensed spectral indicators of tree vitality across scales and forest types

Environmental stresses exacerbated by climate change create increasing pressure on forest ecosystems, challenging their resilience and functioning. As part of the Forestward Observatory to Secure Resilience of European Forests (FORWARDS) we seek to bridge the gap between ground-based measurements of tree vitality and remote sensing methods. Multi- and hyperspectral reflectance data have the potential to map visible damages but also pre-visual vulnerability symptoms (e.g. downregulation of photosynthesis) due to their relation to leaf pigment contents (D’Odorico et al., 2021). However, the interpretation of spectral indicators such as vegetation indices can vary at leaf, crown and stand level, as well as between species and forest structure (Gamon et al., 2023).

We make use of multiple scales of remote sensing observations to investigate how indicators derived from reflectance behave at crown, stand and landscape level and which approaches are promising for operational use in Europe-wide forest vitality monitoring, particularly in the context of the new-generation spaceborne imaging spectrometers.

We present first results from a pilot phase of this project focused on three intensively monitored sites in Switzerland, including a rainfall exclusion experiment for investigating drought stress. For these sites, leaf and tree-level data in the field as well as acquisitions by drone (<0.1 m) and airborne (1 m) multi- and hyperspectral sensors were conducted in August 2023. We supplement remote sensing data with radiative transfer simulations of virtual canopies to demonstrate impacts of forest structure and composition on vitality indicators related to leaf pigment changes.

Preliminary results show that, aggregated to crown-level, shadow masked drone and airborne data reproduce similar variations of the investigated index values between species and individual crowns. Indices normalised for structure (e.g. PRInorm, Zarco-Tejada et al., 2013) appear promising for monitoring stress across species and structural types.

Insights from this work will allow for the improved integration of data from existing forest monitoring networks with airborne and satellite data towards maps of European forest vitality and stress.

 

REFERENCES 

D’Odorico, P., Schönbeck, L., Vitali, V., Meusburger, K., Schaub, M., Ginzler, C., Zweifel, R., Velasco, V. M. E., Gisler, J., Gessler, A., & Ensminger, I. (2021). Drone‐based physiological index reveals long‐term acclimation and drought stress responses in trees. Plant, Cell & Environment, 44(11)

Gamon, J. A., Wang, R., & Russo, S. E. (2023). Contrasting photoprotective responses of forest trees revealed using PRI light responses sampled with airborne imaging spectrometry. New Phytologist, 238(3), 1318-1332.

Zarco-Tejada, P. J., González-Dugo, V., Williams, L. E., Suarez, L., Berni, J. A., Goldhamer, D., & Fereres, E. (2013). A PRI-based water stress index combining structural and chlorophyll effects: Assessment using diurnal narrow-band airborne imagery and the CWSI thermal index. Remote sensing of Environment, 138, 38-50.