Consistent spectral reflectance signatures of photosystem II thermal tolerance in contrasting foundation tree species

Photosystem II (PSII) is among the most thermally sensitive components of
photosynthesis, and emerging evidence suggests that plants in diverse
biomes face increasing risk of PSII damage under future climate change.
However, uncertainties in the distribution and drivers of PSII thermal
tolerance (Tcrit) limit our ability to predict thermal risk in plant
communities across spatial scales. Here, we evaluate whether intraspecific
variation in Tcrit corresponds to leaf reflectance spectra (400-2500nm) to
identify mechanisms associated with Tcrit in field conditions and assess
the potential of its remote estimation using remote sensing platforms. We
measured Tcrit using temperature response curves of minimal fluorescence
(Fo) along with corresponding leaf reflectance spectra in two foundation
tree species: Populus fremontii (US Southwest) and Metrosideros polymorpha
(Hawai‘i). P. fremontii was sampled under both moderate (<40ºC) and
extreme (>45ºC) heat. Consistent spectral signatures of Tcrit
emerged across species and sampling conditions, with the strongest
signatures in P. fremontii under extreme heat. These signatures allowed
Tcrit estimation (R²=0.24-0.30; RMSE<1.0ºC) and classification of
high- versus low-Tcrit (71-77% accuracy) in P. fremontii. Across both
species, Tcrit tended to increase with spectral indices reflecting higher
chlorophyll content and lower carotenoids, nonphotochemical quenching, and
leaf water content. These findings suggest that variation in PSII thermal
tolerance is linked to fundamental biochemical properties of leaves, which
are reflected in their optical traits. As climate extremes intensify,
spectral screening and scaling of Tcrit via remote sensing may support
improved conservation, management, and thermal risk assessment in
vulnerable ecosystems.