Consistent Spectral Reflectance Signatures of Photosystem II Thermal Tolerance (T_crit) 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 an increasing risk of PSII damage under future climate change. However, uncertainties in the distribution and drivers of PSII thermal tolerance (T_crit) limit our ability to predict thermal risk in plant communities across spatial scales. Here, we evaluate whether intraspecific variation in T_crit corresponds to leaf reflectance spectra (400–2,500 nm) to identify mechanisms associated with T_crit in field conditions and assess the potential of its estimation using remote sensing platforms. We measured T_crit using temperature response curves of minimal fluorescence (F_o) 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 (<40°C) and extreme (>45°C) heat. Consistent spectral signatures of T_crit emerged across species and sampling conditions, with the strongest signatures in P. fremontii under extreme heat. In P. fremontii, spectra captured up to roughly half of T_crit variation and allowed T_crit estimation (R² = 0.24–0.30; RMSE < 1.0°C) and classification of high-versus low-T_crit (71%–77% accuracy). Across both species, T_crit 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 T_crit via remote sensing may support improved conservation, management, and risk assessment in vulnerable ecosystems.