Spectral landscape ecology of a foundation tree reveals reduced variation under heat stress

Intraspecific genetic diversity underpins species’ adaptive potential, yet climate change and habitat fragmentation are reshaping spatial genetic structure, potentially limiting species’ capacity to respond to environmental change. We assessed the feasibility of integrating imaging spectroscopy with landscape genetics to characterize intraspecific variation of Frémont cottonwood ( Populus fremontii ), a foundation riparian tree species in the southwestern US. We leveraged imaging spectroscopy data, which captures variation in leaf chemistry, structure, and physiology, to extract canopy spectra of P fremontii across two watersheds. Spectral variability of P fremontii peaked at intermediate maximum temperatures, suggesting that functional variation, and thus adaptive potential, may be reduced at thermal extremes. Between‐site spectral variation was best predicted by a P fremontii resistance‐to‐gene‐flow dataset, which largely included stream connectivity and seasonal precipitation. This finding indicates that spectral differences among populations reflect underlying patterns of genetic connectivity and that human‐altered streamflow and future warming may reduce P fremontii adaptive potential.