Persistent climate maladaptation decreases performance of a keystone tree species (Quercus lobata)

Maladaptation in foundational tree species can undermine ecosystem stability, but few studies have quantified its current magnitude or its consequences under ongoing climate change. Using a range-wide provenance test of valley oak (Quercus lobata), we tested whether historical climate warming has already produced temperature-driven climate-fitness mismatches, whether climatic extremes intensify these mismatches, and whether understanding these patterns can inform management. Growth and survival models from a 12-year common garden study revealed that trees performed best under summer temperatures cooler than their home climates, demonstrating persistent maladaptation to contemporary temperature maxima. This mismatch intensified during hotter years, indicating that climate variability amplifies fitness costs and that stressful years impose lasting effects. However, simulations of assisted gene flow showed that sourcing seeds from high-performing maternal families yields greater projected future performance than climate-matched sourcing across the species range. Together, these findings provide rare phenotypic evidence that maladaptation is already constraining a keystone tree species and show that phenotype-informed management can outperform climate-matched seed sourcing under further temperature increases.