Geographic range is a poor predictor of high-temperature responses among conifers in the boreal-temperate ecotone varying in shade tolerance.

Projected warming and heatwave frequency may disproportionately impact growth and survival of northern tree species at their southern range limits in the boreal-temperate ecotone in North America. However, the extent to which geographic range and shade tolerance influence species' responses to warming remains uncertain. We investigated the effects of 12 levels of ex situ warming on growth, mortality and physiology of seven tree species from the boreal-temperate ecotone, each with different southern range limits and shade tolerances. White pine (Pinus strobus L.), a southern temperate conifer, maintained photosynthetic capacity following prolonged warming and displayed the highest peak growth temperature. However, despite sharing a similar southern range limit, shade-tolerant eastern hemlock (Tsuga canadensis (L.) Carrière) exhibited negligible growth responses and significant foliar damage under warming. Jack pine (Pinus banksiana Lamb.), a light-demanding northern boreal conifer, showed much less foliar damage but reduced photosynthetic capacity under warming. Shade-tolerant species exhibited greater foliar damage and mortality under high temperatures, while light-demanding species exhibited more tolerance regardless of range limits. We conclude that while geographic range explains some responses to warming, shade tolerance may be equally important. These findings provide empirical data to improve forest model accuracy and inform management strategies for forest regeneration and ecosystem stability.