Tropical forests are important drivers of the global carbon cycle, and will experience unprecedented warming in the next few decades. Temperate hardwood forests are experiencing more extreme and frequent heat waves, potentially altering plant physiological processes. Research suggests that upper canopies of tropical and temperate trees are already operating at or near their thermal optimum temperatures for photosynthesis, and we are uncertain how further rising temperatures may affect forest carbon balance. We investigated forest canopy physiological acclimation to experimental warming in a wet tropical forest in Puerto Rico and a temperate deciduous forest in Michigan, U.S.A. We implemented in-situ leaf-level warming along vertical gradients within these forests to investigate photosynthetic acclimation potential of two northern hardwood species and two tropical evergreen species. We found no evidence of photosynthetic acclimation in either forest canopy, but instead we found evidence of photosynthetic decline with warming overall. If these trees are not able to acclimate to warmer temperatures, we could see alterations in distributions and survival of these key species. Upper canopy leaves are responsible for a disproportionate amount of overall forest photosynthesis, and thus incomplete acclimation of exposed sun leaves could have negative implications for the overall carbon balance of these forests.