Elevation gradients are important drivers of divergence in tree populations, yet little is known about which abiotic factors along these gradients are responsible for divergence. The endemic Hawaiian tree, `ōhi`a lehua (Metrosideros polymorpha), has diverged along elevation gradients in the forms of two pubescent varieties (M. polymorpha var. incana and M. polymorpha var. polymorpha) on Hawai`i Island. We are conducting field, greenhouse, and growth-chamber experiments on open-pollinated seedlings from six pubescent populations spanning the full elevation range of this species on Hawai`i Island. We will report the results of the reciprocal transplant experiment, which will allow estimation of the strength of local adaptation across the elevation gradient, and the stress-response experiments of seedlings exposed to environmental extremes (e.g., high UV radiation and low temperature), which will give us a better understanding of the environmental factors responsible for local adaptation. Results will reveal insight into how elevation gradients drive divergence in long-live trees as well as important ecological differences between two of the three dominant forms of M. polymorpha on Hawai`i Island.