With advancing climate change, tree survival increasingly depends on mechanisms that facilitate coping with multiple environmental stressors. At the population level, genetic diversity is a key determinant of a tree species’ capacity to deal with stress. However, little is known about the relative relevance of the different components of genetic diversity for shaping tree stress responses. We compared how two components of genetic diversity, genotypic variation and ploidy level, shape growth, phytochemical, and physiological traits of Populus tremuloides, under environmental stress. In two field experiments we exposed eight diploid and eight triploid aspen genotypes to individual and interactive drought stress and defoliation treatments. We found that: 1) Genotypic differences were critical for explaining variation of most of functional traits and their responses to stress. 2) Ploidy levels generally played a subordinate role for shaping traits, as they were typically obscured by genotypic differences. 3) As an exception to the second finding, we found that triploid trees expressed higher levels of foliar defenses, photosynthesis, and rubisco activity under well-watered conditions, and displayed greater drought resilience than diploids. This research demonstrates that the simultaneous study of multiple sources of genetic diversity is important for understanding how trees will respond to environmental change.