Significant shifts in the timing of life history events (i.e., phenology) of plants corresponding to anthropogenic climate change were reported worldwide, affecting their reproduction, survival, and interactions with other species. Recent studies have uncovered significant intra- and inter-specific variation in plant flowering phenology and its response to changes in climate. However, most of this research has been limited to animal pollinated species with large and/or showy flowers, and variation in phenological responses to climate remain largely unexplored among and within wind-pollinated dioecious species, and across sexes. Based on herbarium specimens and ground observations from volunteers of cottonwood (Populus) species in North America, here we examined how phenological sensitivity to climate varies among species, across species ranges, between sexes, and between different phenophases to assess how climate change may affect their phenology and related synchrony and reproduction of wind-pollinated dioecious species. The timing of flowering varied significantly across and within species, as did their sensitivity to winter temperature. In particular, we found that male flowering generally happens earlier in the season than female flowers. Male flowering phenology was more sensitive to warming than female flowering. Further, the onset of flowering was more sensitive to changes in temperature than leaf out. Increased temporal gaps between male and female flowering time and between the first open flower date and first leaf out date were predicted for the future time periods (2041-2070 and 2071-2100) under two climate change scenarios (SSP2-4.5 and SSP5-8.5). Our study demonstrates significant intra-specific variation in phenology and its responses to environmental cues, across species’ ranges, phenophases, and sex. These variations need to be considered in predicting and mitigating the effects of climate change, due to the critical associations with the ecological synchrony and their community and evolutionary processes.