Previous research collectively demonstrates the importance of taking local moisture availability and biotic threshold responses into account when seeking to reveal the ecological manifestations of climate change within upper treeline ecotones. Yet dendroecological studies that explicitly address the role of slope aspect in this context are non-existent. In this paper, we examine whether slope aspect and related temperature-precipitation interactions mediate abrupt increases in tree establishment and pulses of upper treeline advance (≥10 m) on contrasting north- and south-facing slopes during wet and dry periods of the 20th century. We used regime-shift analysis to quantify episodic changes in the rate of tree regeneration at each site (p < 0.05). We employed a climatic water deficit approach to define fine-scale moisture conditions to compare with dendroecological data from opposite aspects on eleven mountain peaks along a latitudinal gradient in the U.S. Rocky Mountains. Regime-shift analysis measured abrupt, yet asynchronous increases in tree establishment across contrasting slope aspects on 10 of 11 mountain peaks. Upper treeline advance was significantly greater (p < 0.05) during drought on north-facing slopes. On south-facing slopes, ecotonal dynamics varied more with respect to fluxes in the climatic water deficit; namely because of differences in local hydroclimate regimes. Collectively, these results underscore the importance of considering both slope aspect and temperature-moisture interactions when elucidating climate-vegetation interactions within upper treeline ecotones.