This thesis includes two studies focused on quantifying the impacts of climate change, climate variability, and wildfires on forest dynamics. In Chapter 1, I compared the accuracy of field-based methods to precise dendrochronological techniques to age ponderosa pine and Douglas-fir seedlings sampled from three study regions across the western U.S. The use of precise dendrochronological tree aging was well justified, as node counts systematically underestimated ring counts, with bias increasing with tree age.

In Chapter 2, I studied the impacts of climate variability on lower-treeline forests in the northern Rocky Mountains of the U.S., by quantifying how post-fire tree establishment and radial growth varied with seasonal climate over the 20th and early-21st centuries. Climatic conditions favoring regeneration differed between ponderosa pine and Douglas-fir, suggesting species-specific responses to future increases in temperature and drought. Radial growth was also sensitive to moisture availability and temperature, but this sensitivity varied over the past century and between life stages. While adult growth was consistently sensitive to moisture availability, juvenile growth, particularly for ponderosa pine, was sensitive to moisture availability during the warmest and driest decades, suggesting that directional shifts in temperature, accompanied by increasing moisture stress, may be changing climate limitations on growth. This research demonstrates the increased vulnerability of post-fire tree regeneration and decreased growth in dry mixed-conifer forests given increases in temperature and drought. Shifts towards conditions unfavorable for regeneration and growth will likely result in shifts in species composition of lower-treeline forests or transitions to non-forested states.