Throughout the conifer forests of the western United States, wildfires are projected to become larger and more frequent under climate change. The use or prescribed fires is one pathway to mitigate these fires and reduce crown fire hazard. Regardless, whether more prescribed or wildfires, the result is that younger trees (<10 years old) are going to burn more often. A challenge of land managers tasked with making informed decisions is that very little data exists on how fire affects sapling mortality, physiology and recovery. An added challenge is that in some cases, certain tree species may be desirable (e.g., high merchantable value), while in other cases species may be considered weeds. As such knowledge of how much fire behavior kills some species, while protecting others, would be a powerful management tool. To meet this need, this GRIN proposal explored the effects of fire intensity on sapling morphology, physiology and mortality of different sapling size classes of ponderosa pine (Pinus ponderosa). We grew ponderosa pine from seed and conducted experimental burns in the laboratory and monitored the results of these experiments over a series of years. We also conducted prescribed fires and integrated our observations with prior studies we conducted on mature ponderosa pine. We also expanded the original scope of the GRIN to include a wider array of saplings from other important tree species and investigated the interactions of drought alongside fires. We demonstrated that 1.2 MJ/m2 was a lethal fire intensity (as measured via fire radiative energy) threshold for 1-2-year-old saplings of 5 different conifer species (ponderosa pine, lodgepole pine, western larch, Douglas fir, and western white pine.). We further demonstrated that for each of lodgepole pine, western white pine, and ponderosa pine that 100% survival of 1-2-year-old saplings occurred when subjected to low intensity surface fires <0.4 MJ/m2. We also presented conclusive evidence that deformation of xylem cells and loss of xylem hydraulic failure were not the primary mechanisms leading to mortality in ponderosa pine saplings. This result was counter to prevailing scientific knowledge of how fire causes fire-induced tree mortality and to confirm the results, we wholly repeated the experiment. Following 2nd confirmation, we presented evidence to support the counterhypothesis that fire-induced tree mortality in ponderosa pine is due to localized carbon starvation due to lack of phloem transport of sugars and carbohydrates. Given saplings represent the worst-case scenario as trees generally become more fire resistant with age, the identification of a low fire intensity 100% survival threshold will be of considerable use to land managers seeking to use fire as a silvicultural tool. We are actively sharing this data with an international modeling team and developing a data synthesis paper with them. All the project data is being shared via the USFS Research and Development Data Archive. Future research should focus on a wider array of species and trees over a range of ages.