Limber pine (Pinus flexilis James) and Rocky Mountain bristlecone pine (Pinus aristata Engelm.) are two white pines that grow in Colorado. Limber pine has a broad distribution throughout western North America while bristlecone pine’s distribution is almost entirely within the state of Colorado. White pine blister rust (Cronartium ribicola J. C. Fisch.) was discovered in Colorado in 1998 and threatens populations of both species. Available information suggests that these species have several important ecological roles, such as (1) occupying and stabilizing dry habitats not likely to be occupied by other, less drought tolerant tree species, (2) defining ecosystem boundaries (treelines), (3) being among the first to colonize a site after fire, especially fires that cover large areas, (4) facilitating the establishment of high elevation late successional species such as Engelmann spruce and subalpine fir and (5) providing diet and habitat for animals. While the rust is not likely to eliminate five-needle pines from Colorado ecosystems, it is likely to impact species’ distributions, population dynamics and the functioning of the ecosystems. These changes may well affect (1) the distribution of forested land on the landscape, (2) the reforestation dynamics after fire, (3) the rate and possibly fate of forest succession, and (4) habitat for wildlife. Our incomplete understanding of the ecology, genetic structure and adaptive variation of limber pine and Rocky Mountain bristlecone pine constrain our ability to rapidly develop and implement conservation programs.