Ages and diameters were measured in mature stands for each of 507 ponderosa pine, 541 lodgepole pine, 141 limber pine, and 217 Engelmann spruce trees in the Colorado Front Range of the Rocky Mountains, USA. Cumulative age distributions were statistically different for each species. The spruce and ponderosa pine age distributions each exhibited a strong infection point at °210 and 125 yr, respectively, while neither lodgepole nor limber pine showed such a sharp inflection point. We suggest that the presence of this inflection point may be indicative ofclimax type in mature stands and interpret the age distributions of these species as reflective of their climax, colonizing and fugitive ecological patterns, respectively. Diameter distribution curves exhibited patterns markedly different from the age patterns. Ponderosa pine and Englemann spruce showed almost identical distribution despite widely disparate age structure. We interpret this result to simply that to the complex of forces which influence size distributions in forest trees is very similar in ponderosa pine and spruce although they form very different ecological communities. Lodgepole and limber pine diameter distributions were quite different. Our conjectures that the presence of a sharp inflection point in cumulative age distributions indicates aclimax stand and thatclimax species will often have coincident diameter distributions, are offered as empirically testable hypotheses.