Establishment patterns of bitterbrush

Abstract

During the summer of 1969, 50 bitterbrush (Purshia tridentata) plants were collected from each of 25 sites located from approximately 10 to 35 miles west of Fort Collins, Colorado. These sites were selected to represent the range of topographic conditions under which bitterbrush is found in this area. The principle objective was to relate establishment patterns over time, as indicated by the 1969 age structure, to the weather history of the area. The bitterbrush plants were aged by cross-dating techniques using ponderosa pine as a control. The age structure of the sample indicates a severe lack of establishment since 1950. The reason for this reduction in establishment is unknown. There appear to be no differential effects due to weather during the period of good establishment prior to 1950 as compared to the period of poor establishment since. Spring temperature proved to be the best predictor of establishment. The age structures of the 25 plots proved to be significantly different. The growth and vigor were found to be significantly influenced by topography. Maximum stem diameter was found to be the best single predictor of age within the array of measurements taken on the live plant. Even maximum stem diameter is a crude index at best for the ages of plants in the area studied. Both bitterbrush and ponderosa pine were analyzed for growth response to weather. Temperature proved to be more limiting for both species than did moisture.