Spatial and chronological patterns of Purshia tridentata as influenced by Pinus ponderosa overstory Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/m900nx59s

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  • Twelve sites of uniform topography and soils were selected in the Pinus ponderosa/Purshia tridentata/Festuca idahoensis Association. These sites were located in the rain shadow on the east flank of the Oregon Cascades. A fire history in the form of fire scars on ponderosa pine was available for nine of these sites. The bitter-brush populations on these sites were sampled at distances of 0-34.5 feet from a large ponderosa pine which clearly dominated the area for at least 50 feet. Density, cover, age, and clumping data for the bitterbrush were obtained. Density was found to be greatest on plots with the most recent occurrence of fire (22 years past). Cover was extremely variable within any one plot. Only slight variation was found in percent of live crown among the various plots regardless of their density or fire history. Age determinations using growth ring analysis or direct ring counts provided information indicating that although most bitterbrush do not survive a fire, sites are rapidly repopulated and generally show a peak repopulation year within 20 years after burning. Plants on sites with most recent fire tended to show more rapid height growth than did plants on "old" sites. Most "old" sites presently have a rather uniform age class profile with only slight indications of peak germination years. Most age class profiles do not fit a concave survivorship curve. Rodent caching as evidenced by plants growing in clumps was found to occur extensively on all sites. Most of the one year old clumps were large with as many as 21 living plants encountered. Progressive reduction in clump size probably occurs both through rodent browsing of young plants and later through moisture stress. By the time a clump has attained an average age of 30 it is likely that only two or three individuals remain alive. Clump distribution and litter distribution appear to be closely correlated. Most recently planted clumps were found to be outside of major litter deposit areas around ponderosa pine. Apparently rodents prefer to cache seeds in areas of essentially bare soil. The question of the perpetuation of bitterbrush in the absence of fire may be raised. Present fire control practices which allow extensive litter accumulation, especially on mesic sites, are resulting in an elimination of suitable planting sites for bitterbrush and may favor other species from more mesic associations less dependent upon fire and litter removal for their continued existence.
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