Graduate Thesis Or Dissertation

Early plant biomass trends following forest site preparation on the Oregon Coast Range

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  • The vegetation following forest site preparation was examined in ten clearcut units located in the western Oregon Coast Range. The site preparation techniques used in these units included scarification, slash burning, and chemical spraying. The ages of the clearcut units studied varied from one to nine growing seasons for the scarification treatment, two to ten for the slash burning treatment, and two to six for the spraying treatment. The survey of the vegetation was executed during the summer of 1976. Herbaceous and shrub vegetation were characterized by biomass and cover measurements together with the identification of the three principal species in each vegetation layer. In addition, the influence of the density of the previous conifer stand on the occurrence and abundance of shrub individuals following site preparation and the activity of browsers on both planted coniferous transplants and shrub stems were examined. Results were evaluated in relation to the rate of reoccupancy of the site by herbaceous and shrub vegetation. This was used in an attempt to conceptualize the relative roles of both components of the system in the early stages of succession, and to determine the strategies of these vegetation components as secondary plant succession progresses. The vegetation on a clearcut unit is made up of several small vegetative units as a consequence of the different intensities of the site preparation method used and is, therefore, variable in structure and composition. Findings indicate that early successional trends are characterized by an increasing abundance of both herbaceous and shrub species in proportions related to the particular set of site conditions imposed by the method of site preparation. In the initial stages of succession, microenvironmental factors under the influence of herbaceous species appear to control the establishment of conifers while by the fourth year increasing influence from the sprouting shrubs usurps a controlling influence on further succession. The suppressive effect of coniferous overstory on understory development of shrubs indicates that intense brush recovery is more likely to happen on sites previously supporting conifer stands of low density. Such a trend, although variable, identifies the desirability of high stand densities on brush threat sites. Selective feeding on coniferous seedlings and on brush sprouts by deer and rodents appears to be an important consideration in long term succession. Animal populations are high in habitats conditioned by site preparation; the vegetation following scarification and slash burning appeared especially attractive to browsers while that after spraying appeared attractive to rodents. Over all, composition of perennials shortly after disturbance, especially sprouting woody species, has a major effect on long-term community development. Selective consumption by deer and rodents can delete components of low abundance, such as conifers, during the early stages of succession. Choice of site preparation method can affect both the composition and the density of woody cover and the apparent degree of animal use.
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