Graduate Thesis Or Dissertation
 

Population dynamics of three early seral herb species in Pacific Northwest forests

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87pv87n

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  • I investigated the population dynamics of fireweed (Epilobium angustfolium), foxglove (Digitalis purpurea) , and woodland groundsel (Senecio sylvaticus) to understand their colonization, persistence, and extirpation in Pacific Northwest Douglas-fir forests. Factors affecting the species' frequency and abundance in forest communities were evaluated. Their population dynamics were modeled and field experiments examined nitrogen fertilization and competition effects on population parameters. Experimental variation made the population models stochastic and sequential transition matrices incorporated the effects of competition into them. Plant community ordination suggested a negative relationship between foxglove and elevation. Indicator species analysis revealed that foxglove is favored by disturbance, i.e., broadcast silvicultural treatments, and by high site productivity, while fireweed responds to low productivity and lack of disturbance. Regression analysis related the species' frequency and cover to factors such as stand age, elevation, and UTM meters east. Simulated woodland groundsel population increased most rapidly. Increased competition tended to reduce its biomass accumulation and seed production per plant but did not affect seeds produced per unit area. Competition also reduced established immature plant survival. Incorporating these factors into the population model, the rate of population growth declined. The projected foxglove population grew at a lower rate than did woodland groundsel but faster than fireweed. Foxglove was adversely affected by competition, which generally reduced its size, biomass, and seed production. Interspecific competition reduced established plant survival, rosette persistence, perenniation, and flowering. Nitrogen fertilization stimulated seed germination, rosette persistence, and flowering but decreased perenniation. Nitrogen fertilization rate did not influence population growth when modeled but application timing did. Nitrogen applied at transplanting resulted in extinction of the population. Transplanting one year after fertilization resulted in constant population growth. Fireweed, with the most complex life cycle, had the lowest population growth rate. Its persistence was entirely due to vegetative reproduction--without it the population would rapidly become extinct. Competition and fertilization had little effect on this species and population projections based on the field experiments indicated constant growth. These results suggest life history and competitive ability regulate succession of pioneer species in Douglas-fir forests and increases population persistence comes at the expense of their growth rate.
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