The effects of natural fire and recreational disturbance on montane forest ecosystem composition, structure and nitrogen dynamics, Crater Lake National Park, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w9505255w

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  • Disturbance, whether natural or of human origin, modifies to varying degrees many ecosystem attributes. Fire is a natural process in the montane forests of southern Oregon but for much of the 20th century fire was viewed as an apocalypse and thus fervently suppressed. Effective natural resource management requires an understanding of how ecosystems function, including the ecological response to the natural disturbance regime or anthropogenically altered conditions. To describe the effects of natural fire and ecosystem changes with time following fire, I examined plant composition, structure, biomass and soil nitrogen using a chronosequence of sites ranging in age from 1 to >300 years. The ecological importance of human activity is a function of the magnitude and permanence of its effects which were described at active and abandoned recreation sites. Stand-replacing fires killed virtually 100% of the overstory trees, consumed 87% of the forest floor and resulted in a 27% loss in total aboveground biomass. A pulse of mineralized nitrogen was still apparent 2 years following fire. Two years following fire, little plant colonization was established. The availability of local seed sources, particularly seedbanking species, appears to have been an important determinant of the early-successional community composition. The developing community was dominated by nitrogen-fixing species which may play an important role in the post-fire nitrogen dynamics in these ecosystems. Conifer seedling densities measured 667 individuals/ha 8 years following fire and 1714 individuals/ha 24 years after fire. Extended tree maturation and variation in composition characterized the forest structure and composition during secondary succession. Total aboveground biomass remained high and accumulation followed a general U-shaped curve. Recreational use resulted in a reduction in tree density and biomass at all active sites. The greatest reductions occurred at sites set in late-successional forests. Compositional change was also greatest at sites established in late-successional forest with increased dominance of lodgepole pine and graminoids. Within 40 years the abandoned, mid-successional sites had recovered from most of the recreational impacts; however 40 years was not sufficient for late-successional forest recovery. These results suggest that the stage of succession should be considered when assessing potential recreational impacts.
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