The vegetation dynamics of Pinus contorta forest, Crater Lake National Park, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gb19f8304

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  • This study describes the soils, vegetation and age structure of the Pinus contorta forests of Crater Lake National Park. Growth rates of P. contorta, and levels of infection and impact of dwarf mistletoe (Arceuthobium americanu1r) on growth of P. contorta individuals are compared among communities. Vegetation description is based on 81 circular 500 m² sample plots in forest with at least 50% P. contorta. Communities were defined using association tables and a computerized similarity ordination. Tree ages were determined at 30 cm, and age classes were defined. Severity of dwarf mistletoe infection was estimated using a seven class rating system. Eleven communities are defined, of which three are climax, or at least persistent P. contorta, and eight are seral. All but one are found on Steiger soil, or this soil mixed with non-pumiceous rock. An important factor controlling P. contorta climax community distribution may be soil temperature, as determined by length of the snow-free period and topography. Presence and distribution of seral communities may reflect climate and result from higher soil nutrient levels, as determined by the distance to non-pumiceous material. There is considerable variability in size of P. contorta at a given age. Both diameter and height seem to reflect site quality and the two are highly correlated; maximum tree sizes differ among some communities. Growth is greatly reduced after 100 years. Age analyses and historical records suggest that, though fires occurred in Crater Lake National Park before the arrival of white man, they were unusually frequent from 1850 to 1900 because of his activities. Most fires in P. contorta probably were intense enough to destroy, or nearly destroy, a stand, and could occur at intervals less than ten years. A minimum estimate of mean fire frequency is between 25 and 50 years. The area of P. contorta forest probably increased when white-man caused fires burned Abies-Tsuga forests. Invasion patterns of shade tolerant species suggest that repeated low intensity fires were common in two communities. In one, fire-scarred trees support this hypothesis. Widespread fires were probably rate in the P. contorta/Carex-Stipa community. I suggest that true climax forest of Abies and Tsuga does not exist here on pumice soils. A model is presented in which P. contorta develops to Abies-Tsuga forest, which eventually degenerates, burns, and recycles to P. contorta. Dwarf mistletoe infection slightly reduces height and diameter growth in heavily infected trees. Sapwood thickness is reduced only slightly, if at all. Phloem thickness may be reduced in the P. contorta/Carex-Lupinus community by heavy mistletoe infection; trees large enough to be susceptible to mountain pine beetle (Dendroctonus ponderosae) attack have slightly greater dwarf mistletoe infection than smaller trees in this and the Abies lasiocarpa/ Haplopappus/Aster-Elymus communities. Stand age accounts for almost 30% of the variability in mean stand infection level which is highest in old stands and lowest in isolated or repeatedly burned areas. Evidence of bark beetle activity is found in all communities. There probably have been no irreversible changes in vegetation because of fire suppression over the past 75 years. Permitting lightning fires to burn and suppressing man-caused fires should reestablish a more varied age structure and permit some P. contorta to develop to Abies-Tsuga, while some Abies-Tsuga will probably burn and recycle to P. contorta. Meadow area should increase as will the cover of some shrubs, such as Ribes cereum.
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