Relative low temperature tolerance of lodgepole and ponderosa pine seedlings Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qv33s054f

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  • The hypothesis is explored that lodgepole pine at the seedling emergence period is more resistant to low temperatures than ponderosa pine. Differential tolerance between species to low temperatures is then related to the prevalence of lodgepole pine in frost pocket areas in central Oregon to the exclusion of ponderosa pine. Tolerance thresholds to low temperatures for both species were determined in a controlled environment chamber. The chamber, designed specifically for this study, had separate controls for air and soil temperatures. With soil heated to 32°F, air temperature could be lowered, with lights out, to a minimum of about 13°F. Controlled environment chamber treatments consisted of exposing groups of uniformly reared seedlings of each species to nighttime low temperatures of 13°F to 30°F. Seedling groups varied in age by two-week intervals for comparisons of differences between seedling development stages. These experiments showed that differential tolerance to low temperatures occurred at 15°F to 18°F. About twice as many ponderosa seedlings were killed as lodge pole, with ponderosa mortality occasionally approaching 100 percent. At warmer temperatures up to 23°F ponderosa mortality continued to be substantially greater than that of lodgepole, but number of seedling deaths was low. Treatments above 23°F caused little or no injury to either species. During emergence youngest seedlings of each species survived low temperatures better than older seedlings. Fall treatment of seedlings reared in outside planting beds ai1ed to show any signs of injury from 15°F treatments. These results strengthen the theory that the period of species differential tolerance to low temperatures for first year seedlings occurs during the emergence period in the spring. The study of the occurrence of low temperatures in the field was made along a transect starting in a lodgepole pine timbered flat and continuing through a lodgepole-ponderosa transition zone up to a ponderosa timbered slope. On a few occasions during the seedling emergence period, night minimums near the surface were as low as 15°F to 18°F at the lodgepole flat compared to 20°F to 22°F at the ponderosa timbered slope 80 feet above. This spread of five or more degrees was of the same magnitude as the laboratory determined spread of first injury to ponderosa pine to almost total kill. The controlled environment experiments demonstrated differential tolerance between species, and that laboratory-determined lethal temperatures approximated those recorded in field "frost pocket". Although only a small segment of the natural environment complex could be studied, the evidence strongly supports the original hypothesis that lodgepole pine during the seedling emergence period is more resistant to low temperatures than ponderosa pine, thus, explaining in part the "frost pocket" distribution pattern of lodgepole pine in central Oregon.
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