Soil surface effects on soil water, soil temperature, and Douglas-fir seedling injury following radiation frost damage events Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gb19f9093

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  • A microclimatology study was conducted on a high elevation clearcut near the summit of Mt. Ashland in southwest Oregon to evaluate the effects of frost on Douglas-fir seedling growth and survival. Frost and low temperatures cause seedling stress through frost damage, frost desiccation, increased root resistance, and decreased rates of photosynthesis. Five potentially damaging frost events in June and July were identified by evaluating diurnal trends in air temperature, dewpoint temperature, solar radiation and windspeed. Periodic seedling surveys allowed verification of frost damage in response to the identified events. The effects of four surface soil treatments that influence soil temperature and soil water content were compared: burn + scalp, burn (no scalp), scalp (no burn), and a control (no burn, no scalp). No seedling frost damage was noted on any treatment until after a frost event on July 4, even though at least three frost events had occurred earlier in the season during periods when seedling were growing and so susceptible to damage. Soil temperature at 20mm depth dropped below air temperature during the July 4 frost, but not during an earlier frost event on which did not damage seedlings. Air temperature was similar on both dates, indicating that there was a change in soil heat capacity between June 6 and July 4. As long as soil temperature remained above air temperature during frost events, no seedling damage was evident. Seedling condition and damage, soil water loss, soil heat capacity (calculated from soil water loss) and soil temperature changes were compared between the 2 frost events. Seedlings growing on the 2 burn treatments (burn + scalp and burn) showed the least frost damage; seedlings growing on the scalp treatments showed the most. The effect of soil water on soil heat capacity is well documented; by June 4, water loss in the surface 250mm of soil was significantly greater on the scalp treatment than on either burn treatment. By the end of July, treatment ranking for soil water loss was identical to ranking for frost damage - scalp, control, burn, burn + scalp. Control of surface vegetation had the greatest effect on water conservation; burning for vegetation control was a more effective means of conserving soil water than scalping, but combining the two treatments resulted in the lowest soil water loss. Water has a high heat capacity and thermal conductivity relative to air or soil. Therefore, conserving surface soil moisture provides some measure of frost protection to seedlings during the early growing season by buffering soil temperature changes during a frost event.
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