|Abstract or Summary
- 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.