Microclimate measurements were collected from 1993 through 1998 in an uncut control, 0.03, 0.13, and 1.0 ha (20, 40, and 113 m diameter) group selection openings, and a clearcut at three sites in the ESSFwc3 in the Cariboo Forest Region. Near-surface soil and air temperatures were also monitored in a variety of conifer seedling planting microsites in each of these treatments.
Macroclimate has a large effect on microclimate at these cold, wet, high-elevation ESSFwc3 sites. For 7 months of the year, the ground is covered with a deep snowpack. The short growing season lasts from early June through late October. Air temperatures are highly variable during the growing season as a continuous series of Pacific frontal systems strongly influence local weather conditions. Average daily air temperatures typically range from 5 to 15°C during the mid-summer. Extended rainless periods of up to a month can result in warmer weather when temperatures can exceed 25°C. During the winter the air temperature typically ranges between 0 and -10°C, but can drop below -35°C when cold arctic air masses push southward.
Frequent precipitation is characteristic during the growing season. Afternoon convective rainshowers of 1-10 mm typically occur 3-5 times per week and frontal systems often occur with up to 40 mm of rainfall. The humid air masses in the ESSFwc3 cool to the dew point at night, depositing dew on the forest canopy. The dewfall and precipitation result in continuous forest canopy wetting and drying cycles. The forest canopy remains dry for approximately 25% of the days during the growing season. The soil profile also remains wet throughout the growing season, drying significantly only during the occasional extended hot rainless periods.
The air temperature at 15 cm above the ground surface was similar between and within openings. Summer frosts as low as -2°C occurred throughout the growing season at all sites. Minimum nighttime air temperatures were highly variable within each opening, and not well correlated with opening size or location. The upper slope position and steep slopes allowed cold air drainage and prevented more damaging frosts.
Rooting zone soil temperatures were near 0°C for most of the year under the deep snowpack. During the growing season, root zone (15 cm) temperatures typically remained between 6 and 12°C, and increased to as much as 16°C during hot, dry periods. Approximately 400-600 degree days (5°C) normally accumulated during the growing season, although this varied considerably from year to year, depending upon the weather conditions and when the deep snowpack melted.
Soil profile and near-surface air temperatures and accumulated 5°C degree days were found to increase with increased forest canopy opening size. The accumulated degree days were found to nearly double in the clearcut compared with the uncut forest. Planting microsites also had different soil thermal regimes. Microsites that were raised or had exposed mineral soil tended to have increased degree day accumulations.
Working Paper 58 (4188 KB)
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Updated July 24, 2015