Excerpt from the Society of American Foresters
By Michael Taylor, Weyerhaeuser Company; Diane L. Haase, Department of Forest Engineering, Resources & Management; and Robin L. Rose, Department of Forest Engineering, Resources & Management
Reforestation on harsh, high-elevation sites near the crest of the Cascade Mountains can be challenging because of persistent snowpack and extreme climatic variation. To help find a solution to meet this challenge, the use of tree shelters was investigated with two species, Douglas-fir and western larch, on two Yakama Nation sites across three fall planting dates.
High-elevation sites in the eastern Cascades of Washington have a short period of favorable spring planting conditions followed by a short growing season. In addition, rapid changes in soil moisture, temperature, relative humidity, and solar radiation make plantation establishment on these sites more challenging than on lower-elevation sites. Most soil moisture on these sites results from snowmelt, with very little additional precipitation from spring through fall. The snowpack insulates the ground and young seedlings from constantly changing and potentially deadly temperatures and winds during the winter months; but once the snowpack melts, the seedling environment changes rapidly from wet and cold to hot and dry.
Higher-elevation sites can be impossible to plant until late spring because the persistent snowpack prevents access. By the time sites become accessible, soil moisture may drop to unacceptably low levels. The short window between snowmelt and summer drought results in a narrow margin between plantation success and failure. Fall planting is a potential alternative because roads are clear of snow and sites are accessible for planting. However, very few data are available for fall planting on high-elevation sites.
Inherent risks associated with fall planting require careful consideration of local conditions. Tree shelters can increase seedling survival on harsh, high-elevation sites by limiting the intensity of ultraviolet light that may cause damage via desiccation. In addition, tree shelters may increase CO2, temperature, and relative humidity as well as shielding seedlings from animal browse.
Management considerations for using tree shelters include the costs of purchase, and assembly. The increased cost may be offset by increased survival, thereby reducing the need to replant at a later date when competing vegetation is established. The objective of this study was to quantify Douglas-fir field performance as influenced by planting date, tree shelters, and environmental conditions.
HOBO micro stations were installed at each site to monitor air temperature, relative humidity, soil temperature at 15 cm depths, and soil moisture at 10 and 20 cm depths. Measurements were taken every six hours from August 2005 through September 2006.
Results: Environmental Conditions
Soil temperatures on each of the fall planting dates were above 5°C, which is a critical root growth temperature, until less than one week after the third planting date, when temperatures dropped below 5°C, where they remained until May 2006. Soil moisture content on the western larch site was 10% on the first planting date, but increased to nearly 20% for the October planting dates.
On the Douglas-fir site, soil moisture in the root zone was 11% to 12% for all three fall planting dates and did not increase until five days after the three fall planting dates, respectively. Soil moisture peaked at both sites in May 2006 but declined rapidly thereafter.
Air temperature at each site followed seasonal patterns similar to soil temperature.
For both species, seedlings inside tree shelters had increased survival compared with non-sheltered seedlings after one growing season, despite increased damage by crushing snowpack. Volumetric soil moisture content on the western larch site increased from 10% to 17% between the first and second planting dates, resulting in no effect of planting date on western larch survival. On the Douglas-fir site, however, soil moisture content was below 12% for all fall planting dates, resulting in only 2% survival for all seedlings planted on the first planting date. These results indicate the critical importance of soil moisture at time of planting and onset of precipitation after planting. Use of tree shelters may improve seedling survival on harsh, high-elevation sites, but it increases potential seedling damage because of crushing snowpack.