OCR Text |
Show damage from freeze- thaw cycles ( sometimes several a day) occurs in late winter. Dehydration damage can occur throughout the winter because the needles are free from ice at least part of each day and the evapo- transpiration losses cannot be replaced from the frozen soil. Soil temperatures in the sub- alpine zone, so important to growing season and soil biology, are determined mainly by ground relief above the upper forest limit. Wind and radiation, both directly and as they affect snow cover, are also important. Three location types for soil temperature are di sti ngui shed: a. Snow- covered, little winter ground frost, little summer surface heating. b. Deep ground frost resulting from a thin snow cover. c. Excessive heating in sun- exposed habitats. The biggest influence of wind is its effect on snow distribution. Snow distribution decides the length of the growing season and thus the chances and rate of growth of reforestation. Trees on bare ridges suffer winter drought damage while parasitic fungi damage those standing in a long- enduring snow cover. Sparse stands of trees create more uniform snow distribution and also inhibit avalanches. During spring thaw the pattern of snow depth distribution stands out very clearly. Maps showing isolines of snow cover duration are the most important single basis of reforestation. There is a correlation between dominant plant types and snow cover duration. Evaporation is twice as great on wind- exposed ridges as in hollows. Because snow deposition is also minimum on the ridges, these suffer a strong moisture deficiency. Evaporation is greater on shaded windward slopes than on sunny |