This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover.
这是因为中纬度和高纬度地区的林木线受到积雪存留时间和厚度的强烈影响。
When the duration of surface snow cover is long and intensity of surface heating field is weakness in winter, the intensity of surface heating field is strong in next summer.
冬季地面积雪多的年份，地面加热场强度较弱，第二年夏季加热场强度则较强；
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