the permanent snow line and roughly parallel to it.

Causes

Due to their vertical structure, trees are more susceptible to cold than more ground-hugging forms of plants. Summer warmth generally sets the limit to which tree growth can occur: while tree line conifers are very frost-hardy during most of the year, they become sensitive to just 1 or 2 degrees of frost in mid-summer. A series of warm summers in the 1940s seems to have permitted the establishment of

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