However, few studies have examined the impact of skiing at resorts popular in the American west that are typically built at lower altitudes, below the tree line.

但是，几乎没有研究涉及过美国西部那些受欢迎的雪场，其特点是低海拔、建于林木线以下。这类雪场雪道的建法通常是下面两种的其中一种：1。

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Skiers and snowboarders in search of fresh snow often hitchhike from lower elevations to the rocky summit above tree line.

NPR: 5 Snowboarders Killed In Colorado Avalanche