在北美西部，大多数农业活动已被速生针叶树的单一栽培所取代。

In western North America, most have been replaced by monocultures of fast-growing conifers.

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在北美西部的广大地区，日益温暖的冬季正导致树皮甲虫的大规模爆发。

Across vast areas of western North America, increasingly mild winters are causing massive outbreaks of bark beetle.

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在20年的时间内，威廉姆斯和他的同事对北美西部的5000多名美洲印第安人的血液进行了取样。

Williams and his colleagues sampled the blood of over 5,000 American Indians in western North America during a twenty-year period.

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