您好，请问这句话中间部分是什么句型，谢谢European countries are being increasingly concerned by major threats to European forests，threats which know no frontiers other than those of geography or climate：air pollution, soil deterioration, the increasing number of forest fires and sometimes even the mismanagement of our woodland and forest heritage.这句话第一个逗号之后到冒号之间的这部分，完全没明白是什么句型，他为什么能和前半句用逗号隔开，which是定语从句吗，那为什么know没用被动，威胁不应该用 “被认为是” 吗？---------------定语从句有没有主语省略的情况，这句就是 which (we) know ...
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threats which know no frontiers other than those of geography or climate：air pollution, soil deterioration, the increasing number of forest fires and sometimes even the mismanagement of our woodland a...