麻烦解释一下这句话逗号之后的句式结构！

麻烦解释一下这句话逗号之后的句式结构！
European countries are becoming increasingly concerned by major threats to European forests，threats which know no frontiers other than those of geography or climate.
这句话逗号之前是一个句子，所以逗号之后不能再接一个句子了是吧，但是接的是什么啊？？
而且know为什么用的是原型，threats应该是被认为是没有边界，难道know不应该用被动吗？？

上面没说完整，完整的是

European countries are becoming 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. 我说的逗号是第一个逗号。

整段内容，请点击图片！