网友提供的句子有几处错误。今后在提交问题之前，务必认真核对。改正如下：

Over 60% of the land area is mountains, 21% is covered with production forest lands, of which there are some considered the largest in the world.

【翻译】60%以上的陆地面积是山区，21%是经济林地，其中有些被认为是世界上最大的。

moutains 应为：mountains

of which 之前最好加逗号，改为非限制性定语从句。

consider 应为过去分词：considered

【分析】关系代词which 指代前文的production forest lands；

considered the largest in the world，作定语修饰some。

为了理解句子，我把定语从句分离出来,重新改写：

There are some of the production forest lands considered the largest in the world.