the vast forests natural to Northern Europe remained either untouched, or perhaps cleared in small sections by fire, and the land probably used only so long as the ash-enriched soil yielded good crops and then abandoned for some other similarly cleared field.

结构分析：

1、主语the vast forests natural to Northern Europe (其中natural to Northern Europe是形容词短语作后置定语修饰forests)。      

2、remained either untouched, or perhaps cleared 是系表结构，remain为不及物动词这里看成半系动词，连词either…or连接两个表状态的过去分词作表语(其中cleared……为过去分词短语作表语)。by fire和(by)the land是并列的，理解为forests were cleared in small sections by fire and the land.       

3、used、abandoned是过去分词作定语共同修饰the land。 后半句的含义大致如下：北欧自然形成的广袤森林，或者保持完好未损，或者有些小的区域可能因大火被夷为平地，这样的话，当某些土壤因富含灰质能产出优质的农作物时，又会变为耕地被使用，后又因为发现其他类似的平地而被荒弃。