Small genus of densely tufted annual herbs; north temperate regions and South America and tropical Africa and Asia.
一个紧密丛生的一年生草本植物小属；产于北温带和南美及热带非洲和亚洲。
In the reclaimed farmland, the weeds dominated by perennial herbs in the initial stage of reclamation are replaced by the weeds dominated by annual herbs after reclamation for some years.
农田杂草由开垦初期以多年生草本植物占优势到开垦多年以后以一年生草本植物占优势。
Particularly last time tillage and plowed or not were significant elements that affecting the succession way in the locality(. 3)At early stage, the old-fields were often occupied by annual herbs.
最后一次耕种，特别指最后一次耕种的耕种面状况、有无翻耕是影响当地弃耕地植被演替路径的两个重要因素。（3）弃耕早期阶段弃耕地往往会被周边某种离它较近的一年生草本所占据。
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