WEEDS IN AGRICULTURAL CROPS .1. LIFE-FORMS AND OCCURRENCE UNDER SWEDISH CONDITIONS

Weed communities on arable land are briefly discussed as early stages in (secondary) vegetation successions on disturbed ground, and weed co ntrol is looked upon as measures obstructing the successional course. The weeds are grouped into annuals, biennials and perennials and, furt her, into a number of subgroups, modifying some of the definitions giv en in the literature. Groups and subgroups, life-forms, are defined re garding reproduction, longivity and seasonal rhythm of development and growth, emphasizing the resulting tolerance or response to mechanical disturbance by, mainly, soil cultivation in different seasons. Differ ences in the occurrence of weeds of different life-forms in spring- an d autumn-sown annual crops and in perennial crops on arable land, i.e. mainly leys, are illustrated and explained as consequences of the pla nt character. The life-form of a species sometimes varies for genetic or environmental reasons.