WEEDS IN AGRICULTURAL CROPS .3. LIFE-FORMS, C-3 AND C-4 PHOTOSYNTHESIS AND PLANT FAMILIES IN A GLOBAL PERSPECTIVE

Weeds are grouped regarding life-forms and occurrence, C-3 or C-4 phot osynthesis and plant families. Relationships between life-forms and th eir occurrence in fields with annual and perennial crops, i.e. on grou nd differently disturbed by mechanical means, are analysed and compare d with analyses for Sweden and Europe. The analyses are mainly based o n 76 species among ''the world's worst weeds'' described by Helm et al . (1977). In addition, 46 species in the European and 74 in the Swedis h weed flora are considered. Annual weeds mainly occur in annual crops , i.e. on ground frequently disturbed mechanically, but to some extent also in less disturbed areas. Stationary and stoloniferous perennials are more abundant on less disturbed ground, but those with a high see d production and a rapid seedling growth are often important weeds als o in annual crops. Perennials with rhizomes of creeping roots are impo rtant weeds in both annual and perennial crops. The behaviour of a spe cies as an annual or a short-lived perennial often varies. There are p roportionally more C-4 plants among the perennial weeds, foremost amon g the rhizomatous perennials, than among the annuals. Most of them are Poaceae and Cyperaceae plants. Poaceae and Asteraceae are the familie s most represented in the weed flora throughout the world. Poaceae see ms to have a particularly high representation in warm climates.