Manipulation of buried seed reserves by timing of soil tillage in Mediterranean-type pastures

Environmental requirements for seed germination and seedling growth vary appreciably among pasture species. Therefore, it is possible that the botanical composition of pastures may be manipulated by timely seasonal exposure of the soil-seed reserve to conditions that promote germination differentially. This possibility was examined in two Mediterranean type pastures in which plots were tilled once per year, each in a different month, for 2 years. For several pasture species, effects were estimated with respect to numbers of soil seeds, numbers of emergent seedlings and canopy coverage. Individual species showed specific behaviour patterns. Seeds of sorrel (Rumex acetosella) and wireweed (Polygonum aviculare) maintained abundant seed in the soil, those of annual ryegrass (Lolium rigidum) and subterranean clover (Trifolium subterraneum) were less persistent, and seeds of red brome (Bromus rubens) and silvergrass ( Vulpia spp.) occurred only seasonally in the soil-seed pools. Mature plants of red brome, silvergrass and subterranean clover attained dominance only in summer and early autumn-tilled plots. Annual ryegrass was more abundant when soil was cultivated in late autumn and early winter, whereas sorrel and wireweed were significant only in plots ploughed in winter. Thus botanical composition may be altered radically by time of tillage, and this has several practical implications for pasture management and weed control.