THE CONTROL OF RELATIVE ABUNDANCE OF PERENNIALS IN CHALK GRASSLAND - IS ROOT COMPETITION OR SHOOT COMPETITION MORE IMPORTANT

Critical studies have shown that there are significantly constant hier archies of species, based on cover-abundance, for both graminoids and dicotyledons in grazed chalk grassland (Mesobromion) in southern Engla nd. Experiments led to the conclusion that abundant species are abunda nt because of their ability to inhibit the growth of the sparse specie s, while the sparse species persist in the system because (a) they are lower-growing and favoured by each episode of grazing or mowing, and (b) they have a greater ability to establish new individuals from seed . The new work summarized here concerns the question of whether root c ompetition or shoot competition is more important in setting the hiera rchy in species-rich grasslands 2-10 cm tall. It was shown that only a bout 50% of the new nitrogen and phosphorus needed each year by a matu re plant can be met by recycling, and root rum-over is rather slow. Te st species, when grown on chalk grassland soil, showed little response in growth rate to irradiance above 10-40% summer daylight. That propo rtion of daylight penetrates to a height of about 3 cm in turf 5-10 cm fall, i.e. the height above which most leaf laminae are held. Novel k inds of experiment separating root and shoot competition, together wit h the above evidence, led to the conclusion that root competition is m ore important than shoot competition in enabling some species to be re gularly more abundant than others in turf 5-10 cm tall, but that shoot competition is more important in turf c. 20 cm tall, and paradoxicall y also more important locally in very short turf where wide-leaved spe cies form flat rosettes very close to the ground.