The control of relative abundance of perennials in chalk grassland: is root competition or shoot competition more important?
Grubb, Peter J.; Ford, Martin A.; Rochefort, Line
Critical studies have shown that there are significantly constant hierarchies of species, based on cover-abundance, for both graminoids and dicotyledons in grazed chalk grassland (Mesobromion) in southern England. Experiments led to the conclusion that abundant species are abundant because of their ability to inhibit the growth of the sparse species, 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 competition or shoot competition is more important in setting the hierarchy in species-rich grasslands 2-10 cm tall. It was shown that only about 50 % of the new nitrogen and phosphorus needed each year by a mature plant can be met by recycling, and root turn-over is rather slow. Test species, when grown on chalk grassland soil, showed little response in growth rate to irradiance above 10-40 % summer daylight. That proportion of daylight penetrates to a height of about 3 cm in turf 5-10 cm tall, i.e. the height above which most leaf laminae are held. Novel kinds of experiment separating root and shoot competition, together with the above evidence, led to the conclusion that root competition is more important than shoot competition in enabling some species to be regularly more abundant than others in turf 5-10 cm tall, but that shoot competition is more important in turf 20 cm tall, and paradoxically also more important locally in very short turf where wide-leaved species form flat rosettes very close to the ground.