Root and Shoot Competition Between Established Ryegrass and Invading Grass Seedlings

(1) Seeds of Pta annua L., P. trivialis L. and Festuca rubra L. were sown into gaps within established Lolium perenne L. (ryegrass) swards, planted in a systematic pattern to give a range of densities. (2) Competition above-ground was controlled by reflective aluminium tubes, and competition below-ground by polythene tubes. The supply of N and of P and K was varied. (3) When seedlings were in root competition with ryegrass, in the absence of N fertilizer, increasing the density of ryegrass from 2.5 to 40 plants m-2 reduced the dry weight of seedlings by about seventyfold. Increasing ryegrass density from 40 to 160 plants m-2 had little further effect. F. rubra was least affected by root competition and P. annua most affected. (4) Application of N fertilizer (400 kg N ha-1 y-l) alleviated the effects of root competition, even though it also increased the yield of ryegrass approximately threefold at the highest densities. Applications of P and K fertilizer had little effect on the yield of either ryegrass or intersown seedlings at any density. (5) Shoot competition with ryegrass had little effect on seedlings, regardless of ryegrass density or fertilizer application. (6) These results indicate that established ryegrass plants mainly competed with invading seedlings below-ground, probably for nitrogen. (7) The results are discussed in the context of changes in the composition of sown pastures, and the invasion of gaps in plant communities.