Root and shoot competition between established ryegrass and invading grass seedlings.
Seeds of Poa annua, P. trivialis and Festuca rubra were sown into gaps within established Lolium perenne swards, planted in systematic pattern to give a range of densities. Competition aboveground was controlled by reflective Al tubes and competition belowground by polythene tubes. The supply of N, P and K was varied. 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/m2 reduced seedling DW by about 70-fold. Increasing ryegrass density from 40 to 160 plants/m2 had little further effect. F. rubra was least affected by root competition and P. annua most affected. Application of 400 kg N/ha/year alleviated the effects of root competition, even though it also increased the yield of ryegrass approximately 3-fold at the highest densities. Applications of P and K had little effect on the yield of either ryegrass or intersown seedlings at any density. Shoot competition with ryegrass had little effect on seedlings, regardless of ryegrass density or fertilizer application. It was concluded that established ryegrass plants mainly competed with invading seedlings belowground, probably for N. The results are discussed in the context of changes in the composition of sown pastures and the invasion of gaps in plant communities.