Competition Effects And Their Mechanisms Between A Replacing Ryegrass, Lolium Perenne, And An Alien Invasive Eupatorium Adenophorum

Crofton weed, Eupatorium adenophorum Spreng (Compositae) has been invading natural ecosystems throughout portions of the southwestern China. In order to mitigate its harmful impact on natural ecosystems, an ecological approach, Replacement Control, was suggested in 1951 and has been used for about 30 years in China. Ryegrass (Lolium perenne), a perennial herb, is recommended as an alternative economic crop to replace the Crofton weed in some infestation areas, which can be also used as a model plant to research on the competitive potential of invading Crofton weed. However, little research has been done on mechanisms of the competition between E. adenophorum and L. perenne. A variety of questions remained to be answered, such as their relative competitive abilities, the effects of density and proportion on the competition, and the relative importance and interaction of above- and belowground competition between E. adenophorum and L. perenne.The objective of this paper was to quantify the competition effetcs and their mechanisms between E. adenophorum and L. perenne.1. Relative competitive abilitiesA multiple de Wit replacement series was used between Crofton weed and ryegrass. E adenophorum and L perenne were grown at a series of density levels in monoculture and also proportion levels in mixture under each of various density levels using potted plants in the open field. Aboveground dry-weight biomass (yield) was then used to calculate the relative yield per plant, relative yield total, and mean aggressivity index. The relative yield (RY) of E. adenophorum was significantly less than 1.0 under each of treatment regimes except the density 2 plants per pot. The most RY values of L. perenne were significantly greater than 1.0. The relative yield total (RYT) of the plant mixtures was not significantly different from 1.0 at all density and proportion levels, which implies that E. adenophorumand L perenne have the same resource requirements. E. adenophorum had a lower aggressivity value than L. perenne (-2.09 ± 0.29 versus 2.09 ± 0.29). The results showed that E. adenophorum was an inferior competitor with L. perenne. So the greater relative competitive ability of L. perenne should make it an effective plant for replacement of E. adenophorum.2. Effects of density and proportion on growth plasticityE. adenophorum and L. perenne were grown at a series of density levels in monoculture and proportion levels in mixture under each of various density levels using potted plants in the open field. The RY of L. perenne was significantly greater than 1.0 at proportion levels 3: 1 and 1: 1 of each density level but was not significantly different from 1.0 at proportion 1:3. In mixture, the density had significant effect on the RY of E. adenophorum. The RY obviously declined with the increase of density levels while it did not significantly abated with decreasing proportion levels. No interaction between density and proportion was found. When grown with ryegrass, the Crofton weed showed significant decreases in height, root length, number of ramets, aboveground biomass, and belowground biomass, but increases in root to shoot ratio. Under various treatments in mixture, all phenotypic measures except root to shoot ratio significantly declined with increasing density. E. adenophorum showed plastic responses to proportion levels in mixture in height, root length and belowground biomass. Only height of E. adenophorum was significantly affected by the interaction between proportion and density. The results indicated that phenotypic plasticity remained to be studied further to help understand the mechanisms of the invasion of E. adenophorum.3. Above and belowground competitionLarge plastic cups were used to separate roots and metal wire nets to separate shoots, which created above- and below-ground competition between E adenophorum and L perenne. The biomass and root to shoot ratio of E. adenophorum decreased but those of L. perenne increased under all competition treatments except aboveground competition for biomass and belowground competiti...