| The alien plant species Ageratina adenophora(Sprengel),commonly known as Crofton weed, is a worldwide noxious invasive weed.Nowadays,it has become invasive and rapidly spread in more than 30 countries and regions of tropics and subtropics.Since the invasion into China in the 1940s,the rate of spread was faster than anticipated,particularly in the southern and southwestern parts of the country.The invasion of A.adenophora has caused serious economic losses and tremendous detrimental effects on agriculture,forestry,rangeland,and biodiversity.Management actions that control the impacts and spread of the weed become very stringent,considerable effort is being put forward to find innovative methods for integrated management.The replacement control, controlling the weed by other competitive and economic grasses was thought to be most hopeful way. In the present study,the morphological characteristics,competition effects and response of A. adenophora and four southern representative forages(including Lolium perenne L.,Festuca arundinacea Schreb.,Trifolium repens L.,and Sorghum sudanense L.(Piper) Starf) were investigated comparatively on potted plants during their seedling stages.To understand the relative competition between A.adenophora and four forages under different planting patterns,a greenhouse experiment was conducted to explore the effect of planting time,seedling establishment,and the sowing density in conjunction with proportional mixtures on growth of A.adenophora and four forages.The purpose of this study was to quantify the competition between A.adenophora and forages,and to identify species,proportional mixtures under some sowing densities and planting times of forages that are adequate for suppression of A.adenophora invasion but have a minimal impact on forages establishment.The results were interpreted in relation to provide foundation for the replacement control practice of A.adenophora in the future.The main contents and results were as follows:1 Effects of different forage species on the relative competitive ability of A.adenophora and foragesBy using a multiple De wit replacement series design method,four comparative competition experiments between A.adenophora and four forages(i.e.L.perenne,F.arundinacea,T.repens,and S.sudanense) at seedling stages were carried out in the greenhouse.By comparing plant morphology, biomass,relative yield(RY),relative yield total(RYT),and aggressivity(A) difference between A. adenophora and each forage,the results showed that the RY of A.adenophora under most treatment regimes was significantly less than 1.0,but the most RY of forages was greater than 1.0,which suggested that the inter-specific competition of A.adenophora were greater than intra-specific competition,but intra-specific competition of forages were greater than inter-specific competition. The RYT values of the plant mixtures were not significantly different from 1.0 for the greatest part, which implied that A.adenophora and four forages require the similar resources,forages could exclude A.adenophora through competition.The index(A) of A.adenophora was significantly less than 0,suggested that A,adenophora is a kind of weaker competitor against the four forages.Competition effects of four forages on growth of A.adenophora were different,the relative competitive abilities of S.sudanense and L.perenne were stronger,followed by F.arundinacea and T.repens.2 Effects of different planting times of forages on the relative competitive ability of A.adenophora and foragesBy using a multiple De wit replacement series design method,the relative competitive ability between A.adenophora and the four forages with different planting times on the pots were investigated in the greenhouse.Based on the comparison of the changing pattern of indices representing the relative competitive ability,the relative competitive ability of forages was significantly increased with the advance of planting times of forages except for the RY of S. sudanense.In contrast,the relative competitive ability of A.adenophora was notably decreased with the advance of planting times of forages,particularly at the advance of 60 days of planting forages.3 Effects of different mixture proportions on the relative competitive ability of A.adenophora and foragesBy using a multiple De wit replacement series design method,the effects of mixture proportions on the relative competitive ability of A.adenophora and forages were investigated in greemhouse.The results showed that the seedling height and biomass of A.adenophora varied with the different mixture proportions,and they were decreased with the proportion of A.adenophora in the mixtures.But the mixture proportion had no significant effects on the height,biomass,and RY of F.arundinacea.In addition,the changing pattern of RY,RYT and A of A.adenophora were similar as the seedling height,suggested that the negative effects of forages on growth of A.adenophora become greater with the increasing of forages relative density.4 Effects of different planting densities on the relative competitive ability of A.adenophora and foragesBy using a multiple De wit replacement series design method,the effects of different planting densities on the relative competitive ability of A.adenophora and forages were investigated in greenhouse.The results showed that with the inceasing of planting densities,the seedling height and biomass of A.adenophora and forages decreased,suggested that there is a density-dependent effect.In general,the relative competitive ability of A.adenophora is lower at the high densities and high proportions in the mixtures,together with the advance of planting time of forage.It suggested that the inter-specific competition of A.adenophora is greater than intra-specific competition.In contrast,the intra-specific competitions of the four forages are greater than the inter-specific competition.It is concluded therefore that that appropriately increasing the relative density and advance planting time of forages could be used to strengthen the competitive ability of forages agasint A.adenophora.
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