Viewpoints on the study of economic thresholds of cotton bollworm
普通野生稻-长喙毛茛泽泻-柳叶箬混作种群的竞争效应

In the present paper,competitive effects on Oryza rufipogon,Ranalisma rostrata and Isachne globosa populations were studied by orthogonal designed experiment of three species factors and five planting density levels.The results showed great disadvantages for each of the three mixed crops,the bigger the initial density was,the greater the negative effects were.Each population was sensitive to inter specific competition,while intra specific competition should also not be ignored. O.rufipogon was the most important factor among the three species,and follwed by I.globosa . According to the items examined,clonal reproductive ratio and biomas per branch of each population were affected obviously by itself,and also by other factors.The bigger the total initial density was,the smaller the clonal reproductive ratio was.Biomass per branch of O.rufipogon and I.globosa population increased as initial density increased,but that of R.rostrata population showed a contradictionary result.The latter was more sensitive than the other two in biomass allocations to leaf and to sexual reproduction,and its ramet death ratio also increased as total initial density did. Overall, I.globosa has the most powerful competitive ability and than O.rufipogon ,while R.rostrata the least.The latter was always in disadvantage position in competition,and was excluded after one growth season in the experiment of mixed crops.