The Interspecific Competitive And Synergistic Effects In The Process Of Chromolaena Odorata Invasion

Chromolaena odorata(L.) King & Robinson, known as a perennial, herbaceous shrub and chromolaena of the family Asteraceae, originated from middle and South America, and now had spread worldwide, such as Asia, Africa, Oceania and most of the tropical and subtropical regions of the western Pacific islands. It was listed as one of the most harmful 100 kinds of invasive species in the world. In 1934, Chromolaena odoratawas first discovered in mountain areas in Yunnan and Hainan provinces, China, and then quickly spread in southern China, which was considered as a serious threat to the local biodiversity, agricultural production and ecological security.At present, the domestic and foreign scholars have already carried out significant work on its distribution, damage, dispersal, biological characteristics, exploitation, utilization, prevention and control. However, just a few studies focused on the synergistic and competitive effects between Chromolaena odorata and other invasive species, native species. Given that the researchers had observed the coexistence phenomenon of Chromolaena odorataand other invasive species in south China, and all of them can thrive well, which indicated thatthere may exist the mechanism of synergistic invasion. In this study, indoor simulation and greenhouse experiments were carried out, and we studied the ecological adaptability andecology mechanisms of successful invasion of Chromolaena odoratasystematically, i.e., we used Praxelis clematidea(an invasive exotic weed in China)and Urena lobata(native species) as control materials, then studied their biomass allocation, growth characteristics, and related effects to the soil nutrient, soil enzyme activity, soil microbial biomass andanti-oxidation enzymesystem of the leaves of Chromolaena odorataand other two plant species in different planting patterns, with an attempt to explore the invasion mechanism. Main results were shown as follows:(1) When Chromolaena odorata and Praxelis clematidea were interplanted, there was no significant differences in biomass between the two plants, which meant that there were no positive or negative effects, no facilitation or inhibition between them.When Chromolaena odorata and Urena lobataare coexisted, the leaf numbers, total branch numbers of Chromolaena odorata were significantly more than that in their mono-planted treatments correspondingly, which contributes its rapid growth, and more biomass, hence faciliate the Chromolaena odorata invasion.(2) Whether Chromolaena odorata and Praxelis clematidea were single or interplanted, the soil total nutrients and available nutrients were not significantly differed. It illustrated that the competition for nutrients of Chromolaena odorata and Praxelis clematidea was not obvious, or it achieved a certain balance. When Chromolaena odorataand Urena lobata were interplanted, the soil total nitrogen, total phosphorus, available nitrogen, available phosphorus and available potassium contents were not significantly differed compared with the Chromolaena odorata mono-planted except the content of total potassium.Moreover, the change range of the total nitrogen, available nitrogen and available potassium of Urena lobatawas significantly lower than those of Chromolaena odorata when both of them were grown alone. In the case of the total soil nutrient of the habitat stayed consistent, Urena lobata could contribute nutrients for the growth of Chromolaena odorata, or Chromolaena odorata scrambled for the nutrient of Urena lobataand to maintain soil fertility, so that to promote its invasion.(3) When Chromolaena odorata and Praxelis clematidea were interplanted, the soil catalase activity was higher than both of them grew alone. However, the acid phosphatase and urease activitywere not significantly differed whether the two species were grown together or individually. It revealed that there existed no synergistic effect between these two invasion plants. When Chromolaena odorataand Urena lobata were interplanted, the soil catalase activity and urease activity were higher than that of their mono-planted treatments, even though acid phosphatase activity was lower than that of their monoplanted treatments. However, the urease activity and catalase activity increased, which indicated that it could improve soil respiration intensity to realize the coexistence of this two species, and promote the invasion of Chromolaena odorata.(4) When Chromolaena odorata and Praxelis clematidea were interplanted, the soil microbial biomass carbon and nitrogen content were lower than that of Chromolaena odorata mono-planted treatment, but it was not significantly differed for soil microbial biomass phosphorus. It indicated that there were no additive effects when the two invasive plants coexisted. When Chromolaena odorata and Urena lobata were interplanted, the soil microbial biomass carbon and phosphorus content were significantly increased compared with Urena lobata mono-planted treatment, but soil microbial biomass nitrogen had no obvious changes. It indicated that the coexistence of Urena lobata and Chromolaena odorata coild promote soil microbial biomass carbon and phosphorus contents, and result in high levels of soil nitrogen and phosphorus nutrition environment, and hence promote the Chromolaena odorata invasion.(5) When Chromolaena odorata,Praxelis clematidea and Urena lobata were interplanted, the activities of SOD, CAT, and proline of plant leaves were significantly higher than that of Chromolaena odorata mono-planted treatment. However, the MDA of Praxelis clematidea leaves increased significantly compared with Chromolaena odorata and Praxelis clematidea grew together, and MDA of Chromolaena odorata leaves decreased significantly compared with Chromolaena odorata and Urena lobata grew together. The plant protein and POD had no changes. That is to say that SOD, CAT,MDA and Pro contents were main antioxidant system to realize the coexistence of Chromolaena odorata and the other two species. It were these enzymes that adjusted and improved the physiological activities to faciliate the Chromolaena odorata invasion.