Ecological Mechanism Of Invasion For The Forestry Harmful Plant Eupatorium Odoratum

Eupatorium odoratum Linn, is a perennial grass or subshrub of the genus of Eupatorium(Compositae) , which has become a forestry harmful plant in China. Ecological mechanismof invading by this forestry harmful weed had been studied in this paper. The results as follows:1. In different age, habitats, years and seasons, photosynthetic rates of leaves ranged a wide values, from 2.08 to 18.27 μ mol.m^-2s^-1, which indicates that leaves of Eupatorium odoratum exhibit the higher photosynthetic plasticity. Though the photosynthetic rates of leaves were not higher than the control species, there were remarkable differences in temperature adaptabilities of the leaf between E. odoratum and the control species. Comparing to the control species in the same region, E. odoratum adapted better and keep at the highest net photosynthetic rate at the range of leaf temperature 33-39°C. Whereas, when the leaf temperature exceeded the limit of 33°C, it was found that the photosynthetic capacity of the control species decreased straightly. E. odoratum exhibited a better ability to hold high net photosynthetic rate at higher temperature in the subtropical or tropical region, which helps to grow fast, and form monodominant community.2. The photosynthetic rate of non-photosynthetic organs of stem, flower(bud), fruit and green root at the surfacial soil could be up to 53%, 55(59)%, 63% and 31% of that of the leaf, respectively. Thus, the photosynthetic capacity in non-photosynthetic organs may contribute to the formation of seeds when leaf can supply relative limit photosynthates for seed production in this period of reproductive season. All this had been benefited to fast reproduction.3.The root respiration rates and stem gross photosynthetic rates decreased exponentially with diameter size grows. The respiration rate of root hair and other fine root could be up to 150 μ mol.g^-1 DW s^-1, while the respiration rate of main roots was only 20 μ mol.g^-1 DW s^-1. The powerful gas exchange characteristics may directly influence the feature of soil respiration, which has demonstrated by the data between soil respiration and metrical distance to measureing individual, i.e. soil respiration rates decreased with the increase of distance between metrical points and the weed within the ranged of 50cm at different sites.4. The contents of sugar, phytohormone, tannins and total flavonoids in E. odoratum at different developmental stages of plant life cycle (seed, sprouts, seedling, adult) had marked differences. The content of sucrose was 0.23%³.92%, glucose was 0.14%¹.67%, fructose was 0.02%¹.01%, gibberellin was 58.4²46.3μg.g^-1Dw, tannins was 0.10%¹.62% and total flavonoids was 0.44%².60%. This showed that all these indexes performed the functions on the developmental stages of this weed. So accelerating the fast growth and fast propagation, even resisting the bad environment more efficiently. Furthermore, the pH values of stems( 5.5 ) and leaves(5.6) were both lower than that of the other organs(6.9⁸.4), this approved the theoretics that acidic condition could accelerate the fast growth.5. According to different habitats for E. odoratum, different life history forms were adopted by E. odoratum with the different compound(B, A, P)to adapt to the environment. When in the excellent habatit, E. odoratum produced the Structure-Building substance, such as soluble sugar, GA, mainly used to plant cell increasing and volume enlarging, thereby accelerating the vegetative growth, forming the life history forms of V. When in the fragile habitat, E. odoratum produced more antagonistic stress substance(such as alkaloid, tannin) to protect themselves to resist drought and high temperature, forming the life history forms of C. When in the disturbed habitat, E. odoratum produced more propagation-Promoting substance(such as flavonoid) to produce a plenty of seeds, forming the life history forms of S.6. Such a developmental pattern (pollen with the thorn decoration and stigma with many drape ) may enhance reproductive success of E. odoratum by the structural characteristics of pollen and stigma, and produce a lot of seeds. Due to the seeds of E. odoratum matured in dry and windy season, and brief matured time, lighter seeds with pappuses, all these characteristics were beneficial to fast spread and dispersal. Seed dispersal of E. odoratum was mainly by wind and stream in short distance The dispersal distance increased with the wind speed. The seeds of E. odoratum can spread 16²2m, when the wind speed reached two or three grade, and the maximal distance was 25m in four grade. Our results showed that the seeds density was 2-8 individual/lOOcm2 in some streams. Medium distance dispersal was carried by a media such as human, animals and long distance distribution was depended on the developed road network and large-scale vehicles. Furthermore, large-scale soil was destroyed in one time, which made it possible that seed contact with soil surface fast. So the developed road network and large-scale vehicles had become the primary fashion of long distance distribution for E. odoratum.7. In different road types, E. odoratum make the different choice of life history forms. In this study, In different road types, E. odoratum make the different choice of life history forms. In this study, the bareness degree of soil in the roadside of highway was lowest between all the road types (only 3% ) , so the seeds was hard to contact with soil, then seed density and seedling density were both lower in the results, only 6 unit/lOOcm2 and 1.5 individual/m2. but the height of individuals can reach 3⁵m, the life history form was V. Bareness degree of soil in the roadside of provincial and county were higher than the ones in highway (from 50% to 90% ) , so the seed density and seedling density were both increased, 27.5-61.25 unit /100cm2 and 5.5-37.67 individual/m2. but the height of individuals was illness, the life history form was SC. Bareness degree of soil in the roadside of country were higher than the ones in highway (from 14.66% to 86.66% ) , for animals frequently destroyed the soil structure, sothe seed density and seedling density were both increased, 28.33³0 unit /100cm2 and 11.67-35 individual/m2. therefore, the populations of E. odoratum formed monodominant community in the roadside of country, the life history form was S, and would transform from S to C. Bareness degree of soil in village roadside was higher from 50% to 60%, the maximal was 90%. The seed density and seedling density were all higher for the lower soil compaction, 28.33-61.25unit /100cm2 and 11.67-37.67 individual/m2. therefore, the populations of ï¿¡ odoratum formed monodominant community in village roadside, the life history form was S.8. According to different habitats for E. odoratum, different strategies of life histories were adopted by E. odoratum to adapt to the environment, which was the base for forming of diversified life history forms. Based on the methods of habitats division and the principal component analysis of morphological parameters, the life history forms were classified into three types, i.e. V, C and S forms. In this study, plants grown in the hillock habitat was V, in roadside habitat and inner forest habitat would transform from V to S, and the level naked-land habitat was SV. The dynamic change of life histories may be helpful for E. odoratum to fast spread in different habitats.9. Many intelligent behaviors happened in period of physiological metabolism, seed reproduction and distribution, which ensured the predominance on the other plants. This was the basis of matter, energy and ecology for fast growth, fast colonization and fast spread.