Study On Eco-physiological Respones Of Two Aquatic Nvasive Plants To Eutrophication And Herbivory

Alternanthera philoxeroides and Eichhornia crassipes were two of the most dangerous invasive aquatic plants in world, The past century, they colonized and diffused in freshwater from south to north in China. In invaded water, they formed monodominant community, destroyed the local ecological system and led to local extinction of native plants. The present study on their invasive mechanism mainly focus on the flora and community, physiological ecology study on the two species is few. Therefore, we designed control experiments to study the physiological ecology of the two plants under changed sediment nutrient and herbivory and the invasive mechanisms of them. The main contents and conclusions are as follows.(1)The leaf resource capture-and use-related physiological traits of E. crassipes with its confamilial native plant Monochoria vaginalis in response to altered sediment nutrient levels were compared, the photosynthetic rate, leaf nitrogen concentration, and specific leaf area of E. crassipes increased significantly with increasing nutrition, but these traits in M. vaginalis remained unchanged. At low nutrient level, most leaf physiological traits of E. crassipes were lower than M. vaginalis, At high nutrient level, E. crassipes had a higher photosynthetic rate and photosynthetic nitrogen use efficiency than M. vaginalis.(2)The physiological and morphological traits and the resistant strategies of A. philoxeroides after herbivory by the native generalists (larvae of Atractomorpha sinensis and Hymenia recurvalis) and the specialists (larvae and adult of Agasicles hygrophila) in aquatic and terrestrial habitations were investigated. Both specialists and generalists decreased the leaf biomass, photosynthetic rate, leaf nitrogen concentration and leaf total non-structural carbohydrate of A. philoxeroides, but the specialists decreased these traits more intensely than generalists. Both specialists and generalists had not influence on the total biomass of A. philoxeroides in terrestrial habitation owing to they did not damage the stem of terrestrial A. philoxeroides, and the stem biomass contribute to largest proportion in total biomass. The specialists can decrease the total biomass of A. philoxeroides in aquatic habitation due to they can damage the stem and root biomass of aquatic A. philoxeroides. Both aquatic and terrestrial A. philoxeroides increased lignin and cellulose concentrations in leaves to resist generalists, but they only increased lignin concentrations in leaves to resist specialists.(3)The population performance, physiological traits and resistant strategies of A. philoxeroides and Alternanthera sessilis, in the presence of three population densities of a native generalist grasshopper, A. sinensis were investigated. Although A. philoxeroides exhibited better constitutive resistance than A. sessilis(with higher levels of leaf lignin and cellulose), the introduced resistance of A. sessilis was better than A. philoxeroides. When grasshopper was present, the leaf polyphenolic, lignin, and cellulose concentrations all increased in A. Sessilis. In contrast, only the leaf lignin concentration increased in A. philoxeroides. The grasshopper consumed less leaves of A. sessilis owing to the effective introduced resistance of A. sessilis. The photosynthesis and leaf nitrogen concentration of A. philoxeroides were decreased by grasshopper more seriously than A. sessilis. In addition, when population of herbivore was low, the photosynthesis was decreased only on the damaged leaves of A. sessilis, but both damaged and undamaged leaves of A. philoxeroides. Finally, the grasshoppers suppressed the population biomass of A. philoxeroides more intensively than A. sessilis.(4)The tolerance and the physiological and morphological compensatory mechanisms of A. philoxeroides and A. sessilis in different simulated defoliation levels were studied. After compensatory growth, the leaf biomass of two species in each simulated defoliation level recovered to the control level. Compare to control, the stem biomass, root biomass and total biomass of two species in each simulated defoliation level was not influenced by simulated defoliation. These results revealed that both two species have tolerance to the simulated defoliation, and the compensatory growth ability of the two species will increase when defoliation degree grow. We found both species do not increase photosynthesis and special leaf area for plant compensation, but A. philoxeroides can increase resource allocation from root to shoot, A. sessilis can increase the proportion of fleshy tap root in root system. As a conclusion, we suggesting that E. crassipes favors high resource environment, and the invasiveness of E. crassipes was dependent on the availability of resources in environment, it outperformed M. vaginalis only in high resource environment. In addition, our results supported and explained the fluctuating resources hypothesis at plant physiological level. Our study was the first that compared the physiological traits of A. philoxeroides after herbivory by native generalists and specialists, confirmed that when the exotic plants was introduced a new region where specialists herbivores were absent, they can release some herbivory stress despite they were consumed by generalists herbivores. We also found the introduced resistance of A. philoxeroides to resist generalists herbivores was worse than A. sessilis, the tolerance of A. philoxeroides was still not better than A. sessilis. Therefore the generalists herbivores can weaker the advantage of A. philoxeroides competition with A. sessilis, the results supported the biotic resistance hypothesis, implying that native herbivores can constrain the abundance and reduce the adverse effects of invasive species.