The Enhanced Chemical Traits Of Invasive Plants Solidago Canadensis L. And Their Contribution To Competition

Invasion by exotic plant species is recognized as an important ecological problem. Several hypothesises have been proposed to explain why some invasive plants are more vigorous in their introduced range comparing to in their native range."Novel weapons hypothesis" is one of these hypothesises, which poists that some invaders transform because they possess novel biochemical weapons that function as unusually powerful allelopathic agents, or as mediators of new plant—soil microbial interactions. Root exudates that are relatively ineffective against their natural neighbors because of adaptation, may be highly inhibitory to newly encountered plants in invaded communities. And these chemicals are the "Novel Weapons" of invasive plants. However, whether and how the alleochemicals evolve rapidly in the introduced range is not well known.So, I conducted a series of experiments through biogeographical investigation and common garden approaches (eliminating environmental differences) to address the following questions:(1) whether S. canadensis increases its allelopathy in the introduced range compared to in its original range;(2) whether S. canadensis increases its competitive ability in the introduced range compared to in the its original range; and (3) whether allelopathy contributes to the enhanced competititive ability of Solidago. Main results obtained from these experiments are as follows:1. Chemical traits of invasive Solidago canadensisConcentrations of total flavones, total phenolics, and total saponins in soil accumulated as the population of S. canadensis developed in the field experiment and as density of S. canadensis increased in greenhouse experiment. And the contents of these three allelochemicals in the soil grown Solidago before were significantly higher than that in the soil grown native plants before. Seed germination rate of Lactuca tatarica Breitung and growth of Medicago sativa L. were significantly suppressed as the concentration of allelochemicals released from Solidago increased.Total flavones, total phenolics, and total saponins in soil were significantly degraded when the soil was not sterilized. And the contents of three main group compounds declined rapidly6days after incubation, then kept constant at the level of26.04μg/g,17.01μg/g and55.18μg/g respectively.2. The evolution of allelopathy in invasive Solidago canadensisThe results of biogeographical comparisons of Solidago canadensis L. populations and common garden experiments demonstrated that Chinese Solidago have significantly higher shoot height, biomass, the ratio of root to shoot biomass (R/S), leaf numbers, specific leaf area (SLA). Moreover, Solidago of Chinese populations have more34.22%-34.28%total flavones,34.20%-66.53%total phenolics and75.56%-107.80%total saponins than American populations in the aboveground extracts and have more184.93%-209.06%total flavones,60.20%-67.20%total phenolics and49.59%-77.33%total saponins than American populations in the belowground extracts.Allelopathic effects of Chinese Solidago on lettuce significantly increased by17.50%-65.98%(aboveground) and21.03%-70.43%(belowground) than that of American Solidago, indicating that during the evolution of Solidago canadensis, the higher performance of S. canadensis in China was not only attributed to the growth and functional traits but also to an increased allelopathy.3. The evolution of competition in invasive Solidago canadensisWhen native plant Kummerowia striata was set as the target plant, the growth of K. striata was significantly reduced as the density of neighbor plant S. canadensis (rhizome grown) increased, implying that the competitive abilities of S. canadensis enhanced with the increasing density.When Solidago canadensis (seed grown) from different origins (China and America) was set as the target plant, Chinese Solidago had more biomass and height than that of American Solidago. At each density (except for4seedlings/ m2) of neighbor plant K. striata, competitive ability of Chinese Solidago was always higher than American Solidago.When Solidago canadensis was grown with Kummerowia striata (1:1), the competitive ability of Chinese Solidago (rhizome grown) were significantly higher by179.08%(aboveground) and197.68%(belowground) than that of seed grown Solidago from America, and by22.71%(aboveground) and31.89%(belowground) than that of seed grown Solidago from China. The results demonstrated that Chinese Solidago have evolved enhanced competitive ability during the invasion.4. Contributions of allelopathy to the competitionContrary to the results of enhanced competitive abilities of Chinese S. canadensis with target plant Kummerowia striata as the density of S. canadensis increased, allelopathic contributions declined significantly, indicating that resource competition gradually became the dominant form of competition when population of Solidago was very large. But allelopathic contributions were consistent with the results of increased competitive abilities of rhizome grown Chinese Solidago when Solidago grown with Kummerowia striata (1:1), comparing to the seed grown Solidago from China and America. Allelopathy made most contributions to the aboveground (31.09%) and belowground competition (46.12%) of rhizome grown Chinese Solidago. However, allelopathic contributions of seed grown Solidago from America was only up to0.48%(aboveground) and0.80%(belowground), implying that Chinese Solidago had evolved enhanced allelopathic advantage which contributed much more to the enhanced competititive ability.