Ecology Of Arbuscular Mycorrhizal Associations In Solidago Canadensis, An Invasive Alien Plant

Mycorrhiza is a widely distributed plant-fungal symbiosis in natural and man-made ecosystems, in which the host plants provide the fungus with soluble carbon sources, and the fungi help the host plants absorb water and nutrients from the soil, and withdraw environmental stresses. Many aspects of mycorrhizal ecology, including variation in the costs and benefits of carbon and nutrient exchange, the ecological significance of mycelial networks, the role of mycorrhizal symbiosis in multi-species interactions, and the extent and consequences of host-specificity in these associations have also recently been explored. Nevertheless, mycorrhizal ecology is still receiving much attention.Alien species invasion is a worldwide problem, which threatens to ecosystems. However, few studies have investigated the roles of mycorrhizae in plant invasions. Here, I selected an invasive alien plant Solidago canadensis native to North America as study organism, and experimentally examined the associations between S. canadensis and arbuscular mycorrhizal (AM) fungi and the effects of arbuscular mycorrhizae on the performances of S. canadensis under varying growing conditions. The major findings are summarized below.1) As an important type of mutualisms, symbiosis between plant roots and AM fungi is suggested to facilitate plant invasions. We hypothesized that the longer a plant species invades a given area, the more its roots are colonized by AM fungi. To test this hypothesis, we carried out a field study to examine how the mycorrhizal colonization rate of the roots of a mycotrophic alien plant, S. canadensis, and AM fungal spore density and community structure in its rhizosphere soil are affected by the time this plant invaded the habitats of different age on Chongming Island at the mouth of the Yangtze river.There was significant positive relationship between invasion time and AM colonization rate. The longer the time S. canadensis invaded, the higher the rate ofAM colonization was. The total number of AM fungal species increased with increasing invasion time, and was also positively correlated with the number of plant species occurring in plant communities. It suggests that the diversity of AM fungal species was determined by the invasion time and plant diversity.Relative abundance of the two dominant AM fungal species, Glomus mosseae and G. constriction, varied systematically with invasion time. In the dry habitats, the relative abundance (% of the total spore density) of G. mosseae increased with invasion time of S. canadenss (P<0.01), whereas that of G. constriction decreased with invasion time (P<0.01). Various mechanisms are possible, but competition for the roots of the host plant may predominate. However, these relationships did not occur in wet habitats, which may indicate that AM symbiosis and AM fungal spore community structure were related to spatial variation.The results obtained here suggest that certain species of AM fungi might have helped 5". oandensis colonize newly reclaimed habitats, and that succession of the belowground AM fungal Communities occur with the development of the aboveground plant communities.2) Multiple stresses often occurs in natural and man-made ecosystems, which limit the growth and reproduction of plants. Mycorrhizae can enhance the plants' capacity to cope with the environmental stresses. I manipulated P, water and light availabilities to examine the roles of mycorrhizal associations in plant invasions.It was found that arbuscular mycorrhizae could mitigate the influence of environmental stresses on the performances of S. canadensis. At the same time, the habitat conditions determined the degree of host plant benefits derived from mycorrhizae. a) In general, S. canadensis gained little benefit from mycorrhizal associations under favorable conditions, such as sufficient nutrients and normal water condition. On the contrary, AM fungi should derive some of soluble carbon sources from the host plants and thus they reduced the biomass production of S. canadensis. In this case, the influence of mycorrhizal associations was negative on S. canadensis. b) Under such stressful conditions as low nutrient availability and drought, thesymbiosis between S. canadensis and AM fungi mitigated the harmful influence of the stresses on plant performances and increased the biomass production of host plants. In this case, the effect of mycorrhizal associations was positive on S. canadensis. c) At low light availability, the effects of mycorrhizal associations were found to be negative on the host plants in relation to biomass production as AM fungi derived carbohydrates from the host plants.3) One of the most important consequences of plant invasions is the competitive effect of alien species on the natives co-occurring, which may be mediated by AM fungi. Here, a field experiment was conducted to investigate the effects of AM fungi on the competitive relationship between S. canadensis and Artemisia lavandulaefolia (a native species). AM fungi were controlled by fungicide, benomyl.Results showed that S. canadensis is a highly obligate mycotrophic plant. In the absence of interspecific competition, the growth parameters of S. canadensis with mycorrhizal associations were always greater than those treated with benomyl, meaning that the effect of AMF fungi was positive on S. canadensis. Although fungicide treatment had no significant influence on the biomass of A. lavandulaefolia, root/shoot ratio, root weight fraction and stalk weight fraction of A. lavandulaefolia were significantly different between the treatment and the control. In the presence of interspecific competition, AM fungi had significant effects on the relative competitive ability of these two species. Therefore, AM fungi regulates the co-occurring relationship between alien plants and native plants.4) Thiophanate-methyl is a widely used fungicide, whose possible indirect effects on plants are an important concern. In order to examine its safety in the ecosystems, I used thiophanate-methyl to treat A. lavandulaefolia, a widely distributed mycotrophic plant in China. Populations of seven different densities of A. lavandulaefolia, 1, 2, 4, 8, 16, 32, 64 seedlings · m-2 were established in field plots with six replicates. No fungicide or 1 g · m-2 of thiophanate-methyl was applied to each density treatment at 15-day intervals.