Effects Of Spartina Alternflora Genotypic Diversity On Invasive Ability And Ecosystem Function

As global climate changes and more and more species becomes extinct, people pay a close attention to the consequences caused by decreased biodiversity. From1990s, the relationship between biodiversity and ecosystem function has been the focus people concerned. Large amount of experiments indicated that increased plant species (or functional groups) richness can significantly affect ecosystem function (e.g. increasing primary productivity and improving soil nutrient retention). But most studies about this relationship fasten on species (or functional group) level, few studies involved in another level—genotypic diversity. Recently, genotypic identification became more convenient and efficient with development of molecular markers. The effects of genotypic diversity on ecosystem function became new concerns. Similarly to species diversity, genotypic diversity can also increase aboveground biomass, enhance population resilience and resistance, change community structure, accelerate litter decomposition and nutrient release. The effects of genotypic diversity were analyzed through potential mechanisms (Selection effect and Complementary effect) developed from relationship between species diversity and ecosystem function.Existing studies about genotypic diversity effects on ecosystem function focused on native plants. However, for invasive plants, we don't know whether genotypic diversity has the similar effects. As an invasive plant in salt marsh, Spartina alterniflora has spread and replaced native plants in many places of the world. In some places, for example Chongming Dongtan in Shanghai, the higher genetic diversity of S. alterniflora provides good materials for researches in the relationship between genotypic diversity of exotic plant and invasive ability (and ecosystem function). After colonizing in a new habitat, an invasive species occupies empty niches or competes with native species, generally by increasing individual number, biomass and occupied area. Individuals with different genotypes can enhance growth and invasive ability through utilizing different resources. Invasive plant can also expand by producing higher setting rate or more seeds after acclimatization. For S. alterniflora, polycultures are more likely to increase high quality seeds through cross-pollination. So parameters of clonal propagation and sexual propagation can used to estimate invasive ability of S. alterniflora. Field observasions of these two parameters were executed to seek the effects of genotypic diversity of S. alterniflora on invasive ability. Mechanisms of genotypic diversity effects were also discussed.Most direct consequence of enhanced S. alterniflora invasive ability was decreased abundance of native plants. The differences of nutrient assimilate and litter decomposition between S. alterniflora and native plants can further influence nutrient cycle, microbe community and zoobenthos. So we estimated effects of genotypic diversity of S. alterniflora on abundance of native species, nutrient contents in leaves and sediment, macrobenthos diversity, microbe diversity and litter decomposition rate to estimate the influence of genotypic diversity on ecosystem function.Phenotypic variation was also found among different genotypes. Diverse morphology can bring functional differentiation. So morphological characters were compared between different genotypes and were used to further analyze effects of genotypic diversity on invasive ability and nutrient uptake. Main conclusions are as follows:1. Genotypic diversity can enhance invasive ability of S. alterniflora. Genotypic diversity effects were mainly caused by complementary effect and selection effect.Three genotypic diversity levels (1,3,6-genotype) and42plots were set up using six S. alterniflora genotypes and observed for2growing seasons. Results indicated that there was no significant relationship between genotypic diversity and invasive ability parameters in the first growing season. However, from summer of the second growing season, there were significant positive relationships between genotypic diversity and aboveground biomass, maximum spread distance, patch area, shoot number and weight of single seed. The results indicated that the influence of genotypic diversity on invasive ability of S. alterniflora increased with time and became significant after second growing season.The occurrence of transgressive overyielding and non-additive effects in the second growing season indicated that invasive ability was significantly increased by interaction among six genotypes. But no transgressive overyielding or non-additive effects existed in6-genotype patches for stem density and aboveground biomass per mass. This result indicated that S. alterniflora of different genotypes did not enhance growth and spread ability through increasing stem density which were caused by improving resource utilization in6-genotype patches, but enhance invasive ability through facilitation among different genotypes. The significantly higher parameters of invasive ability in some genotypes than in others indicated that selection effect may play a role in genotypic diversity effect.2. There were no significant effects of genotypic diversity of S. alterniflora on nutrient parameters in leaves and sediment, macrobenthos and microbe. However, abundance of native dominant plants decreased with genotypic diversity. The occurrence of significant genotypic diversity effects of S. alterniflora on other processes of ecosystem may need a longer time than on aboveground biomass.Percentage of carbon(C%), nitrogen(N%), phosphros(P%), C/N, C/P and N/P in leaves and sediment were not significantly affected by genotypic diversity of S. alterniflora. These nutrient parameters were also compared among different patches in single-genotype. Significant difference only occurred in C%and C/N in leaves. These results indicated that nutrients distribution was homogeneous in sediment of our study area and nutrient utilization was different among six genotypes.There were no significant genotypic diversity effects on microbe groups and macrobenthos parameters (group number, individual number and diversity index). But microbe groups in S. alterniflora patches were significantly more than those in native Scirpus mariqueter communities.S alterniflora can enhance microbe diversity in growing environment. These macrobenthos parameters were not significantly different between S. alterniflora patches and S. mariqueter communities where Assiminea sp. and Cerithidea sinensis both had the highest abundance. Effects of genotypic diversity of S. alterniflora on nutrient cycle, macrobenthos and microbe may occur after a longer time.3. Genotypic diversity did not significantly affect leaf litter remaining mass of S. alterniflora. But2-and3-genotype litter mixtures decomposed much faster than single-genotype litter at the last retrieve. A longer decompose time may be needed after genotypic diversity effects becaming significant.Leaf litter was collected from six different single-genotype patches. Litter genotype was distinguished for each shoot. Four genotypic diversity levels (1-,2-,3-6-genotype) were set up using six genotypes. Litter bags were retrieved for four times (17,42,86and162day). Linear regressions between genotypic diversity and remaining mass were not significant for all four retrieves. But2-and3-genotype litter mixtures decomposed faster than single-genotype litter and "transgressive over-decomposition" turned up in6-genotype patches in last retrieve. These results indicated that genotypic diversity accelerated litter decomposition and more time may be needed for significant effects.Polynomial regression can fit litter decomposition dynamics of S. alterniflora better than exponential decay model, which may be caused by environmental conditions during decomposition experiment.4. Significant morphological differences appeared among six genotypes of S. alterniflora. Correlationship between morphological index and invasive ability indicate that different genotypes may influence its invasive ability through different growth strategies.There were significant differences among six genotypes in stem height, diameter, nodal diameter, sheath thickness and branch length. For other morphological parameters, significant differences exist between two genotypes. Some morphological parameters have significant correlations with element contents in leaf and invasive ability parameters. S. alterniflora of different genotypes adopted different growth strategies. Morphological parameters can imply invasive ability to some degree.In conclusion, genotypic diversity enhances invasive ability of S. alterniflora and competitive ability with native species through complementary effect and selection effect. But a longer time may be needed for occurrence of genotypic diversity effects on nutrient dynamics, macrobenthos and microbe community structure and litter decomposition.