Phenotypic Plasticity And Exotic Plant Invasiveness: Effects Of Estimators, Species, Traits And Environments

Phenotypic plasticity plays a key role in plant adaptation to the environmental changes,and high phenotypic plasticity has long been considered as an advantageous characteristic for invasive alien plants.However,the results of comparisons of plasticity between invasive and native species(or non-invasive alien species)are inconsistent,and thus,the relationship between plasticity and invasiveness is still quite unclear.Species resource requirement,the environmental resource availability,and the type of traits studied are three important factors influencing the relationship between plant plasticity and its invasiveness,which are not well considered.In addition,different plasticity estimators have occasionally been found to generate different species’ plasticity rankings.However,no effort has been made to determine how frequent this incongruence is,the factors that influence the occurrence of the incongruence,and which estimator is more reliable.In this dissertation,we first theoretically analyzed the difference of reaction norm slope(K)and two plasticity indices(phenotypic plasticity index,PI;relative distances plasticity index,RDPI)when comparing species plasticity,tested the effects of the estimators on the comparative results by reanalyzing relevant data in literature,and discussed the applications of these estimators.Then,we compared phenotypic plasticity of five invasive alien species with different nutrient requirements and their co-occurring phylogenetically related species(same genus or tribe)under five levels of soil nutrients to test the effects of species nutrient requirement,traits,and the environmental resource availability on relationship between plasticity and invasiveness.In order to exclude the confounding effects of intraspecific differences,half-sib families of each species,which were collected from the field and the fullsib families obtained by outcrossing in controlled environment were used.The main results are as follows:1.PI,K,and RDPI are the three most commonly used plasticity estimators.Our theoretically analyses reveals inherent conflicts between K and PI or RDPI when comparing species plasticity,which are influenced by the initial interspecific difference in the target trait before environmental change and the K values of the species compared.The above effects of plasticity estimators were confirmed by our empirical test using data from the literature,and the conflicts occur in 203 of the 1248 comparisons between K and the indices.These results indicate that 16% of the conclusions on species’ plasticity rankings in the literature are estimator-dependent,which would change if different plasticity estimators were used.Consistent with the predictions of our theoretically analyses,the values of the initial interspecific difference in the target trait before environmental change and the K values of the species compared are significantly greater when plasticity rankings are different based on K and PI(RDPI)in 203 comparisons than the values when plasticity ranking are identical.Both our theoretical analyses and empirical test show that inconsistent conclusions were caused by the estimators themselves,not by specific species,traits or environments.When comparing species plasticity,K always has statistical foundation,while indices do not occasionally.However,indices are simple in use and biologically meaningful.2.Our results showed that plant resource requirements influenced its fitness plasticity(total biomass as fitness proxy in this study),and therefore influenced exotic plant invasion under different environmental resource conditions.Species with high resource requirement had higher fitness plasticity than species with low resource demand,which was found in our empirical tests using both half-sib and full-sib familiese of five pairs of invasive and native species with different resource requirments as materials.High fitness plasticity could help invasive plants with high resource demand to invade in resource-rich environments,while prevent their invasions in resource-poor environments.In contrast,low fitness plasticity could help invasive plants with low resource demand to invade in resource-poor environments,but not in resource-rich environments.Under resource-rich environments,the invasive plants Xanthium strumarium and Ambrosia trifida had significantly higher fitness plasticity than their phylogenetically related native plants,which may explain their invasions in resource-rich habitats such as abandoned farmlands,farmlands and wetlands.Similarly,under resource-poor environments lower fitness plasticity of Solanum rostratum and Conyza canadensis compared with their phylogenetically related native species may explain their distribution in resourcepoor habitats such as sandy soil and river beaches.3.Adaptive plasticity was seldom found for functional traits(plasticity was associated with fitness)in our study,which indicates that the phenotypic responses of most traits are passive.We also found that adaptive significance of plastic changes was species-,trait-,and environment-dependent.In addition,no functional traits of any pair of invasive and native species had adaptive plasticity.These results jointly showed the importance of judging the adaptive significance of plasticity of functional traits when testing the relationship between plasticity and invasiveness.Only when confirming the adaptive significance of plasticity of a given functional trait,we can analyse the relationship between plasticity and invasiveness.However,only few studies have tested the adaptive significance of plasticity when determining the relationship between plasticity and invasiveness.In this dissertation,we systematically studied the theoretical and methodological issues that influence the relationship between plasticity and invasiveness,thus provide important references for testing the relationship between plasticity and invasiveness.