Species-and Community-level Differences In Leaf Functional Traits And Nutrient Cycles Between Invasive And Co-occurring Native Plants And The Effects Of Environments

Alien plant invasion has become one of the most severe ecological and environmental problems worldwide,influencing negatively biodiversity security,ecosystem structure and functions,production of agriculture,forestry and animal husbandry,or even endangering human health.To elucidate the mechanisms underlying alien plant invasions,many studies have compared invasive and native plant species.However,the conclusions of the related studies in literature are discrepant,which may be associated with the few number of species included in each case study and the differences in identities of the species compared and the environments under which each case study was conducted.The conclusions of this kind of studies may be species-specific and environment-dependent.Furthermore,community-level comparisons of invasive and native plant species are few although many species-level comparisons have been conducted,and the comparison at both levels is almost blank.It is well known that environments influence functional traits of alien plants and species composition,structure and function of invaded communities,and therefore influence alien plant invasions.Thus,in this study we setup plots in three sites with different soil characteristics and climates,compared leaf functional traits,characteristics of rhizosphere soils,leaf and root litter properties and decomposition between invasive and native species at both species and community levels,and analyzed the effects of environments.To determine invasion mechanisms of exotic plant species,we measured 44 functional traits for 35 invasive and co-occurring native plant species at the three study sites in two years.The main conclusions are as follows:At species level and in all three study sites,the invasive relative to co-occurring native plants had overall significantly higher leaf nutrient concentrations,maximum photosynthetic rates and resources-use efficiencies;nutrient concentrations,activities of nutrient cycle related enzymes and nitrogen transformation rates of rhizosphere soils;higher nitrogen concentrations and decomposition rates of leaf and root litters;but lower carbon and total phenolics contents,(hemi-)cellulose contents,lignin contents,carbon to nitrogen ratios,lignin to nitrogen ratios and nitrogen resorption efficiencies of litters.Almost all of the aforementioned parameters were positively or negatively associated with relative abundance of the species in the community in each site.These results indicate that aforementioned invasive-native differences well explain invasiveness of the invasive plants,and that the inter-category differences are affected by indentities of the species compared.The results also provided a reasonable explanation for the inconsistent results in related literature.Decomposition rates of leaf and root litters were associated positively with their nitrogen concentrations,specific leaf area,and the concentrations of nitrogen,phosphorus and potassium in mature leaves(only with leaf litter decomposition),but negat ively with the contents of litter carbon,(hemi-)cellulose and lignin,carbon to nitrogen ratios and lignin to nitrogen ratios.The results indicate that the invasive-native differences in physical and chemical characteristics of both litters and mature leaves are the important reasons for the invasive-native differences in decomposition rates of leaf and root litters.Carbon and nitrogen contents and carbon to nitrogen ratios of leaf litters were almost positively or negatively associated with nutrient concentrations of mature leaves in each site.The results indicate that the inter-specific differences in mature leaf nutrient characteristics could be used to predict the inter-specific differences in decomposition rates of leaf litters.Nitrogen resorption efficiencies of leaf litters were associated negatively with their nitrogen concentrations,specific leaf area and mature leaf nitrogen and phosphorus concentrations,but positively with litter(hemi-)cellulose and lignin contents,and carbon to nitrogen ratios and lignin to nitrogen ratios.These correlations indicate that leaf litter nitrogen resorption efficiencies may be higher in stressful relative to favorable environments,and that increasing leaf litter nitrogen resorption efficiencies may be one the strategies for plants to acclimate to stressful environments.Our results indicate that the invasive plants have significant advantages over co-occurring natives in all study sites in terms of soil nutrient cycles and leaf functional traits,contributing to invasiveness in all three study sites.The direction and the magnitude of the species-level differences in all parameters between the invasive and co-occurring native plants were influenced by environments.Specific leaf area was one the most typical parameters.For the invasive relative to the co-occurring native plants,specific leaf area was significantly higher in Dandong with higher mean annual temperature and mean annual precipitation,similar in Shenyang with higher soil nitrogen availability,and significantly lower in Chaoyang with lower mean annual precipitation and soil nutrient availability.With the improvement of the environment,the advantages of the invasive plants over co-occurring native plants also increased in other traits.For example,in Dandong the magnitudes of the invasive-native differences in the mean values of specific leaf area,maximum net photosynthetic rates and photosynthetic resources-use efficiencies,net nitrification rates of rhizosphere soils,and decomposition rates of leaf and root litters,(hemi-)cellulose contents of root litter were the greatest.Above results well explained the variations in invasiveness(relative abundances and importance values)of the exotic plants among the three study sites,and also gave a possible explanation for the inconsistent conclusions of comparisons of invasive and native plants.At community level,the advantages of the invasive over co-occurring native plants became more significant.The invasive relative to co-occurring native plants had significantly higher values of community-weighted means of all leaf functional traits(except C:N ratio at Shenyang),all parameters related to rhizosphere soils(except net mineralization at Chaoyang),decomposition rates and N and C concentrations of leaf and root litters,and cellulose contents of root litters.Functional difference indices of the above parameters between the invasive and co-occurring native plants was larger than 0,indicating further that the traits of the two categories of plants were differentiated at community level.These results provided evidence for limited similarity hypothesis,which predicts that trait differentiation between invasive and native plants contributes to invasions of the invasive plant.Similarly as the results at species level,in functional traits between the invasive and co-occurring native plants were also influenced by environments.The trait advantages of the invasive plants increased significantly with the improvement of the environment.In Dandong with favorable environment,for example,the magnitudes of the invasive-native differences in community-weighted means of specific leaf area,leaf phosphorous concentrations,leaf construction costs,maximum photosynthetic rates,photosynthetic resources-use efficiencies,decomposition rates of leaf and root litters,and net mineralization rates of rhizosphere soils were significantly higher than those in Chaoyang and Shenyang.Similar patterns were also found for the functional difference indices of these traits between the invasive and co-occurring native plants.The above results indicate that in favorable relative to stressful environment the differentiation degrees of growth-related traits are higher between the invasive and co-occurring native plants,and that limited similarity hypothesis could better explain invasion success of the alien plants.However,different traits responded to the environments differently.In Chaoyang with stressful environment,for example,the magnitudes of the invasive-native differences in community-weighted means of leaf concentrations of potassium and ash,and C:N ratios,and the functional difference indices o f these traits between the invasive and co-occurring native plants were significantly higher than those in Dandong and Shenyang.The results indicate that the differentiation degrees of these traits are higher in stressful environment,and that different plants have different mechanisms to adapt to water and nutrient stress.We also found the effects of environment on invasions of the alien plants through analyzing the differences in functional composition and diversity of all plants in the communities among study sites.The community-weighted means of specific leaf area,leaf nitrogen and phosphorous concentrations,leaf construction costs,maximum photosynthetic rates,photosynthetic resources-use efficiencies,decomposition rates and carbon contents of leaf and root litters,and ammonium nitrogen contents of rhizosphere soils were the highest in Dandong among the three study sites.Similar patterns were also found for functional difference indices of these traits between the invasive and co-occurring native plants.These results indicate that in community with favorable environment traits for all plant species were divergence,i.e.,differentiation,supporting limited similarity hypothesis.However,in Chaoyang with stressful environment the community-weighted means of the traits such as leaf potassium and ash concentrations and carbon to nitrogen ratios,and the functional difference indices of these traits between the invasive and co-occurring native plants were the highest among the study sites,i.e.,trait differentiation.Our results indicate that trait aggregation or differentiation was influenced by both environments and trait itself.