The Invasive Mechanism Of Tall Form Spartina Alterniflora Loisel:the Role Of Rapid Evolutionary Changes

Biological invasion is one of the most significant global environmental problems in the 21st century. The mechanisms behind biological invasion are still not fully understood. Recently, the potentially important role of evolution in biological invasion has been increasingly recognized, while the evidences are still equivocal. The invasive species, Spartina alterniflora Loisel., native to the Atlantic and Gulf Coast marshes of North America, has been introduced to many countries for the purpose of tideland restoration and creation projects. However, the species has expanded rampantly on many temperate estuaries and coastal wetlands in the world, causing serious negative effects on invaded ecosystems, and now has become an ideal model plant for studying mechanisms of biological invasion.Based on the current status of studies on S. alterniflora and its introduction history in China, we conducted a series of comparative experiments between tall forms of invasive S. alterniflora populations and native populations, and between invasive S. alterniflora and native plants under different nitrogen conditions in a common garden system to reveal the role of rapid evolutionary changes in its invasion success in China. The results indicated that:(1) The total biomass, biomass allocation, and photosynthetic and morphological traits, as well as phenotypic plasticity of invasive tall form S. alterniflora differed significantly from its native conspecific. Invasive S. alterniflora populations were more vigorous than native populations as shown by greater total biomass and higher light-saturated photosynthetic rate. Compared to native populations, invasive populations had significantly higher maximum culm height, total leaf number, but lower leaf area ratio, as well as increased biomass allocation to roots. Invasive populations also showed stronger responses to nitrogen addition in relative growth rate, total biomass, total leaf number, total leaf area and maximum culm height than native populations according to the results of both reaction norms and the relative distance plasticity index. Shifts in these traits and trait plasticity combined to contribute to the invasion of invasive tall form S. alterniflora in its introduced range.(2) Invasive tall form S. alterniflora favored increased nitrogen allocation to photosynthesis in low nitrogen condition as indicated by increased allocation of leaf nitrogen to Rubisco. Nitrogen addition had significant effect on leaf nitrogen allocation in Rubisco and cell walls of native S. alterniflora, but did not influence those of invasive populations. Invasive S. alterniflora had higher leaf nitrogen content on both leaf mass and area basises and increased Rubisco content than native S. alterniflora in both low and high nitrogen treatments, but no significant differences were found in cell wall content, fraction of leaf nitrogen in cell walls and photosynthetic nitrogen use efficiency. The increased photosynthetic capacity of invaders relative to native plants resulted from the significantly higher fraction of nitrogen allocated to Rubisco and higher nitrogen content in the low nitrogen treatment, but only higher nitrogen content in the high nitrogen treatment. Shifts in these traits following introduction of S. alterniflora to a new range combine to result in its high photosynthetic capacity and thus its successful invasion.(3) Invasive tall form S. alterniflora showed greater herbivory tolerance than native populations. Native and invasive tall form S. alterniflora grown under different nitrogen conditions responded to simulated leaf herbivory differently with regard to biomass accumulation and total culm height. Invasive populations had a greater capacity to compensate for leaf damage than native populations in terms of total biomass, underground biomass and aboveground biomass, particularly with low nitrogen availability. Invasive S. alterniflora also showed a greater compensatory capacity than native conspecifics with regard to total culm height in both low and high nitrogen treatments and with regard to tiller number in the high nitrogen treatment. Elevated nitrogen availability significantly increased the compensatory regrowth of underground biomass and total biomass in native S. alterniflora populations but not in invasive populations. The results have important implications for improving management efficiency of tall form S. alterniflora in China.(4) Invasive S. alterniflora differed significantly from native dominant species P. australis in terms of nitrogen uptake and use efficiency. Invasive S. alterniflora had significantly higher biomass, root mass ratio and nitrogen use efficiency but lower 15N uptake rate than P. australis in both monocultures and mixed cultures, regardless of nitrogen levels. Nitrogen supply significantly increased biomass, total 15N uptake and 15N uptake rate but decreased root mass ratio and nitrogen use efficiency of both species regardless of culture form, with biomass and total 15N uptake of S. alterniflora responding more strongly to nitrogen supply than the corresponding traits of P. australis when the two species were grown in a mixed culture. The results suggested that the greater root mass ratio and nitrogen use efficiency played a greater role than root nitrogen capture capacity in the invasion success of this species.According to the results of previous comparative studies between invasive S. alterniflora and native resident plants in terms of traits of morphology, biomass accumulation and allocation, physiology and phenotypic plasticity and our results, it is concluded that rapid evolutionary changes of invasive tall form S. alterniflora in introduced range play vital roles on its competitive advantage over native resident plants, and thus its invasion success:the evolutions of leaf nitrogen partition pattern and herbivory tolerance provide the physiological basements for the vigorous growth; genetic shifts in growth and some morphological traits as well as trait plasticity in terms of growth, morphology, and physiology contribute to high resource use efficiency under different nitrogen environment; the increased biomass allocation to root offsets the disadvantage of lower root resource capacity, enhancing the advantage of belowground competition. These three aspects combined to contribute to higher competitive ability of tall form S. alterniflora, facilitating its invasion in coastal China.