A Study On Physiological And Ecological Mechanisms Of Invasive Plants Secondary Invasion In Stressed Saline Habitats

Widespread invasive plants usually cause serious damage on many differentkinds of ecosystems in their introduced ranges. However, it is usually thought thatstressful habitats are hard to be successfully invaded by exotics. But, recently, moreand more new proofs suggest that some exotic plants can expand into such stressfulhabitats and cause successful secondary invasion as a result of a relatively longinvasion history. In addition, the effects of some global change factors are believed tobe markablely promoting biological invasions. However, so far, we do not know thereasons for secondary invasion and the potantial influence of global change on theexpansion of exotic plants in stressful habitats. How does invasive plant maintain itsfitness and competitive ability when it expands into stressful habitats? Does theanthropogenic nutrient enrichment promote the invisibility of stressful habitats? Howwill invasive plants react to the changing main abiotic factors in stressful habitatsunder global change? All of these big problems need to be studied urgently.In this study, the new expansion of Ipomoea cairica and Triadica sebiferum inendangered coastal ecosystem (habitats with soil salinity stress) of South China and Southern USA are choosed as objects. The physiological and ecological mechannismof the secondary invasion of invasive plants is studied from the following two mainaspects: the mechanism for maintainance of invasiveness (including energe captureand cost, energe allocation, adapative evolution and tolerance) and the invisibility ofcoastal habitats (based on fluctuating resource hypothesis). At the same time, theinfluence of global change factors (anthropogenic nutrient addition, elevated Ndeposition, and CO2enrichment) on secondary invasion was detected and aprospective invasion in coastal area was predicted. Finally, some suggestions onbiocontrol were given. The main results are shown as following:The Amaxof the coastal genotype of I. cairica was relatively higher than theterrestrial genotype, although the difference is not significant. The construction cost ofthe invader was not affected by salinity stress. It suggests that invasive I. cairica canmaintain the balance of energy input and cost along salinity stress gradients. Thereaction norms of the two genotypes of I. cairica on the main traits to the salinitytreatments were significantly different; the fitness of the terrestrial genotypesignificantly decreased with salinity treatments, the growth of the terrestrial genotypewas more severely inhibited by salinity treatment than that of the coastal genotype;however, the fitness and biomass of the coastal genotype were not significantlydecreased by salinity. It suggests the existance of adaptation during the new expansionof I. cairica in high salinity habitats, the new adaptation lead to the difference on theirreaction norms and finally promoted the secondary invasion of I. cairica.The leaf condensed tannin content (CT) of I. cairica was significantly lower thanthat of the three non-invasive plants; the relative growth rate (RGR) of the invaderwas not significantly influenced by salinity and herbivory treatment. The tolerancescore of the invader under all stressed treatments were higher than non-invasive plants.However, the RGRs of the non-invasive plants were significantly decreased by bothsalinity and herbivory treatment. It suggests that the invader allocated more energy ongrowth and tolerance, while less on costly resistance. Consequently, the invader canmaintain a stable fitness in stressed habitats, and consequently promote its secondaryinvasion. The nutrient addition treatment significantly increased the biomass of both nativeand invasive plants, while the salinity treatment decreased their biomass. In mixedpots, the native plant had higher biomass than the invader under scenarios withoutsalinity, while the invader had higher biomass than the native plant under scenarioswith high salinity. There was no significant interaction between salinity and nutrienttreatments. It suggests that the competitive dynamic between invasive and nativeplants is determined by salinity; nutrient addition could not change the competitivedynamic but strengths the difference between them. It is different with the previousstudies which suggested that nutrient addition changed the relative competitive abilityof invader comparing to natives. The invasive plant had higher specific leaf area (SLA)and water use efficiency (WUE) under the scenarios with high salinity stress. Itindicates that invasive I. cairica has higher traits on water capture and use efficiencythan natives, and its secondary invasion was presumably facilitated by it.Consequently, anthropogenic nutrient addition may increase the invisibility of coastalareas.The growth of native and invasive populations of Chinese Tallow was increasedby elevated N deposition, the growth of invasive population increased more than thenative population. However, salt stress diminished the promotion. It suggests that theinvasive population has higher advantage on using elevated N deposition; the invasionof Chinese Tallow in coastal area without salt stress will be promoted by elevated Ndeposition, while secondary invasion in habitats with high salt stress will not happen.In monoculture treatments, elevated CO2concentration significantly increasedChinese Tallow biomass, and salinity decreased the magnitude of the reaction.However, in polyculture treatments, elevated CO2concentration did not increase thebiomass of the invader whenever there was salinity stress or not. Native trees biomassdid not significantly increase with elevated CO2concentration. It suggests thatelevated CO2concentration will promote the invasion of Chinese Tallow in coastalarea, even though soil salinity increased in these areas concurrently. However, it maybe not as high as previous predictions. The exotic grass species showed higher reaction to elevated CO2concentration than native grass. It indicates that thesenon-native grass species would become new invasive species under global change.The native grass functional group reacted lowly to the elevated CO2concentration. However, intersepecific competition significantly inhibited thereactions of Chinese Tallow to elevated N deposition and CO2concentration. Itsuggests that native grass functional group can be used for controlling the invasion ofChinese Tallow.