Effects Of Environmental Factors Related To Water Level On The Invasive Plant Alternanthera Philoxeroides

Biological invasion is increasingly attracting attention of ecologists as it has become a significant threat to biodiversity,environment,and economy in the last decades.Although ecologists have proposed several hypotheses to explain the mechanisms underlying the invasiveness of exotic species,there is still no consensus on invasion mechanisms.Biological invasion is a complex process and many factors including internal and external factors of invasive species promote the success of invasion.Generally,disturbed district is easy to be invaded by exotic species.Alternanthera philoxeroides(Mart.)Griseb is a worldwide,notorious,invasive species,which origins from South America and has invaded most of the world including the south of Yellow River in China.A.philoxeroides can invade many kinds of habitats,especially riparian regions,which are frequently disturbed by natural and artificial activities.Therefore,to study the effects of varying environmental factors in riparian regions is very important to reveal the invasive mechanism of A.philoxeroides,predict the pattern of its distribution and control this species.A sudden downpour or discharge from a dam will increase water levels in rivers and lakes,submerging riparian plants.Heavy rainfall will promote surface runoff,bringing more nutrients from terrestrial to aquatic ecosystems.Nutrient levels in aquatic ecosystems are increasing because of agricultural and urban activities.So in the first experiment,effects of submergence and eutrophication on A.philoxeroides were tested.Three levels of submergence(0,7 and 13 cm)and five levels of eutrophication(1000-,100-,50-,10-,and 1-fold dilution of modified Hoagland solution)were set in the experiment.Results showed that submergence severely inhibited the growth of A.philoxeroides,whereas a great increase in nutrient concentration promoted its growth.Submergence and increasing nutrients both reduced biomass allocation to the roots in A.philoxeroides,which primarily increased investment into stems upon submergence or leaves when not submerged.A.philoxeroides also showed significant phenotypic plasticity,elongating its stems by increasing intermode length rather than number to cope with submergence.These results suggest that A.philoxeroides could actively adapt to submergence and mild eutrophication would not promote its growth,while extremely high nutrient concentrations in water would be beneficial for its adaptive growth.Submergence and eutrophication often co-occur as a result of frequent climate change and artificial disturbance.Submergence will inhibit the growth of A.philoxeroides,but cannot cause their death.Increasing water level will broaden rivers or lakes,increasing riparian area and habitats of A.philoxeroides.Heavy eutrophication will also promote the growth of A.philoxeroides,alleviating stress effects of submergence on A.philoxeroides,all of which will benefit the distribution and spread of A.philoxeroides.Therefore,more attention need be paid to variation of environmental factors and performance of A.philoxeroides in riparian zones to prevent the spread of A.philoxeroides.As disturbance occurs more and more frequently(e.g.continual rainstorm),leading the water level up and down recurrently,as referred to recurrent water level fluctuation.To test effects of recurrent water level fluctuation on A.philoxeroides,three average water level(0,10 and 30 cm)and four fluctuation frequencies(12,6,3,and 0 cycles over a 96-day experimental period)were set in the experiment.Amplitude of water level fluctuation was ±10 cm.Results showed that effects of fluctuation frequency were non-significant,negative,and positive under water levels of 0,10 and 30 cm,respectively.As fluctuation frequency increased,the effects of increasing water level decreased significantly.When water levels were high,A.philoxeroides allocated greater biomass to shoot production probably in order to elongate and escape from submergence.However,as fluctuation frequency increased,biomass investment in roots and leaves also increased,probably in order to maximize nutrient absorption and photosynthesis,respectively.These results suggest that water level fluctuation may alleviate the effects of submergence on A.philoxeroides.In addition,A.philoxeroides showed significant phenotypic plasticity,adjusting its functional traits according to recurrent water level fluctuation.For example,the increase in stem wall thickness,observed under conditions of fluctuation,appears to be a defense against the adverse effects of frequent water level fluctuation.However,at stable water levels(no fluctuation),thinner walls,larger stem diameters and pith cavity diameters can increase plant buoyancy,enabling the plant to float on the water surface,and facilitating gas exchange.Therefore,when water level fluctuation occurs,A.philoxeroides can live in habitats with high water level,increasing the range of the habitat of A.philoxeroides.It is predicted that frequent disturbance will promote the growth and spread of A.philoxeroides in disturbed riparian zones.In aquatic habitats,disturbances due to water movement or human activity frequently fragment plants,and vegetative fragments can serve as an important means for the reproduction and spread of aquatic plants.Clonal aquatic plants are often particularly susceptible to fragmentation and form fragments consisting of one to several ramets that have a high capacity for establishment and growth.If fragments of A.philoxeroides can regrow and reproduce in new habitats,it will invade the new ecosystem soon.So it is important to test under which environments A.philoxeroides can regrow and reproduce.The fragments of A.philoxeroides arrive at new habitats by water flow,so water level is an important factor for the reproduction of A.philoxeroides.The riverbanks fragments arrive at are probably shaded by plants on the riverside.So an experiment was conducted to test effects of water level and light condition on the regrowth and reproduction of A.philoxeroides.In the experiment,three water levels(0,2 and 10 cm)and two light conditions(10%and 80%of natural light)were set for fragments of A.philoxeroides with or without apex.Results showed A.philoxeroides with or without apex performed similarly in most indices including biomass accumulation and allocation.It was found that total biomass of A.philoxeroides under all treatments except 10%of light condition and 10 cm of water level was larger than initial biomass,which suggested that fragments of A.philoxeroides can regrow under most conditions.However,the storage root,the important propagative organ of A.philoxeroides,under high light and 0 or 2 cm of water level,can reproduce new ramets,which suggested high water level and low light can inhibit the growth of storage root in A.philoxeroides,hampering reproduction of A.philoxeroides in new habitats and its dispersal.These studies systematically tested effects of environmental factors related to water level on growth and spread of A.philoxeroides.Varying environmental factors,similar to nature,were set to perform control experiments,revealing effects of water level fluctuation and others on A.philoxeroides.Increasing water level or submergence inhibited the growth of A.philoxeroides while general eutrophication cannot promote the growth of A.philoxeroides unless nutrient concentration increased sharply.In this case,extreme eutrophication can alleviate stress effects of submergence.Water level fluctuation can also offset negative effects of submergence on A.philoxeroides.However,increasing water level will enlarge the area of water surface and wetlands in rivers and lakes,as well as suitable habitats for A.philoxeroides.Submergence cannot cause death of A.philoxeroides and water level fluctuation enhanced the capability of A.philoxeroides in living in environment of high water level.All of these will expand the distribution of A.philoxeroides and aggravate the invasion of this species.In addition,long-distance spread of A.philoxeroides is closely related to environments of its arrival.High water level and low light significantly inhibited the regrowth and reproduction of A.philoxeroides,limiting the spread of A.philoxeroides.The research results contribute to understanding the causes of the invasiveness and wide distribution of A.philoxeroides in disturbed riparian zones.Meanwhile,the results are beneficial to control the growth and spread of A.philoxeroides as well.