The Flowering Phenology Pattern Of The Invasive Spartina Alterniflora Across Latitudinal Gradient In China

Biological invasions influent the biodiversity and ecosystem function and structure significantly.And globalization is exacerbating the biological invasion.Determining the invasiveness of exotic species can help to explain the processes and mechanisms of invasion,and to control the invasion effectively.Spartina alterniflora is native in North America,which was intended introduced into China in 1979.During the past four decades,it had gone through different invasive stages,including dispersal,settle down,and outbreak.And now,S.alterniflora has distributed widely along the coast of China from Liaoning to Hainan province,being one of the most threatening invasive plant species in coastal wetland of China.The adaptive evolution and phenotypic plasticity of biological traits of invasive species are thought to be the main aspects to explain the mechanisms of biological invasion.Within which,the flowering phenology is the key factor to determine the success of invasion.As temperature and plant size are two important factors that influent the flowering phenology,our study focused on the flowering phenology of invasive S.alterniflora along latitudinal gradient and to disentangling the roles of the environmental temperature and plant size in determining the latitudinal flowering pattern,and to reveal whether the flowering phenology of S.alterniflora involved in genetically based latitudinal clines in the introduced range in China.We conducted geographical survey from 2013 to 2017 in the field and a common garden experiment of latitudinal S.alterniflora provenances from 2015 to 2017 in Xiamen,Fujian.We analyzed the growing degree days in each field study sites and the location of common garden,then to explore the relationship between flowering phenology and the growing degree days.Correlation between the plant size and flowering phenology of S.alterniflora was analyzed as well.And we also analyzed the latitudinal variation of shoot density and flowering shoot ratio.Finally,we got the following results:1.Latitudinal flowering phenology pattern of S.alterniflora in the field and common garden:the pattern of the first flowering day of S.alterniflora along latitude were consistent in 5 years(2013-2017),which showed a hump-shaped relationship between the first flowering day and latitude in the field(P<0.0001).In 2017,the first flowering day of S.alterniflora among different sites was significant different(P<0.0001).S.alterniflora flowered earliest(146 days)at Danzhou,the lowest latitudinal site(19.7 °N),and following with193 days at Dandong,the highest latitudinal site(39.9 °N).S.alterniflora flowered latest(246 days)at Yueqing(28.3 °N),which delayed about 3 months than at Danzhou.However,the flowering pattern in the common garden was different with geographical survey,where the first flowering day of S.alterniflora decreased with increasing latitude of the plant provenances in common garden during 3 years(2015-2017),the provenance from high latitude flowering earlier and the first flowering day of S.alterniflora among different provenances was significantly different.And the latitudinal pattern of peak flowering date was same as the first flowering day in both of the field and common garden.We can conclude that the flowering phenology of S.alterniflora has involved in genetically based latitudinal clines in China.2.The relationship between the flowering phenology of S.alterniflora and temperature:firstly,we found there was a significantly hump-shaped relationship between the first flowering day of S.alterniflora and annual average temperature of different sites(P<0.0001),which indicated that higher and lower temperature may driver the flowering time of S.alterniflora in advance.Additionally,we calculated the growing degree days at flowering in the field and common garden,by which we found that whether in the field or common garden,within the low-medium latitude area(20.9-28.3 °N)and high latitude area(34.8-38.0 °N),the growing degree days remained fairly consistent,respectively,and there were significant differences between the two areas(field:P<0.0001,common garden:P = 0.0009).But beyond the areas,the growing degree days at flowering were significantly lower at higher and lower latitude sites.The same regulations of the growing degree days in the field and common garden resulted in their different flowering patterns.3.The relationship between the flowering phenology of S.alterniflora and vegetative growth:there was a hump-shape relationship between the plant height with latitude(P<0.0001)in the field.However,there was a negative linear relationship between the plant height with latitude in the common garden(P =0.0024).And we analyzed the correlativity between the plant height with first flowering today,which showed that plant height of S.alterniflora significantly increased with increasing of the first flowering day(P = 0.0003)in the field,however,there was no relationship between the plant size and first flowering day in common garden,which indicated that this relationship was just driven by environment,but not genetically based.4.Trade-off between clonal growth and sexual reproduction of S.alterniflora:there was a U-shaped relationship between the shoot density of S.alterniflora and latitude(P = 0.0002),and the shoot density was significantly different among different sites(P<0.0001).On the contrast,there was a hump-shaped relationship between the flowering shoots ratio and latitude(P = 0.0396),which reached its minimum at Dandong(high latitude,20%)and a smaller value at Danzhou(low latitude,33%),and maximum at Queqing(intermediate latitude,87%),which was significantly different among different sites(P<0.0001),These results indicated that S.alterniflora invests more energy into clonal reproduction than in sexual reproduction in extreme high and low latitude sites and invests more energy into sexual reproduction in the intermediate sites.Within its introduced range China,the flowering phenology of S.alterniflora along latitude gradient has involved in genetically based latitudinal clines and the growing degree days and vegetative growth play important roles in shaping this pattern.We can predict that in the process of spreading into higher and lower latitude areas,S.alterniflora may flower earlier and have lower plant height,which leads to lower fitness and it may invest more energy into clonal reproduction to resist extreme temperature.All of these may constraint the dispersal of S.alterniflora into higher and lower latitude areas.