Responses Of Phragmites Australis Population Growth To Habitat Changes In Songnen Plain Wetland

In a scenario of global climate change and increasingly dramatic human activities,wetland plants in the Songnen Plain increasingly affected by more frequently extreme climate events(intense rainfall,temperature extremes,etc.),and increased exogenous nitrogen input(agricultural runoff,urban expansion,etc.).The threat of serious environmental change has accelerated the degradation of the Phragmites australis marsh.To explore the response of P.australis wetlands to environment changes,we investigated the response of species dominance of P.australis to habitat factors(including flooding depth,p H,salinity and soil nutrient)in the field,and we also examined the effect of temperature and flooding depth on rhizome bud sprouting and the effect of flooding regime and nitrogen addition on functional traits in the greenhouse.Our study will provide a scientific basis for the protection and management of the P.australis wetland,and is of great significance to the restoration of the Songnen Plain wetland.The main conclusions showed as follows:(1)Flooding depth,soil total nitrogen,soil organic carbon and soil C: N were the main impact factors for the change of P.australis dominance in Songnen Plain wetland.The dominance of P.australis showed a significant skewed unimodal pattern along the flooding depth and soil total nitrogen gradients,and a marked symmetric unimodal pattern along the soil organic carbon content and soil C: N.In addition,according to the results of linear mixed-effects models(LMMs)and generalized additive models(GAM),flooding depth and soil C: N were the dominant factors driving the change of dominance of P.australis,which could explain 37 % of the variation.(2)Temperature and flooding depth had significant effects on P.australis rhizome bud sprouting and early growth.Higher temperature led to shorter mean time to sprout,faster sprouting rate and greater ramet survival.The estimated base temperature for rhizome bud sprouting was 5.0 ± 1.27 ℃.The optimum temperature for rhizome bud sprouting and early growth might be the range of 23 ～ 30 ℃.The final sprouting percentage,relative growth rate and ramet height showed unimodal patterns with flooding depth.The mean time to sprout was increased with deeper flooding depth,while sprouting rate and ramet survival were decreased.The optimum flooding depth for rhizome bud sprouting and early growth might be the range of-7 ～ 0 cm.(3)Flooding regime and nitrogen addition could significantly affect the plant height,ramet number,root biomass,leaf area,total rhizome length and clonal biomass of P.australis.In addition,root: shoot ratio and clonal biomass/root biomass of P.australis also significantly influenced by flooding regime and nitrogen additions.P.australis usually exhibited an apparent quiescent strategy to cope with flooding stress,however,nitrogen addition can turn it into an escape strategy when it suffered from flooding stress at adult stage(5 weeks flooding after the first 5 weeks drought).As for the responses to nitrogen addition,P.australis showed a resource acquisition strategy without flooding stress,whereas the promoting effect of nitrogen addition on the growth of P.australis will be weakened to a certain extent under flooding stress.Flooding regime mainly indirectly affected aboveground biomass by affecting plant height and root biomass,while nitrogen addition not only directly affected aboveground biomass,but also indirectly affected aboveground biomass through enhancing plant height and ramet number,and reducing root biomass.