Study On The Changing Process Of Typical Tundra Ecosystem Under The Invasion Of Veratrum Nigrum

Rhododendron aureum is one of the most important species in the alpine tundra ecosystem of Changbai Mountains.It is widely distributed in alpine tundra with altitude over 2100 m and formed native dwarf shrub communities of alpine tundra,which plays a significant role in maintaining the balance and stability of tundra ecosystem.In the context of global climate change and nitrogen deposition,it has been widely reported that invasion of herbaceous vegetation to alpine tundra emerged all over the world.Changbai Mountain alpine tundra is one of the most sensitive responding areas to global changes.Previous studies have shown that significant changes have been observed in the composition of the vegetation communities in tundra areas.Herbaceous plants,represented by Deyeuxia angustifolia,invaded up towards to the alpine tundra from lower altitudes,forming herbaceous plants patches.As a result,the structure and function of the tundra ecosystem were changed.Veratrum nigrum is a new herbaceous vegetation which was reported as a new invasive species in recent years.Compared with perennial native shrub vegetation Rhododendron aureum and Vaccinium uliginosum,Veratrum nigrum grows at a faster rate and it also has stronger inter-species competition capacity.Since its invasion observed in 2014,it has been expanding rapidly.As a new invasive species,the impacts of the rapidly expanding Veratrum nigrum community on the native shrub ecosystem of alpine tundra have not been well studied.On one hand,observation of growing process of vegetation,as well as research of changing process of soil nutrient,are necessary for understanding of invasion mechanism of herbaceous vegetation during the rapid invading process of Veratrum nigrum.On the other hand,the study will provide a typical case that interspecific competition will affect distribution pattern of vegetation and change habitats.Rhododendron aureum communities of different invasion levels: step 1(no invasion),step 2(mildly invaded),step 3(moderately invaded)and step 4(severely invaded)at different elevation(2150 m and 2350 m)were selected as research subjects.Long-term observation plots with different environmental factors were established for observation of changing process of invasion levels and vegetation growing rates.At the same time,annual changes of soil nutrient were recorded.The study lasted from 2017 to 2019 and conclusions were drawn below:1.Different topographical factors posted different impacts on spreading rates of Veratrum nigrum.At lower elevation(2150 m),gentle slopes(< 20°)and upland Veratrum nigrum would spread at a faster rate rather than at higher elevation(2300 m),steep slopes(>20°)and depressions.It is speculated that Veratrum nigrum invaded from low elevation to high elevation along the track of depressions with gentle slope,uplands with gentle slope and depressions with steep slope to uplands with steep slope.2.Invasion of Veratrum nigrum restricted growth of Rhododendron aureum with decrease emerged in terminal bud length,leaf length,leaf width and leaf area from step 1 to step 4.On the contrary,increase appeared in height,stem diameter,leaf length,leaf width and leaf area from step 2 to step 4,suggesting that invasion of Veratrum nigrum restricted growth of native shrub species while Veratrum nigrum will grow and invade at a faster rate and change of vegetation growth was proportional to spreading rate of Veratrum nigrum,which means the faster Veratrum nigrum spread,the faster growth characteristics of vegetation changed;3.Annual biomass of Rhododendron aureum andnative shrub vegetation decreased significantly(P<0.05)with invasion levels increased while annual biomass of Veratrum nigrum increased significantly(P<0.05).Similarly,change of vegetation ground biomass was proportional to spreading rate,suggesting that the faster Veratrum nigrum spread,the faster soil nutrients changed;4.With increase of invasion levels,soil organic matter(SOM)and total nitrogen(TN)decreased significantly(P<0.05)while available nitrogen(AN)and available phosphorus(AP)increased significantly(P<0.05).Total phosphorus(TP)kept stable(P>0.05)during invasion process.Soil of alpine tundra turned to be more suitable for survival of herbaceous vegetation rather than shrub vegetation.5.Redundancy Analysis(RDA)between environmental factors and vegetation biomass revealed strong positive correlation between biomass of Veratrum nigrum and AN,with positive correlation between biomass of Rhododendron aureum and SOM;Fitting Analysis between biomass of Veratrum nigrum and AN showed that changing process of biomass of Veratrum nigrum conformed to the Logical Growth Model while Fitting Analysis between biomass of Rhododendron aureum and SOM showed that changing process of biomass of Rhododendron aureum was consistent with Cubic Model,suggesting that AN might be the restricting factor for Veratrum nigrum expanding further.