Competition Mechanism Of Herbaceous Encroachment Process In The Alpine Tundra Of Changbai Mountain

Alpine tundra plant communities have undergone rapid changes over the last few decades throughout the world,most commonly in the form of the expansion of low-elevation plants toward higher elevation.Among the expanding plants,herbaceous plants account for24.6% of the global non-native flora.The upward encroachment of low elevation plants has increased the competitive pressure on native plants for resources,and altered the growth and abundance of native plants,thus affecting the spatial distribution of plant communities.Changbai Mountain,the highest mountain in northeastern China,has undergone significant changes in the flora of the alpine tundra located at its summit.Recently,Calamagrostis angustifolia,a warm and nitrogen-loving perennial herb thriving previously in the mountain birch forest zone of Changbai Mountain,has been gradually encroached upward into alpine tundra.The encroachment of C.angustifolia has threatened the survival of native plants and altered the structure and distribution of the plant community in the alpine tundra.It has been reported that the areas encroached by C.angustifolia were once dominated by the native evergreen dwarf shrub Rhododendron aureum,but the original cause of this phenomenon is not yet clear.Additionally,little is about the upward encroachment of C.angustifolia and its controlling factors in the alpine tundra,and it is unclear whether the alpine tundra will be gradually occupied by C.angustifolia under future climate change.C.angustifolia competes with native plants for resources during the upward encroachment,which will certainly lead to strong competition.Generally,in order to avoid the harsh environment,alpine plants will allocate more nutrients to the subsurface.Then it has not been conclusively established whether the C.angustifolia gradually dominates the tundra community by continuously encroaching on the above-ground and below-ground space,limiting the growth of R.aureum and leading to its death.In addition,compared with other native plants in the tundra,C.angustifolia tend to occupy the area of R.aureum first,does this indicate that there is an overlap between the ecological niche of the encroachment plant C.angustifolia and the native plant R.aureum? What kind of environment is more suitable for encroachment of C.angustifolia,and how the encroachment of C.angustifolia affect the distribution of native plants in the tundra? The above questions are important for predicting the future vegetation dynamics and the natural evolution of the tundra ecosystem in Changbai Mountain,and providing a scientific basis for targeting the ecological conservation of the alpine tundra.In order to explore the encroachment process and its controlling factors,we interpreted and analyzed the observation data of 4 ha permeant big plots in the tundra from 2014-2019,so as to obtain the spatial distribution and change of tundra plant cover(spatial resolution up to40×40 cm),and get the accurate topographic map of the big plots by differential GPS,and the spatial distribution map of soil physicochemical properties within the plots by soil sampling at2 m intervals.On the basis,we adopted the method of spatial sequence instead of time successional sequence to assess the impacts of herbaceous encroachment on the biomass allocation of tundra native plants at the species and community scales by analyzing the aboveground and below-ground biomass allocation of C.angustifolia and R.aureum in communities with different encroachment levels of C.angustifolia.Additionally,a decision tree and random forest approach were used to analyze the similarities and differences in the ecological niche characteristics of C.angustifolia and R.aureum from the topographic and soil perspectives,to explore the overlap in the ecological niche characteristic of C.angustifolia and R.aureum,exploring the potential of encroachment in C.angustifolia in the tundra.We used landscape pattern analysis and mathematical statistics to analyze the changes in the distributions of C.angustifolia and R.aureum under different environmental gradients within 5 years(2014-2019)to explore the expansion processes of C.angustifolia and its controlling factors.The major findings were as follows:(1)As the degree of encroachment of C.angustifolia increased,the root-to-shoot ratio of R.aureum and C.angustifolia increased,which resulted in competition between the two plants gradually shifted from above to below ground.The upward encroachment of C.angustifolia led to a significant reduction in fine root biomass of R.aureum,which may limited the growth of R.aureum.In contrast,the fine root biomass of C.angustifolia increased significantly at a higher level of invasion,which provided C.angustifolia a competitive advantage.This suggested that the encroached herbaceous in the alpine tundra alter the biomass distribution of native dominant plant communities in the tundra through competition between above-and below-ground parts.With increasing encroachment,the growth of the native plant,R.aureum,was suppressed.Thus,competition plays an important role in controlling community dynamics.(2)The native tundra plant R.aureum responded to competition from the encroachment plant,C.angustifolia,by increasing its height and leaf area.With increasing encroachment,the height and leaf area of both R.aureum and C.angustifolia increased.The encroachment of C.angustifolia increased the pressure on R.aureum(a low shrub)to obtain resources such as light,forcing R.aureum to change its morphology to obtain resources in competition,mainly by increasing height and leaf area to ensure adequate levels of photosynthesis.This implied that phenotypic plasticity played an important role in the competition between the two.(3)Some of the niche characteristics of the encroachment herbaceous C.angustifolia and the native shrub R.aureum overlapped by more than fifty percent.There was a high similarity between them in the requirements for topography and soil conditions,which confirmed a strong competitive relationship between the two plants.Under stable environmental conditions(e.g.,without significant climate change)encroachment herbaceous and native shrubs may coexist in the future through niche differentiation to reach a dynamic equilibrium.Under such conditions,R.aureum will maintain its position as the dominant species in the tundra by virtue of its wider ecotopic properties.The difference in the niche characteristic of C.angustifolia and R.aureum indicated that the encroaching potential for C.angustifolia would be limited in the future,which was a fundamental factor in determining how the tundra vegetation will change in the future.(4)The niche characteristic of R.aureum were broader than those of C.angustifolia in terms of topography.Especially,R.aureum could grow on steeper slopes and rougher terrain,while C.angustifolia mostly grew on relatively gentle terrain,and it was difficult to colonize steeper terrain and rougher terrain.Combined with the survey data,we found that C.angustifolia mainly occupies areas such as gullies and depressions in the big plot,which were relatively flat sites within complex areas.These sits had less temperature variation and were less affected by strong winds than the surrounding slopes,and the high precipitation in the tundra and the longer snow-covered period in the sites made the sites have higher soil water content and nitrogen content,which together could provide a suitable microenvironment for C.angustifolia to encroach.Then C.angustifolia could dominate the community and promote expansion with its strong competitive ability.However,the cover of C.angustifolia in 2014-2019 showed an increasing trend in areas with steeper topography and higher surface roughness,which indicated that C.angustifolia had expanded from the sites such as gullies and depressions to the surrounding areas with higher slope and roughness.(5)Nitrogen played a very important role in the encroachment of C.angustifolia.The demand for N of C.angustifolia was much larger than that of the native plants R.aureum,and the effective soil N content of C.angustifolia was quantitatively larger than that of the native plants in the distribution area of C.angustifolia.This was mainly due to the fact that C.angustifolia is a fast-growing perennial herb that required large amounts of nitrogen to complete its life cycle.In addition,the nitrogen use efficiency of C.angustifolia was higher than that of native plants,and the increase of external nitrogen deposition can effectively promote the N fixation efficiency of C.angustifolia and thus rapidly increased its biomass.Meanwhile,in places with higher N content,there was a significant trend of increasing cover of C.angustifolia from 2014 to 2019,while there was a significant trend of decreasing cover of bull rhododendron,which also confirmed the importance of N in the expansion of C.angustifolia.(6)The tolerance of R.aureum to low phosphorus conditions was stronger than that of C.angustifolia.Soil phosphorus was the main limiting nutrient at high altitude,mainly from the mineralization of parent soil,and was hardly influenced by external conditions.And the acidic soil in the tundra of Changbai Mountain led to a great limitation of phosphorus availability.As a result,the expansion of C.angustifolia was faster in the lower-altitude of the tundra,and low phosphorus conditions in the high altitude will limit the further encroachment of C.angustifolia.In contrast,the tolerance of R.aureum to low phosphorus conditions determined that it can survive in low phosphorus areas at high altitudes.Meanwhile,we found that in the areas with higher phosphorus content,the cover of C.angustifolia increased faster between2014-2019,while in the areas with lower phosphorus content,R.aureum showed an increasing trend,which also confirms that R.aureum has a low phosphorus demand.This implied that it will be difficult to completely replace native plants by encroachment herbs in the alpine tundra in the future.