Mechanism Research On The Vegetation Changes Of The Sub-alpine Tundra, Changbai Mountains

During the last century, alpine tundra of the Changbai Mountains has experienced threeimportant processes including volcano eruption, climate warming, and typhoon disturbance.The current vegetation distributions on the four slopes showed great variations. On the eastslope, vegetation influenced by volcanic disturbance stayed on the primary stage ofsuccession. On the north slope, mountain birch showed upward shift trend to the alpine tundrawhich lead to the shrink of the tundra zone. On the west slope, migration of Deyeuxiaangustifolia from mountain birch forest to the alpine tundra lead to the meadowization oftundra zone. The steady vegetation on the south slope was different with those of the otherthree slopes. This study analyzed the changes in tree line, vegetation pattern and upper edgeof tundra zone through the gradient from low elevations to high elevations. Take Betula.ermanii and D. angustifolia as the main objects, we aimed to figure out the reason why theymoved to different slopes of tundra zone. Focusing on the herb migration, we illustrated themigration process of D. angustifolia and described the impacts of migration on the alpinevegetation on landscape and community scale. Based on the seed establishment andmulti-factor controlled experiments, we investigated the mechanism of D. angustifoliamigration to discuss the controlling effects of environmental change to the vegetationdynamics.1）Tundra vegetation first experienced volcanic disturbance and then showed differentdistribution pattern on the four slopes. Under the climate warming background, it has beenaffected by the typhoon disturbance. As to climate change, through data analysis, climate ofgrowing season in tundra zone showed warming trend whilst precipitation intensity wasgradually increased during last50years. Moreover, accumulated temperature also showedincreasing trend which may promote the growth of plants. Recovery with human interventionof vegetation on the west slope after typhoon disturbance lasted for30years. Nowadays,woody plant was still scarce while understory herbs growth vigorously. Volcanic disturbanceled to the recession of tree line. After that, vegetation of the four slopes were on differentsuccession stages.2）At the bottom of tundra zone, tree line positions of the four slopes were different.Changes of vegetation at tree line positions of the north and west slopes were obviouscompared with those of east and south slopes. From the interpretation results ofhigh-resolution satellite images, it can be seen that the upper elevation of tree can reach on thenorth slope was2140m. Mountain birch showed significant upward shift trend to the tundrazone. The upper elevation of tree can reach on the west slope was2060m which was lowerthan that of the north slope. The tree line position on the west slope was relatively steady.However, prediction results of logistic model we built showed that trees on the west slope hadthe potential to move to high elevations. Variations of tree line positions on the north and westslopes had high relations with topography. At the upper edge of tundra zone, desert areashowed decreasing trend during1983-2008. Mainly reduction occurred at the regions with high slope. Reduction of desert area indicated that there exist expanding trend of vegetation atthe upper edge of tundra zone. However, volcanic ash still limited the distribution ofvegetation. The effects of volcano eruption still existed on the tundra zone.3）Vegetation change in the middle area of tundra zone mainly occurred at the west slope.Through the comparison with the historical survey data, it can be seen that D. angustifolia hadsuccessfully arrived to the tundra zone and formed dominated plant communities. Based onthe analysis of satellite images, we draw a conclusion that migration of D. angustifolia startedin the late1980s. After the year of2000, D. angustifolia gradually replaced Rhododendronchrysanthum and become dominant. D. angustifolia patches preferred wet habitats in thetundra zone. At low elevations, number of the patches was little with large area whilst therewere few patches distributed at high elevations.4）Current vegetation survey results showed that the impacts of D. angustifolia migrationon the tundra ecosystem were serious. On the landscape scale, D. angustifolia patches led tothe fragmentation which changed the distribution pattern of tundra vegetation. On thecommunity scale, arrive of D. angustifolia changed the community structure and resulted inthe reduction of dwarf shrubs and increase of herb plants. Furthermore, D. angustifoliaaccelerated the soil nutrient cycles. Soil water content of D. angustifolia community washigher than that of native plant community. Changes in soil iron proved that there existedredoxing in the soil of D. angustifolia community which indicated the meadowization oftundra vegetation.5）According to the survey results of seed distribution of D. angustifolia and germinationand transplant experiments, it can be inferred that D. angustifolia in the tundra zone camefrom mountain birch forest zone. In the initial stage of migration, seeds transported by windfell in the tundra region and tends to fall in the habitats with low height and cover of plantcommunity. The germination only occurred at the bare sites. When established, D.angustifolia would spread its range by clonal propagation. Results of simulated warming andnitrogen addition experiments showed that the NUE (nitrogen use efficiency) of D.angustifolia was higher than that of native shrub plants, such as R. chrysanthum. Growth of D.angustifolia needed a large amount of nutrients. High production ability, adaptability andnutrient use efficiency enabled D. angustifolia replaced R. chrysanthum and becomedominant.Variations in the tree line positions and migration of D. angustifolia on the four slopesindicated that variations in vegetation distribution were due to the variations in environmentson the four slopes as a result of natural disturbances.