Research On The Responses Of Subalpine Treeline Of The Changbai Mountains To Global Climate Change

Response of the formation mechanism and change of the alpine and subalpine treeline to climate change is one of the research hotspots in global change research. According to data collection and field experiment, this study obtained a large number of valuable data about the treeline in the Changbai Mountains. On this basis, this study analyzed and discussed the characteristics of climate change at the treeline in the Changbai Mountains, the formation mechanism of treeline, the growth situation of Betula ermanii saplings at the upper rim of treeline, the relationship between the radial growth of Betula ermanii and climate, and the change situation of treeline location. The main results are drawn as the follows:(1) The temperature in summer shows significant increasing trend (p< 0.05) at the treeline of the Changbai Mountains during the last 55 years. Temperature warmed in the early and middle 1980s and further raised in the late 1990s. Precipitation shows insignificant increasing trend.(2) The high content of soluble sugar and starch in plant carbon sink tissue in the end of the growing season is a protective strategy of ensuring successfully overwintering for the Betula ermanii saplings at the upper rim of treeline. However, the ratio of soluble sugar to starch is not the determined factor affecting plant successfully overwinter. Both carbon and growth limitation factors exist in the growth of the Betula ermanii saplings at the upper rim of treeline. The carbon limitation just reduces the content of carbon source, whereas the growth limitation is the determined factor formatting the current upper rim of treeline. Therefore, the growth of the Betula ermanii at high altitudes of the Changbai Mountains is limited by the growth limitation factor, which results in the formation of treeline.(3) The small-scale invasion of the Betula ermanii saplings at the upper rim of treeline to tundra was before 2000, whereas the large-scale invasion was from 2000 onward. The spatial distribution patterns between the younger and older Betula ermanii saplings at different altitudinal gradients are similar, that is, Betula ermanii shows aggregated distribution at small scale but random distribution at large scale. However, the Betula ermanii of the entire sample plot shows aggregated distribution at 0-50 m scale. The habitat heterogeneity, small intraspecific and interspecific competition, and the early stage of succession may be the main cause of aggregation distribution Betula ermanii.(4) This study established the growth curve of Betula ermanii. The radial growth of Betula ermanii increases rapidly first, and then decreases slowly, and will eventually reach a relatively stable level. The biological age of the most apparent of Betula ermanii growth is approximate 40 years. This growth curve can effectively remove the physiological factors from the treering index and retain climate information. No dead Betula ermanii was found in the sample plot. Whether the treeline can shift upward or not may be directly related to whether the seeds of Betula ermanii can germinate at higher altitudes or not under the environment change condition.The radial growth of Betula ermanii is mainly associated with monthly and seasonal temperature and precipitation, specifically, which are the mean temperature in current March, May and summer and precipitation in July. Especially, the relationship between summer mean temperature and the growth of Betula ermanii is significant at the 0.01 level. The summer mean temperature plays a vital role on the radial growth of Betula ermanii. The correlation between summer mean temperature and radial growth of Betula ermanii is positive, so high temperature is in the favor of the radial growth of Betula ermanii. The correlation between precipitation in July and radial growth of Betula ermanii is negative, so high precipitation amount is against to the radial growth of Betula ermanii and the moderate precipitation is in the favor of the radial growth of Betula ermanii. Compared with the 1980s, the radial growth of Betula ermanii at the treeline of the Changbai Mountains would increase approximate 10.6%(1.9%?19.3) under the future climate change scenarios. Therefore, the treeline of the Changbai Mountains would shift upward on this condition in the future.(5) Since 1855, the treeline of the north and west slopes of the Changbai Mountains have been in a upward shift process, and the speed is faster before 1890, when the main driver is the natural succession. From 1890 to the middle 1980s, the succession speed is limited by the more and more external environment with increasing altitude, so the upward shift speed of treeline is slow in this stage. After the middle 1980s, the upward shift speed of treeline become fast again due to the human-made climate warming. During the last 160 years, the treeline of the north and west slopes shifted upward 80 m and 100 m, respectively; the upward shift speeds are 5 m/10a and 6.25 m/10a, respectively. The upward shifts of the treeline of the Changbai Mountains are caused by the combined effects of natural succession and climate warming. The location of the treeline of west slope is higher than that of north slope. The current altitudes of the treeline of west and north slopes are 2200 m and 2140 m, respectively. The treeline of the Changbai Mountains would shift upward under the future climate warming scenarios.The main conclusions are drown as follows:The formation of the current treeline of the Changbai Mountains is caused by the growth limitation. Due to the driven by the succession (natural recovery) after the volcanic eruption of the Changbai Mountains in 1702 and climate warming, the treeline of the Changbai Mountains experienced staged upward shifts during the last 160 years. The radial growth of Betula ermanii of the Changbai Mountains would increase and the treeline of the Changbai Mountains would shift upward under the future climate warming scenarios.