Alpine Treeline Patterns And Its Formation Causes In The Three Parallel Rivers Region, NW Yunnan

Alpine treeline ecotones represent the spatial transition from forested to treeless mountain landscapes and are considered as a well indicator of climate change. Therefore, the alpine treeline spatial patterns and its relationship with environment factors are needed to be known. Currently available data suggest a common isotherm for seasonal mean temperature for natural, climatic tree limits in regions where water permits tree growth (around6.4℃globally with a surprisingly narrow range of±0.7℃from polar to equatorial treelines), However, regional variations of high elevation tree limits have been observed. So these regional variations still need to be explained. The alpine habitat across the Three Parallel Rivers Region of NW Yunnan was classified as an important conservation priority. Because of its special geographical position relative to the direction of the typical monsoonal weather, the mountain system in this region represents an ideal test system to explore both, the spatial pattern and the dynamic of alpine treeline. But the alpine treeline in this region are all in the remote mountain area, resulting in continuous and actual measurement data is hard to obtain. So the alpine treeline study in this region was impeded. Whether the local treeline sites studies can reflect the real circumstance around the whole area is still in doubt.In order to interpret and relate the alpine treeline patterns, and to well understand the underlying ecological processes, we extracted treeline positions from Landsat TM images, DEM (digital elevation model) data and climatic data from Worldclim to analyze the relationship between treeline position and climate. Our study can lay the foundation for further study of alpine treeline response to climate change in this region, offer help to alpine ecosystem conservation and provide data proof for globle treeline theory. Results of the study indicate as followings:(1) The elevations of treeline in this area vary between3800and4300m a.s.l., the east two mountain ranges (Baima Snow Mountain and Mt. Daxueshan) are distinctly higher than the westerns (Mt. Gaoligongshan and Mt. Nushan). This is match with the description made by other studies that the alpine treeline elevation in this region is raising along the monsoon climate route. The distinct summer monsoon climate exerts a W to E, maritime to continentality gradient, along which treeline position is rising in elevation.(2) The results revealed no significant exposure effect. The alpine treelines are not situated in higher positions on south-facing slopes that receive solar radiation under a steeper angle. Counter to expectation, south-facing slopes showed a slightly lower position than the others (less than30m), which may be caused by greater monsoon exposure (cloud cover) And the human disturbance seems more obvious in Mt. Nushan and Baima Snow Mountain Ranges.(3) The mean growing season temperature at treeline showed the same pattern as was obtained globally in earlier assessments (6.7℃). The current treeline position is likely to lag behind the climate of recent decades. And because of the climate change, the mean annual temperature and the mean summer temperature in the study area experienced a warming of0.014K per year and0.0198K per year, respectively, from1901to2001. So the treeline isotherm is slightly higher than the new globe average (6.4℃) maybe because of the impact of climate change. The mean annual precipitation (MAP) at treeline is around1,100mmacross the whole study area and decreases from west to east. It ensures that there is neither drought nor heavy snow pack which could affect the forest limit. The decreased rainfall caused by the summer monsoon which is reaching this region from SW with rainfall decreasing along its route to NE, and along with it, the clouds get less and the solar radiation get enhanced, and the treeline isotherm slightly rises to higher elevations from west to east as well. This gradual change brings alpine treeline position to higher elevation.Our results confirmed the theory that the treeline is strongly associated with atmospheric circulation and does not benefit from greater radiative warming on south-facing slopes during the growing season; once a forest cover is established, the variation in slope-specific solar radiation can be ignored. The common treeline isotherm hypothesis matches with treeline position in this monsoon-driven climate regime. The close association between air temperature and tree temperature, explaining the globally rather uniform isotherm temperatures at the treeline. Moreover the alpine treeline database we established in the study, not only fulfilled this study needs, but also has great significance for the further treeline pattern and dynamic studies in this region. The database could also provide data support for the alpine forest recovery.