A Study On The Vegetation Ecology Of Subalpine Timberline Ecotone

Subalpine timberline is an apparent ecotone which controlled by climate. Various ecologistssuggested that timberline may be used for the detection of global climatic change . So it is of greatimportance in studying response of timberline ecotone to climate change. Vegetation ecology isthe basic approach to reveal the interrelation between ecological process of ecotone and itsresponse to climate change. This study is conducted on timberline of Abies faxoniana in BalangMountain, Wolong Natural Reserve of Sichuan province and that of Betula ermanii in ChangbaiMountain of Jilin province. By using the methods of transect survey and typical sampling of theecological gradient across timberline ecotone, in combined with qualitative and quantitativemethods to collect the field data, simultaneously the past research results and regional vegetationmaterials are also used for comprehensive analysis. The methodologies of population demographyand quantitative vegetation ecology were adopted for the analysis of the survey data. On the basisof comparison of results of research sites in Balang Mountain and Changbai Mountain, theecological characteristics and distribution pattern of timberline ecotone are summarized. The mainresults of the research are as follows:The species composition of trees in timberline are abundant. Spruce-fir, larch, birch, Quercus,Juniperus and alpine cypress are the main species at timberline. The uplifting and environmentalchange of Qinghai-Xizang Plateau are the motives which lead to the differentiation andspecialization of subalpine coniferous species. The timberline species are of great diversity in thesoutheast part of the plateau. The flora at timberline ecotone, mainly consisting of NorthTemperate and Cosmopolitan distribution type, are of typical temperate feature.The distribution of timberline in the Qinghai-Xizang Plateau coincides with three dimensionalzonation model: H=exp( a + bx +cy + dxy + ex2 + fy2). Among the thermal indices, the averagetemperature in warmest month is the main restrictive factor for timberline distribution. Thehydro-thermal conditions at timberline ecotone is categorized as frost-humid type.Quantitative vegetation ecology is the important means for identifying ecotone. Movingsplit-window (MSW) and vegetation differential profiles (VDP) are two effective methods todetermine ecotone. The peaks and their widths of the MSW distance functions are sensitiveindicators of the locations of ecotone and its ranges. The VDP can also show the environmentalheterogeneity of timberline ecotone.Timberline is a sharp ecotone that the community structure changes drastically. The density,mean height and diameter of stands decrease remarkably. The synusia structure tend to besimplified and the herbaceous species composition tends to be complicated. The herbs are mixtureand infiltration of adjacent communities. The size class of trees at the timberline ecotone presentsan increasing population age structure. This is the basis for stability and increase of timberline.Tree growth is sensitive and susceptible to unfavorable environment. Subalpine is the zone inwhich tree growth decreases sharply and growth form change swiftly. The trees are on multipleadaptation strategies in respect of reproduction.The boundary of timberline ecotone presents apparent edge effect which is reflected by highecological diversity at ecotone in the communities along the transect. Timberline is in the peakarea of either species diversity or β diversity. The boundary form plays a great role on the diversityat timberline in Changbai Mountain. The α diversity in convex and straight boundary is lower thanthat of concave, while the β of the former two boundary forms are higher than that of the latter one.The species turnover rates are more rapid in convex and straight boundaries, while that of concaveboundary is much slower.Gap disturbance is the driving force for tree establishment and forest regeneration at Abiesfaxoniana timberline. The death and establishment process at timberline ecotone are sensitive topast climate change featuring with peak periods of tree death and regeneration in cold period andwarm period respectively. The timberline of Abies faxoniana in Balang Mountain is stable. InChangbai Mountain, the boundary forms at timberline have great impact on seedlingsestablish-ment pattern and colonization in tundra. The concave boundaries are more favorable forseedlings establishment. Boundary form exert powerful control over the colonization of birch totundra. The "concave-convex reversal" of boundary forms may be the possible mechanism oftimberline rise.Finally, based on climate change tendency in recent decades in the Qinghai-Xizang Plateau,the climate change scenarios in the plateau are designed. The response of timberline to climatechange is analyzed and the vertical rise range of 150-600m is predicted. The rise range oftimberline has a tendency of decrease from west to east in southeast plateau.