贡嘎山雅家埂峨眉冷杉林线种群的时空动态

Spatial-temporal dynamics of an Abies fabri population near the alpine treeline in the Yajiageng area of Gongga Mountain,China

通过对贡嘎山雅家埂峨眉冷杉种群林线附近6个3000 m2样地(阴阳坡各3个)中峨眉冷杉(Abies fabri Craib)种群的定位调查,分析了过去100a间该区峨眉冷杉种群的时间-空间动态。结果表明:1)雅家埂林线附近峨眉冷杉种群密度在过去100 a(主要是近50 a)有显著的升高,但树线的海拔位置并无明显的爬升;2)阴阳坡林线格局存在显著的坡向分异:阴坡林线和树线的海拔高度显著高于阳坡(分别比阳坡高152.5 m和135.8 m),阳坡林线附近峨眉冷杉早期的生长速率在大于阴坡,但后期的生长速率却低于阴坡;3)热量(温度)控制假说不能完全解释雅家埂目前的树线格局,除气候因素之外,其它因素也限制了雅家梗地区树线位置的变化。

The high-altitude limit of forests,commonly referred to as the treeline,timberline,or forest line,represents one of the most obvious vegetation boundaries. In most cases,however,the transition from the uppermost closed montane forest to the treeless alpine vegetation is not a line,but an ecotone characterized by stand fragmentation and stuntedness. Owing to its high altitude and complex ecological dynamics,the alpine treeline ecotone is very sensitive to climate changes. This ecotone has accordingly been extensively studied,and a growing body of evidence has already revealed that treelines are moving upslope in the Swedish Scandes,North America,and Eurasia. Although there is some photographic evidence of a rapid treeline advance in the Hengduan Mountains on the southeastern Tibetan Plateau,its dynamics have not been studied in detail. To model how alpine forests will adapt under the predicted temperature increase,a thorough understanding of alpine treeline dynamics is essential. In this study,we chose Gongga Mountain( 29°20'—30°20'N,101°30'—102°15'E,7556 m asl),a typical mountain in the Hengduan Mountains,southwest China,as our study area. Six rectangular plots( each 3000 m2,30 m × 100 m) were established within the natural alpine treeline ecotone on shady and sunny slopes of the Yajiageng area,on the eastern slope of Gongga Mountain. On the basis of a detailed study of the age and distribution of treeline trees( Abies fabri) in the alpine treeline ecotone,the spatial-temporal dynamics of the A. fabri population wereanalyzed over a 50-year period. The results suggested that the six study plots showed a similar pattern of regeneration dynamics,characterized by a gradually increased recruitment in the last 100 years and an abrupt increase in the last 50 years. However,the position of the treeline has moved only slightly and insignificantly upslope,despite the apparent warming on the east slope of Gongga Mountain. Moreover,we found that there were some differences in the spatial-temporal dynamics of the A. fabri population between the sunny and shady slope. The altitude of the timberline and treeline on the shady slope was( 3770.4±6.6) m and( 3771±7.7) m,respectively,whereas that on the sunny slope was( 3617.9±10.0)m and( 3635.7±7.8) m. The altitude of the timberline and treeline on the shady slope was significantly higher than that on sunny slope( 152. 5 m and 135. 8 m for timberline and treeline,respectively). In addition,we also discovered that the growth rate of A. fabri on the shady slope was higher than that on the sunny slope during the early stage of tree growth( tree age below 110 years),but lower during the later stage of growth. Our results showed that the thermal limitation hypothesis alone could not explain the present treeline pattern in the Yajiageng area. In addition to temperature,other environmental factors might also affect the formation of the treeline in Yajiageng. Hence,in the case of non-climatic climax treelines,such as those in our study area,both climate-driven model projections of future treeline positions and the use of the treeline position for bioclimatic monitoring should be used with caution.