中文摘要：

从人口分布变迁入手探讨植被变迁,发现距今500年以前,长江上游亚热带山地海拔3000～1500m的中低山地区多是乔木林为主而间与草甸混交景观,而非现在的以草甸、灌草坡为主的景观。人类主要居住在海拔1500m以下的平坝、台地和丘陵地区,人类活动对海拔1500m以上的山地地区影响甚小。现代地理学界静态描述的长江上游地区海拔1500～3000m的中低山湿地草坡带,在500多年前仍多是以冷杉、云杉为主的针阔叶林与草甸混交景观。近500年来人类垦殖、商业砍伐、皇木采办等活动是造成这种变化的主要原因。皇木采办、商业采办影响了长江上游中低山的森林资源的品质,主要对高大的楠杉的砍伐,较明显地影响了森林的郁闭度,但并不给森林资源带来不可回归的影响。人类垦殖活动,特别是固定的农耕活动,大量砍挖树根,使森林资源的自我恢复失去了可能。气候变化的影响可能在其中,一时还难以区分,需继续关注。

英文摘要：

In this paper,the author holds that the sub-tropical medium and low mountainous areas,at an altitude of 3000-1500m above the sea level along the upper reaches of the Yangtze River were mainly covered with arbors mixed with meadow landscape 500 years ago,rather than meadow and shrub-grass-based landscape today.Since man mostly live on flat grounds,platforms and hilly regions at an altitude below 1500 m,human activities have little impact on the mountainous areas over 1500 m.In short,along the upper reaches of the Yangtze River,wetland and grass-slope zone in the low hills with 1500-3000 m above the sea level,which was described statically by modern geologists,was actually mixed coniferous and deciduous forests dominated by abies and picea 500 years ago.The main reasons for the change are land cultivation,commercial logging and timber gathering for the imperial palaces in recent 500 years.The ecological environment thus cannot be completely restored,either by artificial or natural reconstruction. Timber gathering for the imperial palaces and commercial logging had a great impact on the forest of medium and low mountains along the upper reaches of the Yangtze River.Phoebe and China Fir chopping greatly affected the canopy density of the forest in particular,but it exerted no introgression to the forest resources.Land cultivation,especially the stable farming activities which had dug a large amount of roots,made it impossible for the recovery of the forest resources.So far we have not found out the causes of the vegetation alteration,of which how much was caused by the climate change,so further study is needed.