ZONAL TRANSITION OF EVERGREEN, DECIDUOUS, AND CONIFEROUS FORESTS ALONG THE ALTITUDINAL GRADIENT ON A HUMID SUBTROPICAL MOUNTAIN, MT. EMEI, SICHUAN, CHINA

Altitudinal zonation of evergreen, deciduous and coniferous forests on Mt. Emei (3099 m asl, 29 degrees 34.5' N, 103 degrees 21.5' E), Sichu an, China was studied to understand the transition of vegetation zonat ion from tropical to temperate mountains in humid Asia. On the basis o f quantitative data on floristic composition and community structure s ampled at ten plots selected in different altitudes on the eastern slo pe of the mountain, forest zonation and the inter-relationships among different Life-forms of trees in each zonal forest community were stud ied quantitatively. Three forest zones were identified physiognomicall y along the altitudinal gradient, viz. (i) the evergreen broad-leaved forest zone (660-1500 m asl), (ii) the mixed forest zone (1500-2500 m asl), and (iii) the coniferous forest zone (2500-3099 m asl). Great co mpositional changes were observed along elevation, and the zonal fores t communities were characterized by their dominants and floristic comp osition. Maximum tree height decreased from 33 m at lower middle altit ude (965 m asl) to 13 m near the summit (2945 m asl). There was no app arent deciduous forest zone along the altitudinal gradient, but true m ixed forests of three life-forms (evergreen, deciduous, and coniferous ) were formed around 2000-2500 m asl. Patches of deciduous forest were found in a lower part of the mixed forest zone, particularly on scree slopes, between 1450 m and 1900 m asl. These patches were dominated b y the Tertiary relic deciduous trees, such as Davidia involucrata, Tet racentron sinense, and Cercidiphyllum japonicum var. sinense. High spe cies diversity in the mixed forest zone resulted from the overlapping of different life-forms at middle altitudes, which is partly due to wi der variety of temperature-altitude correlations. A comparison of the altitudinal zonation with the ether east Asian mountain vegetation cla rified that Mt. Emei is located exactly at the ecotone between tropica l and temperate zonation types in eastern Asia.