Abstract:

Beeches are important constructive species in humid temperate forests in the Northern Hemisphere. China contains rich species of genus Fagus, 5 out of the 11 distributed in the country. Although there were many reports about Fagus in China, most of them referred to the south of the Fagus distribution region, and specifically to F. lucida. In this paper, we examined the size class structure and distribution pattern of a deciduous and evergreen mixed beech forest dominated by F. engleriana and Cyclobalanopsis oxyodon to discuss the relationship of the two species, and the community dynamic. The research plot was located in Shennongjia area of Hubei province (31°19′04″ N, 110°29′44″ E), which was in the northern part of the Fagus distribution region. This area crosses the mid-subtropical and north-subtropical zones of China, and is also a transition area of landform. The unique geographic position causes its particular plant species diversity and antiquity. Beech community is an important type of mountain forest in the local vegetation. F. engleriana, F. long ipetiolata and F. lucida are all distributed there, and F. engleriana is the most common species. The 0.96 hm2 permanence plot was established in 2001, and was divided into 384 subplots. All the stems over 1 cm in DBH were identified, measured, tagged, and mapped.The community consisted of 46 woody plant species, belonging to 22 families and 27 genera. The dominant families were Fagaceae, Aceraceae, Ericaceae, Cornaceae and Rosaceae. The tree layer could be divided into three sub-layers. In the first layer, deciduous trees made up 80.7% of the layer, and evergreen species were 19.3%; dominant species were Fagus engleriana and Cyclobalanopsis oxyodon. In the second and third layers, evergreen species increased from 55.9% to 80.5%.C．oxyodon and Rhododendron hypoglaucum were dominant species. The size-class distribution of Fagus engleriana and Cyclobalanopsis oxyodon showed positive pyramidal type, which indicated that both species had rich sapling banks and regenerated well. Three hundred and fourteen stems including 289 sprouting ones were counted for Fagus engleriana in the 0.96 hm2 plot. The sprouting stems were 90.8% of the total stems, and some researches showed sprouting could be the major method of recruitment of the population. C.oxyodon had 1 576 stems, 74.6% sprouting. Although sprouting was a large part of the stems, C．oxyodon was mainly regenerated by seed. Distribution patterns were analyzed using David and Moore’s index, Lloyd’s index and Mosisita index in plot sizes of 1 m × 1 m, 3 m × 3 m, 5 m × 5 m, 7 m × 7 m, 9 m × 9 m, 11 m × 11 m, 13 m ×13 m, and 15 m × 15 m. The Cyclobalanopsis oxyodon population was divided into three size classes as DBH 1-4 cm, DBH 4－8 cm and DBH > 8 cm. The distributions of all of these classes were clumped. This may be because the life span of C．oxyodon was so short that no self-thinning happened during this period. The distribution pattern of F．engleriana was also clumped. The possible reasons were: 1) the recruitment of F. engleriana needed open canopies. The pattern in fact showed the disturbance history. 2) compared with other beech forests, this community had not reached maturity. In this stage of succession the distribution pattern was clumped. 3) the distribution could be influenced by topography. The slope of the plot is 49.5 degree on average. This could highly affect seed distribution, and so also affect the pattern of the population. The studied Fagus engleriana-Cyclobalanopsis oxyodon community had rich species diversity and clear structure. AlthoughC．oxyodon was the most important species in the community, it was not a canopy tree at this elevation. So F．engleriana was the constructive species and C．oxyodon was a co-dominator. The two species both had sufficient regeneration storage that the community could exist stably.