Structure and dynamics of subalpine forests in giant panda habitat

The structure and regeneration mechanism of subalpine forest, one of the most important habitats for Giant panda was determined by the abundance of understory bamboo and gap disturbance regime. Four sample plots ranged from 5000 m~2 to 4000 m~2 in size and sixty-two gaps were investigated through sample plot and Point-centred quarter Method to explore the forest characteristics and the relationships of forest regeneration with bamboo and gap disturbance in Wanglang Nature Reserve. Abies faxoniana and Picea purpurea were the most abundant tree species in the forest. The density of Abies faxoniana was 2～8 times higher than those of Picea purpurea. The forests displayed continuous age distribution pattern or discontinuous pattern in the period of 250 and 500 years. In three of the plots, non-regeneration of Picea purpurea lasted for more than 100 years in the past 550～600 years. Gap survey indicated the prevalence of a gap-phrase mode of regeneration and canopy disturbance of Abies faxoniana, Picea purpurea and Betula utilis. Saplings, small and large seedling and trees regenerated in 90% of the gaps. Density of Abies faxoniana were the highest, with percentage of 82.5% compared with other tree such as Picea purpurea with 11.3% and Betula Utilis with 6.2%. The sapling and seedling densities of these trees were correlated with gap size(P0.05), and the correlation of Abies faxoniana with gap size was the highest(r=0.48%) followed by Picea purpurea(r=0.37) and Betula utilis (r=0.32). The gaps, which had a density of 17.49个/hm~2, turned over every 469 year, which meant 0.199/ hm~2 gaps were formed each year. Gap sized from 49 m~2 to 675 m~2 and mean size of the gaps was 26 m~2.In the meantime, the forest regeneration was determined by the growth competition between the bamboo and seedlings. The fact that low density of saplings and seedlings of trees when the bamboo cover is high indicated that bamboo growth impeded the forest regeneration process. The abundance of saplings and seedlings of Abies faxoniana in plot 1 and 3 with low bamboo density was 5～6 times higher than those of Abies faxoniana in plot 2 and 4 with high bamboo density, and the abundance of saplings and seedlings of Picea purpurea in plot 1 and 3 was 10 times those of Picea purpurea in plot 2 and 4. The density of Abies faxoniana seedlings was 20～40 times those of Picea purpurea in plot 1 and 3 with low bamboo cover, and the seedling density of Abies faxoniana was 6～9 times of those of Picea purpurea in plot 2 and 4 with high bamboo cover. These suggested that bamboo cover effectted Abies faxoniana more than Picea purpurea. The life history traits of bamboo that it flowered intermittently and die-off periodically every few decades led to the synchronism of tree regeneration: in plot 1 and 3 with declining bamboo cover, the forest regeneration ability was high between last 100～140 years.