Little information is available for forest gap structure and its forming mechanism in alpine forests, which is essential to understanding of forest regeneration in the scenario of global climate change. Therefore, a field investigation to study forest gap structure and its disturbances was carried out in alpine forest in western Sichuan. Forest gaps with density 14.67 per hm2 in this alpine forest were dominated by medium and small gaps. About 63.64% of gap formation was caused by stem breakage in gap center. The expanded gap (EG) and canopy gap (CP) covered 12.60% and 23.05% of forest area, respectively. The natural disturbance frequency was 115.25 m2 hm-2 a-1 and 63.02 m2 hm-2 a-1 for EG and CG, respectively.

The return interval of canopy gap was about 260.30 a. About 50.09% of gaps were formed by single gap maker, whose DBH and height ranged from 40 cm to 60 cm and from 25 m to 30 m, respectively. Every gap had 1.53 gap makers on average, with each gap maker forming 103.20 m2 EG and 56.43 m2 CG, respectively. The average DBH of gap border tree was 50.16 cm, which exhibited a curve distribution with a significant left peak. Moreover, power function relationships were observed between average DBH and height and the area of EG and CG. Taken together, alpine forest gaps formed in the last two decades occupied a higher proportion compared with those of subalpine forest, with gap maker and border tree having lower height and larger DBH in alpine forest than in subalpine forest. The results indicated that bad weather conditions (such as strong wind and heavy snow) might be an important contributor to regeneration of alpine forest.