| Betula albo-sinensis forest was the most universal forest type in Qinling Mountains, but there was an oppositional idea on the plant position, some ecologists thought it was the climax community, but others regarded it as the transition community and will be replaced by Abies fargesii forest. We researched the small-scale disturbance in terms of gap dynamic theory. The study of forest gaps is very important for natural regeneration and dynamic changes of Betula albo-sinensis community. It could offer the theoretical foundation to Qinling Mountains forest vegetation zone.1. Gap characteristic and disturbance regime in the Betula albo-sinensis forest in Taibai mountain areas were studied, including the quantity, type and causal factors of gaps and the type, number, species composition and size structure of gap makers (GM). The results indicated that the main manner of gap formation in the forest was standing death. The linear gap density in the forest was 23.5 gaps km~-1. The canopy gaps (CG) and expand gaps (EG) covered 4.7% and 11.9% of total land area, respectively. Canopy gaps varied from 18.9 m2 to 289.2 m2 and the majority were less than 80 m2. Expanded gaps varied from 45.2 m2 to 398.2 m2 with the most between 50 m2 and 200 m2. The average number of GM per gap was 1.5 and almost all gaps were formed by 1-3 gap makers. The major gap makers were Betula albo-sinensis, and it was most possible for them to create gaps in the manner of uprooting or standing death when they were 20cm-40cm in D.B.H. The decay class of maximum gap maker was chiefly C class (about 35-45 years old). The ratio of the CG diameter vs. gap height of canopy gap and the ratio of the EG diameter vs. gap height centralized at about 0.30-0.45 and 0.60-0.80, respectively. The seedlings in the gap were mainly Betula albo-sinensis and Pinus armandii, and the large gap size was suitable for regeneration of Betula albo-sinensis.2. The community character gap and non-gap in the Betula.albo-sinensis forest in Taibai mountain areas were studied. The frequency of main species and importance value was different between gap and non-gap. The percentage of Phaenerophyte in gap was higher than that in non-gap, while the percentage of Hemicryptophyte was lower. There was no significant difference of species diversity between gap and non-gap in arbor layer. In shrub layer , however, the diversity index of D, H/ , Dm, E, R, Ma in gap were higher comparing with that of non-gap, and in herb layer the diversity index of D, H' , E, R, Ma in gap were higher comparing with that of non-gap. There were significant difference in water content, organic matter, total N and available P between the upper soil in gap and in non-gap, whereas these characteristics of lower soil layer in gap were not significantly different with non-gap.3. The soil seed bank reserve could offer sufficient seeds of Betula albo-sinensis in gaps and non-gaps. But, only the seeds in gaps with bright light could sprout. In general, forest gaps greater than 80 m2 in the area could offer the sufficient illumination condition for growth of the seedling upgrowthed. By analyzing the curve of structure and dynamics, population dynamics trend shows that the mortality rate of the Betula albo-sinensis population is bigger in 0-4 age. This paper presents the spatial distribution pattern of Betula albo-sinensis population using C\ i\ /*-, M*/x aggregate indices and distance method, the result of measurement indicate that the spatial patterns of the population are clump type.Base on the above analyse: Betula albo-sinensis was the heliophilous tree, only the seeds in gaps with bright light could sprout and the seedlings need light to grow. The gap area, the rate of gap making, the micro-environmental transform and the population dynamics could fulfil the forestupdate in Taibai Mountains. Therefore, gap disturbance was important in forest dynamics.
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