| Evergreen broad-leaved forest widely distributes in China, and is one of the most representative forest vegetation types in subtropical region in China. However, the vegetation has been destroyed due to human's unreasonable exploitation and utilization for a long time. Subtropical evergreen broad-leaved forest which grows in east coastal low mountain area of china has its own unique characters to confront natural disturbances, such as typhoons and occasional snowstorm. In addition, the complex geomorphic condition of valley and ridge also contribute to the characteristics of gap disturbance, which play a great role in study the recovery of evergreen broad-leaved forest and the succession of forest ecological system. Based on the data of20hm2dynamic plot of evergreen broad-leaved forest in Tiantong, Zhejiang Province, eastern China, this paper used geographic information system software to study gaps basic characteristics, topographical distribution pattern and species diversity maintenance mechanism of Tiantong evergreen broad-leaved forest. The main conclusions are as follows:(1) A total of174gaps with the area of23980.87m2were investigated in Tiantong20hm2plot of evergreen broad-leaved forest. Results show that the gap fraction is13.06%and gap density is9.5/hm2, and the average size was137.82m2. All the gaps in Tiantong evergreen broad-leaved forest are irregular polygon shape. Generally speaking, the shape of most gaps is similar oval. But overall the shape of gap was nearly. Tiantong evergreen broad-leaved forest is a relatively stable forest community.85.06%of the gaps with an area of less than200m2and most of gaps were formed in a short time, and81%of the gaps with the height of less than4m.(2) There are656gap markers for the174gaps in Tiantong evergreen broad-leaved forest, with the number of gap markers ranged from1～16. The average number of gap markers for each gap is3.77. Gaps are mainly formed by uprooting, standing death, and stem breakage. Most gaps are formed by two formation types, followed by three and one formation types. In gap with one formation type, overturning collapse branches had the highest proportion (53.5%); In gap with two formation types, combining form of uprooting+stem breakage and uprooting+standing death have the highest proportion (34.8%and33.3%). In gap with three formation types, the combining form of uprooting+standing death+stem breakage have the highest proportion (73.1%).(3) The gap fraction and gap density in high elevation (≥500m) was significantly greater than that in medium and low elevation. Greater typhoon disturbances in high elevation were the major contributor to this circumstance. The gap fraction and gap density within the valley area was significantly greater than that of the slope area, while the average gap area within the valley area was significantly greater than that of slope and ridge area. Slop failure caused by intense rainfall during typhoon weather was the major cause. In summary, typhoon and accompanied intense rainfall was the major cause for the significant difference of the three gap parameters along the gradient of elevation and slope position.(4) The results of rarefaction species richness test and Wilcoxon signed-rank test show that the gap only increase the number of overall species and shrub species, while the species richness has not been increased. But species richness of sunlight preferred species within the gap was significantly great than within non-gap. This shows that in early phase of gap formation, it is mainly the sunlight preferred species increase the species diversity in the gap. |
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