| Summary: | 碩士 === 國立嘉義大學 === 生物資源學系研究所 === 105 === Tree death is among the key processes of forest dynamics. Identifying factors affecting tree deaths can help us understand forest dynamics. Typhoon disturbance has been considered as one of the factors that cause tree death. The influences of typhoons on forest dynamics has been widely studied in the Atlantic regions, whereas relatively few studies were conducted in the Northwestern Pacific regions, where the frequency of typhoon disturbances is the highest in the world. Our study site (Nanjenshan Forest Dynamics Plot；2.1 ha) is in a forest frequently disturbed by typhoons (1.3 typhoon hits per year). From July 2015 to October 2016, the mortality rate of trees with diameter at beast height (DBH) ≥ 1 in three transects (10 × 100 m2) were recorded in every three months. Typhoon wind speed data from the Hengchun weather station over the past 22 years (converted into typhoon destructive power) were used to identify the relationship between the typhoon destructive power and tree mortality rates. Tree by tree census data collected from 1993 to 2013 in the intervals of five to seven years were also analyzed by survival analysis to identify which environmental factors (elevation, slope, convexity, gap) and intrinsic factors (tree species, DBH, wood density) are influential to tree mortality. Results showed that the relationship between the mortality rate and the destructive power of typhoons was not significant (P = 0.67). Although the destructive power of typhoons had fluctuated greatly in the past 22 years (the maximum was twice of the minimum), the tree mortality rates are similarly ranged from 2.1 to 2.6% yr-1. Survival analysis showed that tree mortality was negatively correlated with DBH and elevation, and positively related with slope. Species had a significant impact on tree mortality, while wood density and gaps were not significant. The lower the DBH of trees, the higher their tree mortality rates. It may be attributed to the high risk of death caused by light deficit associated with small tree size. Trees on steep slopes were also prone to die, which may be the results of landslides and treefall triggered by heavy rain events. Early successional species were more likely to die because of their short life span. Compared with regions with low frequency of typhoons, tree mortality caused by typhoons was negligible in the Nanjenshan forest. Influential factors for tree mortality were intrinsic properties of trees and topography. The impact of typhoons on the forest is mainly defoliation and branches broken.
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