The effect of climate warming on the structure characteristic of the timberline in Laotudingzi Mountain
气候变暖对老秃顶子林线结构特征的影响

Ecotones, the boundary zones between major ecosystems, are important sites for examining the effects of climate change on ecosystem structure. The forest-tundra ecotone, formed by alpine timberline, may be especially sensitive to climate change. This paper analyzed the timberline stand structure related climate factors at Laotudingzi Mountain in Dahailin region by using transect investigation and dendrochronological methods. There was a trend of climate warming during the past 30 years, especially in winter seasons. February was the most sensitive month in a year. Both summer and winter temperature increased, and the later was greater. The temperature difference between summer and winter decreased, but the accumulated temperature slightly increased, indicating that the yearly heat increased. The climate warming in Dahailin region greatly influenced the structure characteristic of the timberline. The rate of regeneration and survival in the upper timberline ecotone and forest coverage increase, but stand age decreased. The trees class was clumped distributed structure with and an inversed "J" type age structive. The regeneration in the lower timberline was poor and exhibits scattered distributed. The diameter and height growth increased because of the climate warming. The trend of the ring index was consistent with the temperature variation during the past 30 years. The tree ring index at timberline was strongly correlated to the temperature and slightly to precipitation. The warm period temperature and accumulated temperature were positively correlated to the ring index, while the cold period temperature and mean annul temperature were negatively correlated to the ring index. These suggested that the warm period temperature control the timberline elevation, and the cold period temperature determine the type of tree species at timberline. The sensitivity analysis indicated that the tree at the upper timberline was more sensitive than those at lower timberline, but not at the most upper timberline. The sensitivity of Betula ermanii was better than spruce-fir by comparing, which mainly related with the location of Betula ermanii, and the mid-age tree had the better sensitivity.