Responses of radial growth to fire disturbance in alpine pine (Pinus densata) of different age classes in Nang County, Xizang, China

Aims Forest fire plays a complex and important role in affecting forest regeneration, tree growth, and stand development. Despite the importance of forest fire in modulating forest dynamics, researches on the response patterns of trees of different age-classes to fire disturbances are scarce. This study was conducted to determine the growth patterns of surviving trees of different age-classes in an alpine pine (Pinus densata) forest in the southeastern Xizang Plateau, where a moderate surface fire occurred in 2005. Methods We collected tree-ring samples of P. densata in the Gong-Zi-Nong valley in Nang County, Xizang Autonomous Region, in western China. Based on the diameter at breast height (DBH), the sampling trees were divided into saplings (DBH < 10 cm) and mature trees (DBH ≥10 cm). The tree-ring samples were subsequently polished, measured, cross-dated, and detrended to produce sequences of tree-ring width indices. The detrended tree-ring sequences were averaged using bi-weight robust method to develop chronologies for saplings and mature trees separately. Pearson correlation coefficients and response coefficients between the sequences of tree-ring width indices and climate factors (air temperature and precipitation) were calculated for examination of the responses of tree-ring sequences to monthly mean air temperature and monthly total precipitation both before and after the fire event. Indices of growth resistance and growth recovery were calculated based on the relative changes of trees’ raw ring width before and after the fire event, for trees in different age-classes. These indices were then compared between the trees in different age classes to assess the impacts of fire on trees. Important findings Before the fire event, the radial growth of saplings showed a significantly negative response to the monthly mean minimum temperature of the preceding November, whereas the radial growth of mature trees ©植物生态学报 Chinese Journal of Plant Ecology 李宝等: 西藏朗县地区不同龄级高山松林木径向生长对火干扰的响应 437 doi: 10.17521/cjpe.2015.0440 showed a significantly positive response to monthly mean minimum temperature and monthly mean temperature of current September; following the fire event, radial growth of both the saplings and the mature trees showed a significantly negative response to monthly mean temperature and monthly mean maximum temperature of January of the tree-ring formation year. Based on the ratios of mean tree-ring widths of 5 post-fire years to those of 5 pre-fire years, the mature trees were significantly more fire resistant than the saplings. Moreover, the mature trees also showed greater ability in post-fire recoveries than the burnt saplings. Our results demonstrated that moderate surface fire stimulated the radial growth of both saplings and mature trees, and that the mature trees better recovered from the fire event than the saplings. The changes in growth-climate relationships following the fire event may attribute to changes in understory vegetation and microenvironments.