The Responses Of Abies Faxoniana And Picea Purpurea To Climate Factors In Subalpine Of Western Sichuan Province, China

Climate change in the20th century, especially the significant climate warming, has deeply influenced the dynamics of forests in the high latitudes and high altitudes. Subalpine forest is one of the most important and sensitive indicators for global climate change. It is very important for us to understand and grasp global change, to study the response of subalpine forest ecosystem to climate change. As one of the most sensitive areas to global change, subalpine in western Sichuan province owns prominent standing and role in the research about global climate change. Wanglang Nature Reserve belongs to the typical subalpine area of western Sichuan province on the eastern edge of Qinghai-Tibet Plateau. The unique landscape and the vast virgin forest supply the advantaged conditions for us to study forestry adapting to climate change. To carry out systematic dendroclimatology study in this area is beneficial for gaining profound knowledge on the influences and consequences of climate change to forest ecosystem. It is also conductive to forecasting the time-space distribution patterns of forest ecosystem in the future.Regarding Abies faxoniana and Picea purpurea as the research object, we established five sampling transects for Abies faxoniana and one sampling transect for Picea purpurea. After surveyed and sampled within and around the six transects, we developed the corresponding chronologies and age structures for both the tree species. At the same time, we simulated the month climate factors for each sampling site using the software of MTCLIM. And then, through correlation and component analysis, correlation function and response function analysis, pointer year analysis, and growth release analysis, we revealed tree radial growth patterns and recruitment dynamics influenced by climate, slope, elevation and age. This study filled the research gap of treeline dynamics in the humid and semi-humid climate sensitive areas in subalphine of western Sichuan province. The main conclusions can be drawn as follows: (1) The annual-growth of Abies faxoniana in all sampling sites had certain common characteristics, such as the lowest value appeared around1937and there was a down trend after1990. The pointer narrow years for all the chronologies of the two different aspect occurred in the same time period, such as1935-1937,1967,1976and1982. The correlation and principal component analysis among all the chronologies showed that tree growth was controlled by common factors at different altitudes and different aspect. Fir growth among different sampling transects was influenced by common climatic factors:the warming temperature in the previous growing season (July and August) restrained fir radial growth; adequate precipitation in the current January promoted growth. Only in the two low-altitudes transects, fir growth was significant affected by growing season precipitation:the previous September precipitation promoted fir radial growth significantly in the low-altitude of northwestern slope; fir growth in the low-altitude of southeast slope was significantly positively correlated with the current July precipitation.(2) There was some consistency about relationships between radial growth of Picea purpurea and climate factors at different altitudes:the three tree-ring indexes were all negatively correlated with December precipitation, and were all positively correlated with monthly mean temperature and monthly mean maximum temperature in current June. The sensitivity of radial growth to climate factors was also different in some ways with the elevation gradient, spruce growth in the low and mid-elevation were inhibited by the temperatures of previous growing season (July and August); tree growth in the high-altitude sampling site was obviously promoted by the temperatures before the growing season (December, February and April) and the temperatures in the growing season (June and July).(3) That water loss in the growing season caused by the warming of the past30years might be the main reason for the "abruption" between the fir growth and temperature at the low-elevation. At the same habitat of low altitude:fir radial growth was mainly negatively influenced by the temperatures of previous growing season (July and August) and in the end of current growing season (September); spruce growth only showed significant positive correlations with monthly mean minimum temperatures of previous December and current September; fir and spruce were all showed significant negative correlations with previous October precipitation. At the same habitat of high altitude:fir growth was obviously inhibited by the high temperatures in previous growing season (June and August), and was promoted by the higher monthly mean minimum temperatures of current growing season (July); spruce radial growth showed significant positive correlations with the monthly mean minimum temperatures before growing season (December, February and April) and in the growing season (June and July), and the high temperatures in current February and June also promoted radial growth.(4) With tree age increasing, the sensitivity of spruce reduced. The responses of young spruce were significant with temperatures before growing season and in growing season. The chronology of the middle-aged spruce showed significant and positive correlation with monthly mean minimum temperatures in current April and July. Ring width index of old spruce was significantly negatively correlated with monthly mean temperature and monthly mean minimum temperature of previous August. The "lag effect" of high temperatures in previous growing season was prominent in the old spruce. Spruce within young group and mid-age group showed a significant negative correlation with current June precipitation. Adequate precipitation in December was not beneficial for the radial growth of young and aged spruce.(5) Small-scale interferences occurred in low frequency were not the main factors to restrict fir recruitment. In the northwestern treeline with sparse bamboo, the relatively rich seedling recruitment in recent60years was beneficial for higher monthly mean minimum temperatures before growing season (January, February and March) and in the growing season (May, June, September and October). Adequate precipitation before the growing season (January and February) was also beneficial for seedlings’ survival. Recruitment was greatly restricted by competition with dense bamboos in the southeast treeline.