Differences In Response To Climate Change Of Picea Purpurea And Abies Faxoniana At Different Elevations And Slopes In Western Sichuan Plateau, China

Subalpine coniferous forests in Western Sichuan was the main part of the Southwest Forest and a typical environment fragile zone. Even slight climate change would have a significant impact on the forests in Western Sichuan. Nearly 50 years, Western Sichuan Plateau in most regions appeared significant warming, so it formed the warmest on record period. In this context, we carried out a major study of the following two aspects by using the tree ring data of Picea purpurea and Abies faxoniana:(1) This paper discussed the different aspect spruce-fir in response to rapid heating of the difference by using tree-ring chronologies of different slopes aspect of P. purpurea and A. faxoniana. (2) In this paper, in order to investigate the response of A. faxoniana tree rings at different elevations to the threshold temperature and accumulated temperature on the radial growth, we established tree ring width chronology of A. faxoniana at different elevations in Western Sichuan Plateau of China.Tree growth of P. purpurea at east slope aspect was obviously accelerated (0.011/a) since rapid warming in 1980, whereas those at northwest slope aspect was significantly reduced (-0.006/a). Tree growth of P. purpurea at southeast and A. faxoniana at northwest slope aspect decreased insignificantly. With the rapid warming, growth-climate relationships of P. purpurea and A. faxoniana at different slope aspects changed significantly. After rapid warming in 1980, the promoting effects of growing season temperature (GST) on P. purpurea growth at east slope increased significantly, while the inhibitory effects of GST on its growth at at southeast and northwest slope also increased significantly. However, the effects of GST on A. faxoniana growth at northwest slope did not change significantly before and after rapid warming. The effects of precipitation in May (PM) on P. purpurea growth at east slope was changed from inhibition before rapid warming to significant promotion after rapid warming, while the inhibitory effects of PM on P. purpurea growth at southeast and northwest slope increased significantly. For A. faxoniana at northwest slope, however, it did not change obviously before and after rapid warming. The response analysis between tree growth and the Palmer Drought Severity Index (PDSI) showed that soil moisture variations at different slope aspects is an important reason of tree-ring growth response difference since rapid warming. In addition, the results of multivariate analysis of variance indicated that the combined effect of slope and climatic factors was most important limited factors for tree growth variability in western Sichuan. Therefore, we should consider the combined effects of different slope aspects and climatic factors when simulating and predicting tree-growth response to the recent climate warming trend.The most sensitive variable to the radial growth of A. faxoniana in high elevations was the temperature, whereas A. faxoniana at the low-altitude site was marginally influenced by temperature. The correlation analyses showed that the tree-ring width of A. faxoniana of low elevations was negatively correlated to the temperatures of the growing season and the tree-ring width of A. faxoniana at high elevation was positively correlated to the temperatures of the growing season(July, August and September month), winter(December, January and February month) and last year in September and October month, however, weak correlation with precipitation. Radial growth of A. faxoniana at different elevations was a significant negative correlation with initial dates of different earlier initial temperatures,6.5℃ temperature on the radial growth of A. faxoniana at different elevations had the strongest negative correlation. Besides, A. faxoniana at high elevation had the significant negative correlation with initial dates when the growing season was defined as the days above the temperature threshold of 9.5 ℃, and the significant positive correlation with later final dates of 6.5℃. Radial growth of A. faxoniana at different elevations was a significant positive correlation with later final dates of different earlier initial temperatures,8.5℃ temperature on the radial growth of A. faxoniana at different elevations had the strongest positive correlation. The analyses between the chronology and other growing season parameters showed that tree growth responded positively to the cumulative temperature (the temperature threshold of 6.5～9.5℃) and continuous days (9.5℃) of the growing season. So 9.5℃ was threshold temperature of cambium activity and 6.5℃ was threshold temperature of treeline A. faxoniana in photosynthesis. Defined this way, the growing season typically started in early-May and ended in early-October. Results showed that as temperature rose after 1980, the growing season was lengthened (6.4d/10a, R2=0.19, P=0.01) with both earlier initial dates (4.6d/10a, R2=0.19,P=0.01) and later final dates (1.8d/10a, R2=0.04, P=0.25). Longer growing season increased the growth of A. faxoniana in tree-line.