Divergence Of Responses Of Tree Ring From Main Conifer Species To Climate In Subalpine Region Of Western Sichuan, China

The Earth is undergoing global warming in the latter half of past century. Under this background, the annual mean temperature of Qinghai-Tibet Plateau has risen by more than 1.3 ℃ during the past 45 years, along with an abrupt warming in the mid-1990 s, which has influenced forest growth and the carbon cycle. Subalpine forest region in western Sichuan, located at the southeast margin of Qinghai-Tibet Plateau and serving as the major headwater area in China, is sensitive to climatic change. It is very helpful to study the response of trees to climate change for the vegetation management and protection in this region.In this study, three places, Miyaluo, Markang and Songpan, were selected as our study area to survey and sample. Using ARSTAN, we not only get standard chronologies for each site but also develop sequences of tree-ring width indices for each sample tree by detrending with negative exponential curve or linear regression fits and then normalizing with double-weighted average. Correlations between tree-ring index sequences and climatic factors(temperature & precipitation) were calculated respectively in one period(1955-2011), two periods(1955-1994, and 1995-2012), and four periods(1955-1982, 1963-1992, 1973-2002 and 1983-2012) to examine consistency of both individual and group responses to climate change. The results are as following.(1) Taking Miyaluo as an example, during 1955-2011, chronologies at low altitudes were negatively correlated with air temperature in April and May and positively correlated with precipitation in the same months, which indicated that spring drought stress limited tree growth. Different tree species showed significant variations. Tsuga chinensis was most severely affected by drought stress, followed by Abies faxoniana and Picea purpurea. However, trees at high altitudes were mainly affected by growing season temperature. Tree-ring index of A. faxoniana was positively correlated with monthly minimum temperature in July. Radial growth of Larix mastersiana was positively correlated with monthly maximum temperature in May.(2) Taking Markang as an example, the growth of A. faxoniana at middle and high altitude sites had a positive temperature response in 1955–1994 and the tree-ring index was relatively high in this period, whereas Cupressus chengiana at the low altitude site was marginally influenced by temperature in 1955–1994 and the tree-ring index was apparently high at this period. The “divergence phenomenon” is expressed as an offset between instrumental temperatures and their underestimation based on tree rings in these sites. The ring width index of A. faxoniana at high altitude site was positively correlated with winter temperature prior to 1995, while it was significantly negatively correlated with April temperature and significantly positively correlated with April precipitation after 1995. This indicated the potential existence of spring drought stress at the high elevation site. The temperature sensitivity of A. faxoniana at the middle altitude site disappeared in 1995–2012, accompanied by the intensification of the effect of spring drought stress on radial growth. The growth of C. chengiana at the low altitude site was mainly controlled by precipitation, and the spring drought stress has a noticeable influence on tree growth after 1995.(3) At tree line in Miyaluo, Markang and Songpan, the consistency of growth-climate relations among tree-ring index sequences of A. faxoniana in 1983-2012 was lowest in Songpan, followed by Markang and Miyaluo. Trees showed divergent growth trends and responses to temperature in growth season in 1983-2012, which was more obvious in Songpan than that in Markang and Miyaluo. The temperature sensitivity in June declined, even disappeared during 1983-2012 in all the three sites. In detail, the positive correlation between tree-ring index and temperature from July to September significantly increased in Miyaluo. The temperature sensitivity in last November and December declined in Markang. More than half of the sampled trees(20 of 34) showed an significant increasing positive correlation with temperature from July to September, while others turned to be a significantly negatively correlation with temperature at the same months.