Responses Of Picea And Abies At The Treeline On The Eastern Edge Of The Tibetan Plateau To Climate Change

The earth is undergoing global warming in the latter half of 20th century.The Tibetan Plateau,as one of the most sensitive regions,has experienced significant warming.Treeline trees in the eastern Tibetan Plateau(TP)are very susceptible to warming and rising atmospheric CO2 concentration,but it is unclear how the radial growth of treeline trees respond to changing climate and the underlying physiological processes has been known little.A study on the response of trees at treeline to climate change is helpful for the vegetation adaptive management and forest protection in this region.In this study,we collected 534 ring cores of spruce(Picea spp.)and fir(Abies spp.)genera from 17 sites across 10 latitude gradients on the east edge of the Tibetan Plateau(Qinghai,Sichuan and Yunnan)to study the radial growth-climate relationship.Using ARSTAN,we not only got standard chronologies for each site but also developed sequences of tree-ring width indices for each sample tree by detrending with negative exponential curve or linear regression fits.The growth trends were detected by using the unitary linearity regression model and Pearson correlation was performed to analyze the relationship between individual tree growth and monthly mean temperature before and after the rapid warming.The proportions of positive response to the spring and growing season air temperature were also ascertained.Paired sample t test and single factor analysis of variance were used to detect trees’ response to climate warming.The relationships between basal area increment(BAI),intrinsic water use efficiency(iWUE)and air temperature were analyzed and quantified by the commonality analysis.How the net photosynthetic rate and stomatal conductance affect iWUE was also examined.The main results are as follows:(1)The mean annual temperature rapidly increased after 1980,modeled by the generalized additive model.The annual precipitation did not change significantly.However,the precipitation varied with sampling sites:Miyaluo and Markang in western Sichuan have the highest annual precipitation,while Jianza and Zeku in Qinghai have the lowest annual precipitation.(2)The radial growth of spruce increased significantly at most sites except for that in the extremely dry Jianzha,whereas it was not significant for fir except for that in the wetter Xianggelila.(3)The proportion of spruce positively responding to spring and growing season temperature increased from 6%and 18%to 24%and 43%,respectively,while that of fir did not change.The spatial variability of trees’ response to the spring and growing season air temperature coincided with the precipitation decreasing.That is,the proportion of positive response individuals increased with the precipitation increase.(4)From 1900 to 2016,the basal area increment(BAI)of spruce increased significantly with a rapid of 0.74 cm2·decade-1.Whereas,after tipping point of 1980,the BAI of fir showed the two opposing trends(increased trend and decrease trend).The effect of intrinsic water use efficiency(iWUE)explained most of the enhancement of BAI for spruce(20.15%).And the increasing mean annual air temperature and atmospheric CO2 concentration synergistically explained most(47.88%)of the increased iWUE.The previous autumn and winter temperature and the current growing season temperature explained most of the increase of BAI for fir(11.42%).Whereas,the pure effect of iWUE,which was mainly explained by the rising atmospheric CO2 concentration(36.87%),was just 0.77%.The iWUE of the decreasing fir trees was much lower than that of the increasing ones(P<0.001)before the tipping point of rapid warming.After then,the Ci/Ca and Δ13C of the decreasing fir trees decreased significantly leading to an increase of iWUE which was even higher than that of the increasing ones after 1998(P<0.01).The main conclusions are as follows:(1)Higher spring and growing season temperatures contributed the radial growth of spruce but not for fir.The fir trees showed the decreasing positive response,increased negative response and even the two opposing responses were found.Precipitation determined the spatial variability of trees’ response to the air temperature.(2)Rising temperature and atmospheric CO2 concentration synergistically stimulated the increases of iWUE leading to the stimulated growth of spruce while the increase of iWUE is useless for fir in Songpan site.For the fir trees,the increase of BAI was mainly caused by the previous autumn,winter and the current growing season temperature.For the decreased fir trees,the stomatal conductance decreased significantly after the tipping point of rapid warming and the increased iWUE could not improve tree growth sufficiently to compensate for warminginduced water stress.This study provides a theoretical basis for further understanding the response of forests in the eastern edge of the Tibetan plateau to climate change.