Difference Of Climatic Response Of Tree Rings Between Larix Gmelinii Timber Line And Non-timber Line

Timber line is the ecological boundary of forest distribution,and the radial growth of trees is affected and limited by their own physiological factors and external factors.Under the background of global warming,the relationship between tree growth and climate change response may be different between timber line and non-timber line.In this paper,taking Larix gmelinii in Okedui Mountain area of Daxing’an Mountains as the research object,the standard chronology of Larix gmelinii was established by using the method of tree ring climatology.By analyzing the variation characteristics of temperature and precipitation and the climate response law of radial growth of Larix gmelinii between timber line and non-timber line,the difference of growth response of Larix gmelinii to climatic factors in timber line and non-timber line was analyzed and compared.The results show that:(1)In terms of climate change,the highest temperature and the lowest temperature in the northern Daxing’an Mountains show a significant upward trend,and the maximum temperature and the lowest temperature are consistent in the whole time series.The precipitation shows an increasing trend as a whole,but the change law is not the same in different periods.The fluctuation range of dry and wet period is large in Aokudui Mountain area,and the dry and wet condition tends to drought as a whole.The Palmer Drought Severity index(PDSI)showed a slight downward trend as a whole,and the change trend was different in each stage,showing a trend of first decline,then rise and then decline.According to the change law of PDSI index,the whole time series can be divided into three stages:the drought period from 1960 to 1980,the humid period from 1981 to 2000 and the drought period from 2001 to 2016.(2)The variation of ring width index of Larix gmelinii at timber line and non-timber line showed obvious difference in the public area.The growth of Larix gmelinii at the timber line showed an obvious upward trend,but at the non-timber line,the change of Larix gmelinii was not obvious in the whole period of time,and the radial growth of trees did not increase with the increase of temperature.The results show that there is a difference in the response of tree growth to global warming between the timber line and the non-timber line,the temperature condition of the timber line is worse than that of the non-timber line,and the temperature rise has a greater impact on the tree growth at the timber line.(3)The main limiting factor affecting the radial growth of Larix gmelinii in timber line and non-timber line is temperature.The tree growth at the timber line and non-timber line has relatively consistent response characteristics to the lowest temperature,but it is different from the highest temperature response:there is a significant positive correlation between tree growth and the highest temperature at the timber line.the correlation between tree growth and the highest temperature at non-timber line is not significant.In terms of response coefficient intensity,the response coefficient of tree growth and the lowest temperature at the timber line is significantly higher than that at the non-timber line,indicating that the tree growth at the timber line is more sensitive to climate change than the non-timber line under the background of climate warming.(4)The correlation between the radial growth of trees at the timber line and non-timber line and the characteristic values of dry and wet period did not reach a significant level.The results showed that short-term drought or moisture did not significantly promote or inhibit the radial growth of Larix gmelinii at timber line and non-timber line,and the growth of Larix gmelinii was mainly affected by medium-term or long-term drought or moisture.(5)The effect of the coupling of hydrothermal conditions on the growth of Larix gmelinii at the timber line and non-timber line is more significant than that of temperature or precipitation alone,and the warming aggravates the drought stress in the study area,and the effect of water stress is more obvious in the drought period when precipitation decreases.The tree growth at the timber line and non-timber line had a significant positive correlation with the PDSI index during the drought period from 1960 to 1980,and the correlation with the PDSI index changed significantly to a consistent and insignificant level during the humid period from 1981 to 2000.During the humid period after 2001,there was a significant positive correlation between tree growth and PDSI index in the non-timber line zone(6)The relationship between timber line and non-timber line trees and climatic factors showed a more consistent differentiation response characteristics.With the increase of temperature,the sensitivity of tree growth to the highest and lowest temperature in last autumn increased gradually,and the sensitivity to the lowest mild precipitation in the growing season increased at first and then decreased.Among them,the rising rate of sensitivity of tree growth in timber line to the highest temperature in last autumn was higher than that of non-timber line.It shows that attention should be paid to the stability of the relationship between tree growth and climate change when modeling and climate reconstruction based on tree rings and predicting the development trend of forest ecosystem under the background of global warming in Daxing’an Mountains.