The Mechanism Of Tree Radial Growth Response To Climate Change At High Latitude And Alpine Treeline Areas

In recent decades,tree radial growth response to climate change has been one of the hot spots in ecological study areas,which is the foundation of predicting the potential effect of future climate warming on forest ecosystems.It is well known that tree growth is affected by climate conditions and stand structure together,however,the studies about the effects of stand factors on tree radial growth were rarely considered on large scales in past tree ring researsh.In addition,how climate factors and stand factors effected forest growth response to climate change along geographical gradients and their relative effects were still unclear.This research used dendroecological methods to analyze tree radial growth response to climate change along climate gradients in northern China,including altitudinal gradient of Mt.Changbai,latitudinal gradients covering the distribution of Korean pine forests in northeast of China and alpine treeline areas of main mountain ecosystems in northern China.Our research focused two aspects of tree radial growth response to climate change,the first was identified by growth variability and growth trend,which reflected long-term climate change on tree growth,another was the correlation coefficients of ring width indices(RWI)and monthly climate variables(e.g.RWI-climate relationship),reflected the effect of historical high frequency climate fluctuations on tree radial growth.The research examined the response and mechanism of tree radial growth to climate change on large scales,and aimed to discussing the following questions:(1)How does growth variability,growth trend and RWI-climate relationship changed along climate gradients.(2)What dose the relative effect of climate factors,climate trend and stand factors on effecting geographical patterns of growth-climate relationships?(3)What are the differences of the effects on different aspects of the radial growth-climate response to climate change? The important findings are as follows:(1)The research conducted along altitudinal gradient of Mt.Changbai showed that: both growth variability and RWI-climate relationships changed significantly with altitude.Forest height was the most important predictor for altitudinal changes of growth variability.For RWI-climate relationships,forest type was more important than climate gradient,while forest height and stem density were weak but necessary predictors.We showed that the altitudinal differences in growth response to climate change cannot be explained by climate gradient alone,and highlight the necessity to examine the influence of biotic factors(which covary with climate across geographic gradient)to better understand forest response to climate change.(2)The part of research conducted along latitudinal gradient covering the whole distribution of Korean pine forest in China,considering the differences of climate warming rates before and after the year of 1984,our research examined tree radial growth responses to climate change along latitudinal gradients in two time intervals,respectively.The results showed that,in the period of1959 to 1983,the growth of Korean pine forests increased in most sampling plots,and growth trends tended to faster towards higher latitude,climate warming trend had significant effects on growth variability and growth trend,especially for mean sensitivity(the proxy of high frequency growth variability).Growth variability during the year between 1984 and 2012 was higher than before,and Korean pine forests showed a growth decline in most of the sampling plots,and downtrend of Korean pine growth increased with higer latitude,both climate conditions and climate trends played significant roles on growth variability,climate warming had both positive and negative effects on tree growth.For RWI-climate relationship,the moisture sensitivity of Korean pine radial growth was higher in the period of 1984 to 2012 than that in the period of 1959 to 1983.In two periods,the precipitation trend had a high explanatory power for most RWI-climate relationships,indicated that in addition to climate conditions,the importance of the effect of climate trend on latitudinal pattern of tree radial growth reponse to climate change.In general,the decrease of water availability caused by continuing warming was not conducive to tree growth and might exacerbate vulnerability of matured Koeran pine forest at higher latitude.(3)The result of the research conducted along geographical gradient of alpine treeline in northern China showed that: growth variability decresed from humid to arid regions,the radial growth revealed temperature sentivity in west-arid regions but changed to water sensitivity towards east-humid treelines.These “strange” patterns were partly because precipitation increased in the west while decreased in the east since 1960.Stand factors revealed similar importance as precipitation trendin shaping these broad-scale patterns.Forest height(difficulty in vertical water transportation)seem to be important for growth variability,while stand density(competition for water)was key in modulating the temperature/water sensitivity of radial growth.Climate trend and stand factors were more important in affecting growth response to climate change than climate gradient.Growth at many alpine treelines are limited by water availability,implying that the widespread forests at lower altitudes are more threatened by warm-induced drought.However,the importance of stand factors found here suggested forest management as a promising way to alleviate forest vulnerability to climate change.This research systematically analyzed the regulation of climate-related factors and stand factors on tree radial growth reponse to climate change on large geographic scales,revealed that the importance of climate trend in different regions through large-scale studies.The results showed that the climate trend had the primary impact on growth variability.When the climate trend tended to be similar,the role of stand factors highlighted,while the climate factors comes the second.For RWIclimate relationships,the effect of climate conditions,climate trend and stand factors work together.In general,tree radial growth is more limited by water condition(e.g.precipitation and precipitation trend)in north part of China,while stand structure(e.g.species composition,stand density etc.)also necessary to radial growth.In the context of climate warming,the adverse effects can be mitigated by adjusting species composition and rational thinning in future forest management.