Differences In The Sensitivity And Response Dynamics Of Upper Elevational Tree-limits To Climate Change

The upper elevational tree limit,as a type of transition zones or ecotones between ecosystem types,is an area where vegetation can change drastically in a short spatial distance,and it is very sensitive to climate change.In this study,we studied the upper elevational tree limits at two different spatial scales.At the local scale,this research focused on the upper elevational distribution limits of three tree species(i.e.,lightdemanding deciduous coniferous Larix chinensis at treeline,shade-tolerant evergreen coniferous Abies fargesii and shade-intolerant deciduous broad-leaved Betula albosinensis below treeline)along elevational gradients in the Qinling Mountains.The methods of dendrochronology and point pattern analysis were used,and the factors of climate,topography,and soil were collected,to explore and compare the spatiotemporal dynamics of upper tree limits in response to climate change,comprehensively.At the global scale,a basic dataset of 273 elevational treelines after 1900 worldwide was compiled to determine the main influencing factor of treeline migration,construct the global pattern of treeline tracking climate change,and discuss the impact of climate warming on this global pattern.In addition,a basic dataset of stand density and treeline shifts after 1950 in 150 sites worldwide was compiled to construct a global pattern of the differences between elevational upward shifts and tree densification at the treeline ecotones,and explore the impact of climate warming on this pattern.This research mainly examined the dynamic changes of the upper elevational tree limits from many aspects and aimed to compare the response of upper tree limit to climate change at different spatial scales,to provide a scientific basis for assessing the impact of climate change on structure,function,and dynamics of alpine forest communities.The results are as follows:(1)In the Qinling Mountains,three elevational transects were established across the whole elevational distribution range of all three tree species,respectively,and the dynamic structures of upper tree limits were reconstructed.Over the past 300 years,all the upper species limits shifted upslope as a response to climate warming.However,the warming-induced upslope migrations showed substantial differences,displaying the maximum upward shift of larch with an average elevation of 24.7 m during the past century,while only a slight advance of the non-treeline tree species.In addition,the results of point pattern analysis showed that the disparity in elevational advance of upper species limits might be attributable to the presence of interspecific competition,showing that the non-treeline tree species experienced intermediate interspecific competition while the treeline tree species experienced no interspecific competition.(2)To further examine the effects of competition on the shifts of upper tree limits,the stem competition intensity at upper tree limits was quantified using the Hegyi competition index.The results showed that climate warming has increased the intensity of competition among trees,which has shown a significant inhibitory effect on the migration of upper tree limits.(3)To clarify the impact of species-specific and site-dependent local factors on the dynamics of upper tree limits,factors such as biological interaction,topography,and soil were included in the analysis.The results showed that the vegetation thickness index(TI),soil characteristics,and slope together explained 71% of the variable in the migration of upper tree limits,while vegetation thickness index explained more than78% of the explained variables.A significant negative relationship was detected between the migration of upper tree limits and vegetation thickness index.This indicates that in addition to temperature,biological interaction is also an important factor altering the migration of upper tree limit at the local scale.(4)Most of the studies about treeline dynamics were only based on the observed treeline position changes.Here,this study expanded the focus and compared the shift rates of global treelines among different time periods,determined the extent to which treeline tracked temperature increase over the past century by identifying the indicator of treeline shift ratio(i.e.,the ratios of the observed shift to the expected shift due to climate warming),and examined if and how different levels of climate warming had affected treeline shift ratios.This study found that temperature was a principal factor driving the changes of treeline positions at a global scale,and treeline shift rate tended to increase over time.Only 25.3% of the treelines worldwide could shift upward keeping pace with the rate of climate warming.Treeline shift ratio showed consistent responses to climate warming in different macroclimatic regimes,displaying that it was negatively associated with increasing temperature.Our findings revealed that the accelerating climate warming has advanced treeline positions but reduced the potential of treelines to track their upper thermal limits worldwide.(5)A global pattern of the differences between tree densification and elevational upward shifts at treeline ecotone were constructed by comparing the rates of standardized treeline shift and stand densification.Factors such as climate,geographic distribution,species classification,and study time were also included in the analysis,to explore the main reasons for the formation of this global pattern.Only 37.3% of treelines worldwide showed no mismatch between treeline shifts and stand densification in response to climate warming.In addition,as a response to climate warming,29.3% of global treelines showed migration faster than stand densification,while 33.3% of global treelines exhibited an upward shift rate lower than the densification rate.Temperature change(especially summer temperature change)and the differences between the global treeline shifts and densification showed a significant negative correlation,and such a negative correlation was also detected in different geographic regions(i.e.,eastern,and western hemispheres),which indicated that temperature was critical to the formation of such a global pattern of treeline dynamics.