Response Of Radial Growth And Xylem Characteristics Of Typical Subtropical Tree Species To Climate Change

The subtropical region has a large forest area,complex environmental conditions,and a diverse range of species.Recently,the climate in subtropical region has changed dramatically,and extreme weather events have become more common,having a significant impact on the subtropical forest ecosystem.Because tree rings are widely distributed and have high resolution,they are excellent materials for studying tree growth and climate change.In comparison to the traditional width index,the anatomical features of the tree ring xylem can provide a microscopic mechanism explanation for the macro-tree ring-climate relationship.Dashanchong National Forest park in Changsha,Hunan province,was chosen as the research site for this study,and 12 typical subtropical tree species were chosen based on different tree species differentiation methods(different wood properties,conifer/broad leaves,evergreen/deciduous leaves),and the response mechanism of subtropical forest to climate change was explained using macroscopic tree ring width and microscopic xylem anatomical characteristics.The research results are as follows:(1)Under the same habitat conditions,the tree growth-climate response is species-specific.In the study area,Sassafras tzumu grows fastest(0.168cm/yr),Quercus fabri grows slowest(0.051cm/yr),and pinus massoniana grows slowest(0.066cm/yr)among the four conifers.Overall,the increment of basal area(BAI)of broad-leaved trees is higher than that of conifers.There is a stage similarity between the change of ring width index of different tree species and the change of basal area increment.After 1980,most tree species showed rapid growth in different degrees.During the drought period from 2000 to 2015,most tree species showed slow growth in different degrees.Coniferous tree species are more sensitive to climate change than broad-leaved tree species.Different tree species grow differently in the same habitat,and the same or opposite growth patterns among tree species will show similar or different growth characteristics.(2)Temperature was the main climatic factor that limits the radial growth of different tree species in the study area,and the response of each tree species to temperature is more obvious than that of precipitation.The influence of seasonal climate change on tree species is different due to the differences of species,wood properties,and life forms of tree species.Meanwhile,the responses of different tree species growing in the same habitat to climate change have similarity.The low temperature in winter,high temperature in summer,and excessive precipitation in autumn can inhibit the radial growth of most tree species,while the previous winter precipitation and high relative humidity in spring can promote the radial growth of most tree species.(3)The anatomical characteristics of the xylem of Taxodium ascendens,Choerospondias axillaris,and Liriodendron chinense were obviously different.The average ring width of three wood tree species has a significant positive correlation with ring area,tracheid/conduit number,tracheid/conduit area,and tree ring hydraulic conductivity,which means that the more tracheid/conduit number and tracheid/conduit area,the wider the ring area and average ring width,and the stronger the tree ring hydraulic conductivity.The anatomical characteristics of xylem of three wood species can further proved that temperature is the most important climatic factor limiting the growth of tree species in the study area.High temperature in the growing season has a negative correlation with the average ring width,ring area,tracheid/conduit number,and tracheid/conduit area of three tree species,and the higher the temperature,the less favorable it is for tree growth.The relatively humid climate environment was more conducive to the growth of the three trees.(4)Extreme weather occurred frequently after 1980,and the occurrence probability of drought was higher than that of low temperature.The occurrence probability of extremely narrow rings of various tree species increased obviously after 2000.The high temperature was the main reason for the extremely narrow years of different tree species in this area,and the double effects of "low temperature and drought" in 2008 are more significant than those of a single extreme climate(low temperature in winter in 2005 and extreme drought in 2013).Non-porous wood species are more sensitive to extreme climates,and porous wood species are more adaptable to extreme climates.Conifer species are more sensitive to drought than broadleaf species.Compared with deciduous trees,evergreen trees have stronger adaptability to "the low temperature+drought"climate in 2008,and deciduous trees have stronger resistance to the extremely high-temperature climate in 2013.The adaptability of slow-growing trees to extreme climates is stronger than that of fast-growing trees.Through the study of xylem characteristics of three wood tree species,it is found that extreme climate often leads to the decrease of vessel/conduit number and vessel/conduit area,which leads to the narrowing of ring width.Taxodium ascendens is more resistant to the "low temperature+drought" climate in 2008,but weaker to 2013,while Choerospondias axillaris is the opposite,and Liriodendron chinense is in the middle.Taxodium ascendens’ resilience after a "low temperature+drought" extreme climate is not as good as that of Choerospondias axillaris and Liriodendron chinense,but it is better than that of Choerospondias axillaris and Liriodendron chinense after extreme drought.The tree species with a larger conduit diameter has stronger drought resistance,but it also increases the risk of embolism and cavitation,resulting in weak resilience.Generally speaking,the macro and micro indicators of growth characteristics and growth climate responses of different tree species in the same lifetime were obviously different.Coniferous tree species were more sensitive to climate response.Relatively humid climate environment was more conducive to the growth of trees in the study area,and the extremely narrow rings of different tree species are affected by extreme climate,especially extreme drought.Some tree species not only show slow growth but also show a declining trend.The anatomical characteristics of the tree ring xylem can further verify the response relationship of "tree ring-climate",and the continuous temperature rise is a great challenge to the future forest management in this study area.