Response Of Tree Growth To Drought Based On Stable Carbon Isotope In Guizhou Plateau

The global climate is experiencing significant changes characterized by rapid warming and increased extreme weather events（droughts,floods,etc.）,which have a huge impact on the ecological environment and human society.Forests are the main body and important resource of ecosystems,and have important impacts on climate system changes,carbon cycle,water conservation,and ecological environment protection.The Yunnan-Guizhou region is one of the regions with the widest distribution of subsistence agriculture,the most poverty-stricken population,and the most concentrated ecological environment.It is also one of the regions with the most serious natural disasters such as soil erosion and ecological damage.In recent years,we have found a large number of cherished deaths or growth declines in coniferous forests in different parts of the Yunnan-Guizhou Plateau.Tree growth decline and death are one of the important processes of forest ecosystems,and have long-term feedback effects on forest ecosystem population dynamics,interspecific interactions,atmospheric CO₂ concentrations and climate.Therefore,the relationship between tree growth decline and death and climate change in the study area is of great significance to the region’s biodiversity and ecological environment protection,forest carbon sinks,forest management and management.We choose the Guizhou Plateau as our research area,and use the dead and living natural Fokienia hodginsii which was collected in the Xishui National Nature Reserve northern Guizhou Provence and Pinus kwangtungensis Chun ex Tsiang which was collected in the Maolan National Nature Reserve in the southeastern part of Guizhou Provence.The first stable carbon isotope chronologies were established of this two regions.Then we calculated the ratio of intercellular carbon dioxide concentration（C_i）,the ratio of intercellular carbon dioxide concentration to atmospheric carbon dioxide concentration（C_i/C_a）,base area increment（BAI）and other physiological indicators by combining theδ¹³C and tree ring width.The correlation analysis,linear regression analysis and other statistical methods was used to analyzed the response characteristics of theδ¹³C of this two different tree species to climate change.The effects of age on theδ¹³C value of Pinus kwangtungensis Chun ex Tsiang was also analyzed.Finally,the relationship between the changes of radial growth,δ¹³C and water use efficiency of this two tree species were compared with climate change respectively may help us discuss the reason of the tree death.The main conclusions are as follows:Our results demonstrate that carbon assimilation and radial growth of Fokienia hodginsii are limited by cumulated moisture from previous growing seasons,as it affects early growth in subsequent years.The stable relationship between radial growth and water use efficiency（iWUE）in the period 1960s–1980s indicates that trees can take advantage from favorable climate conditions.However,the intensification of warming and drought since the 1980s affected those trees that were more sensitive to drought,revealed by a consistent decrease in radial growth paralleled by an unprecedented increase in iWUE,and a diminishing sensitivity to rising C_a between1985 and 2010 that suggests that trees were experiencing water stress.Moreover,the reduction in F.hodginsii radial growth and associated tree mortality in the early 21^st century due to water limitations could have serious implications for a drought–sensitive species which only remnant patches remaining within some protected areas.Given that drought in the region is expected to further increase in frequency and intensity in the near future,urgent measures should be taken to ensure the preservation of this valuable trees species.The Pinus kwangtungensis Chun ex Tsiangδ¹³C is mainly affected by the water conditions in the early growing season and growing season.From 1960 to 1981,due to the relatively humid climate,water use efficiency increased with the increase of atmospheric CO₂ concentration,BAI increased rapidly due to CO₂ fertilization effect;From 1982 to 2011,the climate gradually dried up,and Pinus kwangtungensis Chun ex Tsiang reduced stomata to reduce transpiration and water loss,resulting in a significant increase in iWUE amd a decrease in photosynthetic products,so the radial growth was maintained a relative low level.Subsequently,the climate turned wet,the iWUE did not increase but remained constant and the radial growth began to increase again.The results of the study indicate that the extreme drought events of 2009-2011are the main cause of the large number of deaths of young Pinus kwangtungensis Chun ex Tsiang.The results also indicate that young trees are more susceptible to drought than old ones in the harsh karst dry mountain environment.