Intra-annual Radial Growth Of Three Coniferous Species And Their Responses To Climate Factors In The Eastern Qilian Mountains

Climate change has had an important impact on the global environment in recent years,and the significance of warming and extreme climate events has become increasingly important in ecology.As an important component of the terrestrial ecosystem,trees are closely linked to climate change and interact with each other.On the one hand,climate change affects the process of radial growth.On the other hand,trees are constantly adjusting their growth strategies to climate change and regulating climate change through the carbon cycle.In the context of global warming,it is helpful to understand their ecological adaptability and predict stand structure dynamics by clarifying the relationships between cambial phenology,radial growth and climate.The Qilian Mountains is an ecological security barrier in western China,and the trees in this area are sensitive to climate change.Although a large number of studies on dendroclimatology and dendroecology have been carried out over the years,the mechanisms of radial growth of tree species in this region remain unclear.Based on this,two sampling sites were set to monitor cambial phenology and growth dynamics of three coniferous species(Pinus tabuliformis,Picea wilsonii,and Juniperus przewalskii)at the Liancheng Field Station in the eastern Qilian Mountains during 2014-2021.Additionally,the relationships between radial growth of these tree species and climate were also explored.As intra-annual density fluctuations(IADFs)were found both in spruces and pines,we also revealed the formation mechanisms of IADFs and discussed the adaptability of tree species to drought conditions.The results will provide the scientific basis for forest management and conservation in the study region under climate change.The main conclusions of this study are as follows:(1)There were significant differences in the cambial phenology among tree species.The onset of xylogenesis occurred earliest in P.tabuliformis while latest in J.przewalskii,and between them in P.wilsonii.The end of xylogenesis was opposite to the onset.The xylem cells of three species depended on the rate rather than the duration of cell production,while their xylogenesis dynamics were significantly different.The total number of xylem cells was the most in P.wilsonii while the least in P.tabuliformis,and between them in J.przewalskii.The duration was the longest in P.tabuliformis,while the shortest in J.przewalskii,and between them in P.wilsonii.The difference in rate among tree species was opposite to the duration.Consequently,P.tabuliformis adopted an "extensive strategy" with a long duration but low rate,while J.przewalskii adopted an "intensive strategy" with a shorter duration but higher rate,and P.wilsonii was in the middle.(2)Temperature was an important factor determining the onset of cambial activity.Therefore,warming will lead to an earlier onset of xylogenesis and a longer growing season.The end and rate of P.tabuliformis and J.przewalskii were limited by moisture,while P.wilsonii was limited by temperature.The difference in the responses of radial growth to climate in tree species may be related to their growth habits and microenvironment.In addition,the maximum growth rate of trees occurred around the summer solstice,synchronized with the photoperiod.That may be a growth adaptation to photoperiod constraint,thus allowing trees to safely complete secondary cell wall lignification before winter.(3)Adaptability and plasticity to drought of tree species were different.There was no intra-annual density fluctuations(IADFs)in J.przewalskii.P.tabuliformis was easier than P.wilsonii to form IADFs,revealing that it had a stronger sensitivity and adaptability to drought.Therefore,in the context of global warming,it is expected that the frequency of IADFs in P.tabuliformis will further increase.