Cambium Activity And Radial Growth Dynamics Monitoring Under Different Hydrothermal Conditions

Understanding the process of tree radial growth and its response to environmental factors is fundamental to tree physiology.Therefore,many scholars are trying to explain the physiological process of tree radial growth and reveal the mechanism by which tree rings record environmental and climate information.Numerous research results highlight the indispensable role of such research.In recent years,microcores and high-resolution tree radial growth monitors have been used to monitor trees in different growth environments,to study the mechanism of tree radial growth,and to reveal the response relationship between tree growth and climate,which has become an international research hotspot.In order to explore the relationship between the growth of major conifer species and climate factors in the Qilian Mountains of the northeastern Tibetan Plateau.In 2019 and 2020,The cambium phenology and radial growth of Picea Crassifolia and Sabina Przewalskii were monitored in the eastern（Tulugou,Tuliping）and western（Langchaigou）boundaries of their distribution in the Qilian Mountains by using microcores,tree radial growth monitors and automatic weather stations.The responses of cambium phenology and radial growth to the environment were revealed,and the physiological mechanism of the radial growth patterns of the two species were discussed.The unique radial growth patterns of different conifer species in this region were found.The following are the four main conclusions of this study:（1）The onset of cambium activity of P.crassifolia and S.Przewalskii occurred mostly in mid and late May,except for Tuluping P.crassifolia in 2020,which occurred in late April.The end of the growing season varies greatly between species,regions and year.From the perspective of tree species,P.crassifolia ended later than S.Przewalskii.From the perspective of regions,the eastern boundary ended later than the western boundary;From the perspective of year,2019 ended later than 2020 due to the overall drought in the Qilian Mountains in 2020.The cambium phenology stages in different regions and tree species differed not only in the time of onset and end,but also in the growth pattern.The cell accumulation of P.crassifolia showed a certain consistency between years and regions,and it grew rapidly in relatively suitable time to obtain higher income in harsh environment.In contrast,the S.Przewalskii showed similar inter-annual variation and obvious regional variation.In the region with harsh environment,the growth rate and duration of the growing season were reduced to reduce the risk of environmental abrupt changes and to grow rapidly when favorable environmental conditions arrived.（2）The cambium activity of coniferous trees in Qilian Mountains is very sensitive to environmental factors,and the beginning of the growing season is mainly regulated by temperature.The mean temperature threshold of P.crassifolia and S.Przewalskii is7.0 ℃ and 7.3 ℃,respectively,while precipitation controlled the end of the growing season.This study was the first to evaluate conifers in qilian Mountains by sensitivity analysis.The results showed that the wood yield of conifers in this region was mainly determined by growth rate,accounting for 68.9%.At the same time,the growth rate was negatively correlated with the mean maximum temperature in the growing,and positively correlated with precipitation,indicating that drought or not determines the growth amount in the growing season.（3）There is a certain difference in judging cambium phenology by radial growth measuring instrument and microcores method,which is caused by the reversible swelling of stems caused by rehydration of plants in spring and precipitation after the end of the growing season.These factors make it easy to produce large errors in judging the growing season by radial growth measuring instrument.Compared with P.crassifolia,S.Przewalskii frequently dehydrated and rehydrated to protect the elastic tissue cells when the ambient temperature dropped rapidly.Timely rehydration when the ambient temperature rises to keep the cells filled,thus making the best preparation for the growth of the possible suitable environment.In general,S.Przewalskii can better adapt to the cold climate in high altitude areas than P.crassifolia because of its unique radial change mechanism.With the increase of time scales,S.Przewalskii has always maintained a low radial change rate in the growing season,but it also makes the S.Przewalskii can respond to harsh environment timely.Whether it is a decrease in temperature or a decrease in precipitation,the water potential of the xylem will be reduced to stimulate the dehydration mechanism,which in turn protects the fragile elastic tissue cells,temporarily reducing or stopping their life activities,waiting for suitable conditions to reappear.In short time scale,the radial growth of conifers was limited by precipitation,but with the increase of time scale,conifers at the eastern boundary were no longer restricted by water,while conifers at the western sample were more restricted by water.（4）There were significantly positive correlation between conifer radial growth and precipitation of May and June in every site,but there are regional differences in response to vapor pressure deficit.The correlation is lower between the growth of coniferous trees in arid regions and saturated vapor pressure deficit.It is speculated that due to a lower dependence of the plants in arid areas on moisture,they reduce the stomatal opening time as much as possible to minimize water loss.Prolonged drought can stop conifers from growing,reducing their sensitivity to precipitation.