Responses Of Pinus Tabuliformis Xylem Anatomical Characteristics To Changes In Moisture And Temperature In Northern China

Pinus tabuliformis pure forests and P.tabuliformis–broadleaved mixed forests in northern China are of great importance in avoiding soil erosion and water loss,stoping desertification,and maintaining regional ecological security.Northern China is experiencing pronounced warming and drying,with potentially dire implications for the health and productivity of the region’s forests.It is crucial to investigate the response mechanisms of tree growth to climate change.Most previous dendrochronological studies have only used macroscopic index,such as tree-ring width,characterizing the inter-annual radial growth of trees.The intra-annual variation of tree growth and stem hydraulic structure adjustment in response to climate warming and drying are still unclear.In the typical distribution of P.tabuliformis,we selected18 natural P.tabuliformis pure forests and 3 P.tabuliformis–broadleaved mixed forests in northern China as the target sampling sites.Applying dendrochronological,wood anatomical methods and Vaganov–Shashkin（VS）models,we investigated the stem xylem anatomical structures differences in different moisture conditions,in earlywood and latewood,in Mature and Juvenile P.tabuliformis,in different tree species from P.tabuliformis–broadleaved mixed forests,their different responses to climate change,and the relationships between the canopy features and stem xylem anatomical parameters.The main research results are as follows:（1）Water availablity is the main climate factor limiting earlywood and latewood formation of P.tabuliformis.Both soil and atmospheric temperature have significant impacts on the formation of earlywood,only the increase in soil temperature significantly affects the formation of latewood.Earlywood formation is most sensitive to climate around the 150^th day of the year,which is when P.tabuliformis grows most vigorously.The most serious impact of water stress on latewood formation occurs around the 200^th day,i.e.,when both temperatures and P.tabuliformis growth are at their highest.The adjustments in tree-ring anatomical structure to climate change are found primarily in earlywood rather than in latewood.The Mork’s index（r TSR）of earlywood declined significantly from dry to wet sites（r²=0.87,p<0.001）.In a continuously warming climate,the adjustments of xylem anatomical structure of P.tabuliformis will balance between hydraulic efficiency and safety and differ in dry and wet areas.Frequent severe droughts in dry areas can enhance hydraulic safety and induce P.tabuliformis to form smaller lumens and thicker cell walls（higher r TSR）to survive.P.tabuliformis in wet areas can improve hydraulic efficiency by forming larger lumens and thinner cell walls（lower r TSR）.Considering more frequent droughts in the future,forest managers should pay enough attention to the moisture conditions when earlywood formation occurs.（2）Age trends existed obviously in wood anatomical parameters of P.tabuliformis.In line with the age trends,younger P.tabuliformis grew faster than older trees in the two drier sites（KQB and LYS）(RW_Younger=864.04μm,RW_Older=606.84μm,Wilcoxon rank-sum test,p<0.01).But younger P.tabuliformis in wet site（TTH）grew incredibly slower than older trees(RW_Younger=98.40μm,RW_Older=751.54μm,Wilcoxon rank-sum test,p<0.01),indicating that younger P.tabuliformis growth was under more environmental pressure,which entirely offset the age trend in tree growth itself.Younger trees own safer stem hydraulic structure,the mean lumen area and hydraulic conductivity（radial thickness-to-span ratio）of younger trees were lower（higher）than that of older trees.The greatest differences in the anatomical parameters between age classes were observed in the wet site.VS models showed a significantly longer growing season of younger trees than older trees.The longer growing season may benefit younger P.tabuliformis growth in the drier sites but limit younger P.tabuliformis growth in the wet sites.Therefore,P.tabuliformis forests may potentially develop into younger and shorter forest stands in dry areas.But serious growth decline of younger P.tabuliformis was found in wet areas.Therefore,the protection of older P.tabuliformis in dry areas and younger P.tabuliformis in wet areas may be effective management measures to maintain the productivity and drought resilience of P.tabuliformis forest stands.（3）The radial growth（RW）and xylem-specific hydraulic conductivity（Ks）of P.tabuliformis,Betula platyphylla,and Quercus liaotungensis closely correlated with the relative humidity during the growing season.The moisture sensitivity of P.tabuliformis(r_max=0.38,p<0.05)was lower than that of B.platyphylla(r_max=0.43,p<0.05)and higher than that of Q.liaotungensis（p>0.05）.It revealed that P.tabuliformis growth and stem hydraulic conductivity were under inter-mediate water stress,while B.platyphylla and Q.liaotungensis were under maximum and minimum water stress.In facing the extreme drought in 2001,P.tabuliformis had the highest resistance(Rt _{P.tabuliformis}=0.45,Rt _{B.platyphylla}=0.36,Rt _{Q.liaotungensis}=0.28,Wilcoxon rank-sum,p<0.05),and Q.liaotungensis had the highest recovery(Rc _{Q.liaotungensis}=4.24,Rc _{B.platyphylla}=3.64,Rc _{P.tabuliformis}=3.04,p<0.05).Therefore,climate warming and drying may lead to a severe and rapid growth decline of the pioneer species,B.platyphylla.More frequent occurrences of extreme droughts in the future may lead to a sharp reduction in the growth of P.tabuliformis.There is a possibility that Q.liaotungensis will take the dominant role over P.tabuliformis in the mixed forests,or even mixed forests directly translate to pure Q.liaotungensis forests.Therefore,focused protection of Pinus tabuliformis may be the key to maintaining biodiversity and forest productivity in P.tabuliformis–Q.liaotungensis mixed forests in this region in the future.（4）Carbon sink activity（variations in cell wall thickness and cell number）of P.tabuliformis stems is critical in controlling the interannual variability of wood growth.The xylem anatomical parameters of P.tabuliformis differed in dry and wet sites.P.tabuliformis in dry sites had lower mean,total cell wall thickness and cell number than that in wet sites（Wilcoxon rank-sum,p<0.01）.P.tabuliformis in wet sites showed increasing mean cell wall thickness（r=0.17,p<0.05）and decreasing total cell wall thickness（r=-0.12,p=0.08）and cell number（r=-0.14,p<0.05）with increasing NDVI.The results can preliminarily allow us to confirm the two diffetent mechanisms explaining the phenomenon that forest greening but tree growth decline.In dry areas,water deficit due to warming and greening forced trees to allocate more recent photosynthates to underground carbon pools（soil organic carbon）.In wet sites,vegetation greening prior to the onset of cell wall thickening activity consumed soil moisture significantly,further inhibiting P.tabuliformis stem growth during its rapid growing periods.Therefore,we can conclude that the earlier onset of growth and greener canopy,induced by warming and CO₂ fertilization,can not significantly promote the accumulation of aboveground biomass in the temperate Pinus tabuliformis forests in northern China.In sum,our results improve the eco-physiological understanding of the tree growth-climate relationships in the P.tabuliformis pure forests and P.tabuliformis–broadleaved forests in northern China.The findings can provide valuable insights predicting the potential development and formulating reasonable forest management strategies under climate change.