Hydraulic Traits And Acclimatizing Strategies Under Low-temperature For The Main Temperate Tree Species In Northeast China

Global climate change has been more significant in mid-and high-latitude regions than others,especially on low-temperature environment,which in turn affects the survival and growth of temperate forest trees.However,how temperate trees cope with low-temperature is poorly understood,which impedes the modeling and prediction of tree growth and distribution under global climate change scenarios.In this study,we selected seven trees species that represent different wood properties in the temperate forests in Northeast China,i.e.,semi-ringporous species[Manchurian walnut(Juglans mandshurica)],diffuse-porous species[white birch(Betula plaryphylla)and Korean aspen(Populus davidiana)],and non-porous species[Mongolian pine(Pinus sylvestris var.mongolica),Korean spruce(Picea koraiensis),Korean pine(Pinus koraiensis)and Dahurian larch(Larix gmelinii)],we measured the hydraulic traits and nonstructural carbohydrate(NSC)concentration of branches of the trees at five lowtemperature stages[i.e.,frozen stage(January),beginning of freeze-thaw cycles(FTCs)in spring(February);ending of FTCs in spring(April);beginning of FTCs in autumn(October);ending of FTCs in autumn(November)],with reference to the growing season(July),and explored the hydraulic traits and acclimatizing strategies under low-temperature.The main results were as follows:(1)There were significant interspecific variations in hydraulic traits under lowtemperature(P<0.05),which were mainly driven by the differences in conduit diameter.The native sapwood-specific hydraulic conductance(Ks)of the birch and aspen(diffuse-porous)were significant higher than those of the other trees(P<0.05).Walnut(semi-ring-porous,56.3%)had the highest percentage of loss hydraulic conductance(PLCs),while non-porous trees(25.5%)had the lowest.Embolism vulnerability of branch(P50S,negative)showed a similar interspecific pattern as PLCs.The non-porous tree species had the greatest saturated water content(WC).These results demonstrate that non-porous trees species have less hydraulic damages and stronger embolism resistance under low-temperature,which make them maintain higher WC.(2)The hydraulic traits examined varied with low-temperature stages.The lowest Ks showed in April,but the highest was in October(0.61 vs 1.35 kg·s-1·m-1·MPa-1).No significant differences in PLCs were found among January,February and April with a mean PLCs of 40.4%;but PLCs decreased in July,October and November to 23.3%.P50S increased from Jaunary to April,but the most negative P50S was found in October(-2.91 MPa).These results indicate that the higher hydraulic damages occurred in frozen stage than in growing season.Compared with autumn,spring FTCs induced more hydraulic damages,weaker embolism resisitance and frost fatigue,which may be related with initial environments and tree metabolism.(3)A leaf-branch vulnerability segmentation was detected for the evergreen trees(Mongolian pine,Korean spruce and Korean pine)under low-temperature.The degrees of VS of all the three species were positive under low-temperature,with the VS values being higher in spring or autumn FTCs(1.57 MPa)than in growing season(1.31 MPa),which might be associated with the lower temperature and higher frenquency of FTCs for leaves than branches.PLCs significantly decreased with VS increasing(P<0.05),but not with hydraulic segmentation.These results suggest that higher VS decreases the branch hydraulic damages and thus improves the branch hydraulic safety for the evergreen trees under low-temperature.(4)The positive relationships between wood density(SD)and PLCs,P50S or WC were significant under low-temperature(P<0.05),but negative relationship with Ks(P<0.05);leaf mass per area(LMA)was significantly and negatively correlated with embolism vulnerability of leaf for the evegreen trees under low-temperature(P<0.05),suggesting that LMA and SD are correlated with leaf and branch hydraulic traits under low-temperature,respectively,which extends the application of economic trait spectrum.Increasing carbon investment under lowtemperature not only enhanced the ability of embolism resistance,but also increased the hydraulic conductance.(5)Integrating all the data under low-temperature,we found that PLCs was positively correlated with conduit diameter and P50S,but negatively with WC(P<0.05),and these relationships also showed at different low-temperature stages.This result suggests that tree tolerance to low-temperature significantly affects on the hydraulic damages,i.e.,trees with smaller conduit diameter and stronger embolism resistance have lower hydraulic damages.PLCs decreasing with soluble sugar concentration only showed in deciduous trees at the ending of spring FTCs(P<0.05),representing a repairing strategy.No relatioinships between soluble sugar concentration and PLCs was found for the evergreen trees,but their soluble sugar concentrations increased from the frozen stage to the ending of spring FTCs,suggesting that photosynthesis may provide soluble sugar source to the branches.In addition to the deciduousness,the larch had stronger embolism resistance and more starch accumulation in the branches in order to have similar hydraulic damages and hydraulic conductance under lowtemperature as the evergreen trees.These results suggest that tolerance,repairing and avoidance strategies are important for temperate trees to acclimatize low-temperature,but their relative importance varies with species and low-temperature stages.Overall,our findings not only deeped the understanding of how tree hydraulics respond and acclimatize to low-temperature,but also provide new perspective for explaining the biodiversity maintenance in temperate forest ecosystems.