Study On Xylem Hydraulics Of Afforestation Trees Species In Lower Subtropical Region

In the subtropical region,there are large areas of diverse plantations.However,these plantations have simply community structure,and are sensitive to environmental changes.Under the background of climate drying in this region,afforestation tree species will experience increasing water stress,which may affect ecosystem function.Tree hydraulics mainly elucidates the water transport of xylem and its resistance to drought-induced embolism.Therefore,study on the hydraulic structure and hydraulic safety of afforestation tree species help us understand their growth performance and drought risks,which will provide scientific basis and theoretical guidance for the sustainable management of plantation under climate change.In this study,we selected ten afforestation tree species（six native species and four exotic species,respectively）as well as eucalyptus species under four planting patterns,including Eucalyptus grandis×urophylla pure plantation（PE）,E.grandis×urophylla and Castanopsis hystrix mixed plantation（MEC）,E.grandis×urophylla and Dalbergia odorifera mixed plantation（MED）,E.grandis×urophylla and Parashorea chinensis mixed plantation（MEP）in this region.We surveyed long-term mean growth rate,measured wood density,xylem anatomy,hydraulic traits and leaf water potential during the dry season.By analyzing the trait-growth relationships and comparing the interspecific and intraspecific differences in HSM₅₀and HSM_tlpunder drought period,we found the following results:（1）For the ten afforestation tree species,long-term growth rate was significantly and positively associated with hydraulic conductivity,but not with wood density,specific leaf area,and hydraulic safety-related traits.Compared to the six native tree species,Eucalyptus and Acacia of exotic tree species showed higher hydraulically-weighted vessel diameter(D_h),sapwood and leaf specific hydraulic conductivity（k_sand k_l,respectively）,fiber fraction and vessel to parenchyma connectivity,and growth rate.There was no significant trade-off between hydraulic efficiency and safety,with two exotic species（i.e.,Eucalyptus grandis×urophylla and Acacia crassicarpa）showing both high hydraulic conductivity and cavitation resistance.We found significant interspecific differences in the HSM₅₀and HSM_tlpamong afforestation tree species.During the dry season,Acacia auriculiformis,Castanopsis hystrix,Mytilaria laosensis and Cinnamomum burmannii were more likely to experience hydraulic failure,thus indicating an urgent requirement of stand transformation.（2）There was no significant intraspecific difference in hydraulic conductivity,Huber value,and wood density for E.grandis×urophylla under different planting patterns.However,we found significant intraspecific difference in D_h,vessel to parenchyma connectivity,vasicentric tracheids fraction,leaf water potential,leaf water potential at turgor loss point,HSM₅₀and HSM_tlp.Compared to other two planting patterns,E.grandis×urophylla from MEC,and MED showed more vulnerable to cavitation and negative hydraulic safety margin.This may be due to the high water demand and strong interspecific competition for belowground resources,and suggests that the three mixed models significantly increased the drought risk of Eucalyptus species.In conclusion,this study revealed the relationships between the growth rate and hydraulic traits across afforestation tree species,and different drought risks among different plantations.With increasing frequency and intensity of extreme droughts,some afforested tree species may encounter high possibility of drought mortality.Therefore,we suggest that tree hydraulic traits should be included into the index system of ecological monitoring of subtropical plantations,which can provide theoretical basis for the development patterns of subtropical plantations.