Hydraulic Structure And Function In Woody Plants Across Different Slope Positions From A Tropical Karst Forest

Karst is a major geomorphic landform in Southwest China.Due to unique hydrological process,the environmental factors such as soil water availability,soil depth and soil distribution continuity generally decrease from valley to hilltop,thus the spatial distribution of species show obvious niche differentiation.The hydraulic trait is the underlying mechanism that determines species distribution,but it remains unclear whether the hydraulic traits reflect the spatial distribution across species on karst hills.This study was conducted at the Dalian Karst Forest Park,Pingxiang city,Guangxi Zhuang Autonomous Region,Southwest China,which was located in the northern tropics.The study explored how karst species adapt to heterogeneity from embolism resistance,storage capacity and stomatal regulation.(1)This study measured leaf and stem hydraulic characteristics in 17 common species with a different topographical distribution.It found both predawn water potential and minimum water potential sharply decreased from the valley to the hilltop in dry season,indicating the gradual increase of water deficit.Meanwhile,the stem of species living in more severe water stress showed narrower vessel,higher vessel reinforcement index,higher wood density,and stronger embolism resistance,but there was no significant change in hydraulic efficiency.It was mean there was no trade-off between hydraulic safety and hydraulic efficiency in branch.Moreover,in leaves,hydraulic pathways both in xylem and in mesophyll were enhanced to resist hydraulic failure.These were demonstrated in species with have more negative?_50leaf,higher vessel reinforcement in minor veins,more negative?_TLP,and higher LMA were living in higher relative altitude.Though embolism resistance enhanced both in stems and in leaves,the difference between?_50leaf and?_50branch became larger from valley to hilltop,which supported hydraulic vulnerability segmentation hypothesis.(2)In the same 17 species,the saturated water content decreased from valley to hilltop,which consisted with the trade-off between storage capacity and embolism resistance,and indicated species in the arid karst environment did not depend on water storage to ensure hydraulic safety.However,the living cell proportion in stem xylem did not change significantly along altitude,which meant more water storage in species living in lower altitude area in intercellular space and fibrous lumen.Additionally,because axial parenchyma was correlated with embolism repair and embolism resistance,its proportion was positive correlated with vessel diameter,and then it can optimize water transport efficiency.Five species have abundant axial parenchyma due to band living cells,thus these species had wider vessel and higher hydraulic efficiency.This may be the main reason for decoupling between hydraulic safety and efficiency.(3)By measuring stomatal behavior in 3 species per microhabitat,giving a total of 9species growing in valley,slope and hilltop respectively,we found the stomatal behavior only in 5 species was significant linked to stem water potential.Nevertheless,to maximum carbon gain,the leaf water potential at which 50%of stomatal conductance remains(?_gs50l)in all species were correlated to embolism resistance both in leaf and in stem.Similarly,carbon assimilation capacity(A_max,g_smax)in karst species was limited by vascular hydraulic supply(Ks?Kl),but not related to conductance in outside-xylem pathway of leaf.The difference between?_gs50l and?_50branch ranged from-0.87 MPa to 1.22 MPa,which indicated the stomatal water regulations were diversity in karst forest.However,this indicators in species living in hilltop were positive,which meant species generally adopt relatively conservative regulation to occupy arid area.(4)This study also investigated the plasticity in hydraulic traits of three local widespread species.Only two species,Delavaya toxocarpa and Tirpitzia ovoidea showed plasticity,and which occurred only at the driest hilltop.Moreover,there was obvious hydraulic synergy between branches and leaves.For the remaining species(Litsea glutinosa),although there was no significant difference in hydraulic traits among populations living in three slope positions,the conservative stomatal regulation led low hydraulic risk in all microhabitats.Therefore,there may be a trade-off between plasticity in hydraulic traits and stomatal water regulation.Additionally,in karst forest,plasticity in vulnerability to embolism of species was related to its competitiveness in the community.As a result,the phenotypic plasticity index of?_50branch in these three species was positive related to their importance value in the community.Conclusion:In karst forest,soil water availability decreased with the increasing relative altitude.To overcome the increasing water deficit,plant enhanced resistance to hydraulic dysfunction and adopt conservative stomatal regulation behaviors.Moreover,compare to species in moist environment,species living in arid hilltop did not depend on storage water to ensure hydraulic safety.The variation in plant hydraulics and stomatal regulation in different karst species are closely related to the niche differentiation and coexistence of species in water-deficient habitats.The study will provide important guidance for the selection and allocation of plant species in the ecological restoration of karst rock desertification.