Study On The Relationship Between The Xylem Functional Traits And Tree Growth Of Subtropical Karst Natural Forest Species

The Mulun karst formation in Guangxi is part of the“Southern China karst formations”which is a natural World Heritage.The protected area of Mulun hosts a relatively preserved natural subtropical mixed evergreen and deciduous broad-leaved karst forest.The species diversity is high and have high ecological value.The karsts in Mulun are complex,from the crest to the valley:the bareland fraction is high,the soil layer is thin and aridity is obvious.Plant hydraulics mainly study the processes and mechanisms of water transport.It is fundamental for researches on plant ecophysiology,forest and tree physiology.It is valuable towards the understanding of the adaptation of native trees species to karst environment.Based on data from a karst forest community survey permanent plot,22common species were selected,including 10 deciduous and 12 evergreen species.Their stem xylem anatomy,saturated water content(SWC),sapwood-specific hydraulic conductivity(Ks)and their embolism resistance(P₅₀)were measured.Two years stem diametral growth information were collected.The results showed that(1)Compared to the global xylem tissue partitioning,the karst forests had higher axial parenchyma fraction.The storage capacity of the xylem of the karst species may constitute an vital adaptation to the aridity of the environment.(2)The relationship between their xylem anatomy and related functional traits were analyzed.It revealed positive correlations between the xylem SWC,the vessel hydraulic diameter,the vessel-axial parenchyma connectivity and Ks.These relationships suggest the contribution of the water storage to increasing the xylem hydraulic efficiency.They also weakened the xylem hydraulic efficiency and safety trade-off.(3)The comparison of the structure and hydraulic traits of the evergreen and deciduous species did not show any significant difference in P₅₀ and Ks between the two leaf phenology groups.However,the SWC in evergreen species was correlated with were observed between SWC the wood density were higher in evergreen species Ks and P₅₀.(4)The variation of the relation between hydraulic traits and stem diametric growth was analyzed.Without considering the stem diameter classes,only the Ks was positively correlated with the stem diametral growth and the species total basal area.Species with fast water transport rate exhibited high DBH growth and were relatively dominant.For small trees(5-10 cm DBH)none of the studied hydraulic traits was correlated with the diametal growth.For average size trees(10-15 cm),Ks was positively correlated with the DBH growth.For larger trees(>15 cm)P₅₀and DBH growth were positively correlated.Besides,the absolute value of the P₅₀ was positively correlated with the diametral growth acceleration(the DBH-dependent diametral growth).In other words,embolism resistance species growth rate increased with tree size.This study investigated the intergrated link between the xylem structure,hydraulic function and ecological performance and exposed the adaptation strategy of karst tree species to the environment and tentatively discussed the theoretical basis of the dynamic of karst forest species under climate change.Such understanding constitutes a reference for protection of karst forest biodiversity.