Vessel Variation Of Woody Plants And Their Coupling With Leaf-wood Traits Across Islands In Eastern China

Plant leaf and wood economics spectra represent the trade-off between material investment and return at leaf and stem levels.Numerous studies have confirmed the existence of economics spectrum at the level of single organ,but rarely integrate leaf and wood traits in a whole-plant perspective to explore the coupling relationship between them,and there are only a few researches about the potential physiological mechanism for the relationship between leaf and wood traits.As a physiological process that runs through various organs of plants,water transport is closely related to leaf and wood traits,and vessel diameter is the most important factor to represent water transport efficiency and resistance,so maybe it will mediate the coordination between leaf and wood traits.Therefore,it is particularly vital to explore the hydraulic transport mechanism formed through vessel diameter,as well as its variation pattern and driving factors.Woody plants common to 17 islands in eastern China were selected,and were measured for tree height,leaf traits(leaf dry matter content,specific leaf area,mean leaf area and Huber value),wood traits(twig wood density,specific twig length,twig water content and wood density),vessel anatomical characteristics(mean vessel area,mean vessel diameter,vessel density,and vessel void-to-wood area ratio),and collected meteorological factors for each island from WorldClim(http://www.worldclim.org/),including mean annual temperature,mean annual precipitation,mean annual solar radiation,mean annual vapor pressure and mean annual wind speed.First,the linear mixed-effect model was used to decompose the variation of 13 functional traits on four nested ecological scales(sites,islands,species,and individuals),then used the same method again to explore the main factors driving the vessel diameter variation,the species were set as random variables,and the environmental factors,tree height and the leaf and wood traits related to hydraulic dynamics were set as fixed variables.Then,Pearson's correlation analysis was used to verify whether there was coupling between leaf and wood traits at the whole plant level.Finally,using causal mediation analysis to explore the mediating effect of vessel diameter on coordination between leaf size and woody tissue density,revealing the physiological mechanism of leaf size-woody tissue density synergy.The main results are as follows:(1)The variation patterns of 13 plant functional traits on climatic zone,island,species and individual scale were consistent,that is,the variation at species level was the greatest,and at climatic zone and island level were weak,while the variation on individual scale was almost negligible.After the variance decomposition of the overlapping species of Pinus thunbergii,the results showed that traits variability at climatic zone and individual level were generally low,and the variation among different islands was larger.These results indicated that the environmental gradient along largescale climatic zones was still insufficient to cause trait variation,and the main source of variation is the difference among species properties at local scale.(2)The linear mixed-effect model showed that environmental factors had no significant effect on vessel diameter variation(p > 0.05).After selecting tree height,leaf traits related to transpiration and wood traits as fixed variables,it was found that only tree height,specific leaf area,vessel density and wood density can significantly affect vessel diameter.Specifically,vessel diameter increased with tree height and specific leaf area(p < 0.05)and decreased with vessel density and wood density(p < 0.05).The results indicated that the variation of vessel diameter of woody plant was not affected by the environment,but was more constrained by the traits limiting their own hydraulic transport.(3)At leaf level,there was a significant positive correlation between specific leaf area and mean leaf area,and both decreased with the increasing leaf dry matter content(p < 0.01).There were also significant correlations between three economic traits and Huber value(p < 0.01),indicating the coupling between economic traits and hydraulic traits.At the level of woody tissue,as wood density and twig wood density decreased,water transport capacity of xylem increased,resulting in an increase in twig water content,which promoted its rapid growth and increase in specific twig length(p < 0.05).These results indicated that at leaf and woody tissue levels,each formed a trade-off between material investment and return.(4)From the whole-plant perspective,there existed close relationships between leaf traits and wood traits.Hydraulic conductivity and water content increased as woody tissue density decreased,then xylem hydraulic system transported water to the canopy quickly and efficiently,increasing the water potential at the end of the branch,enabling larger leaves and satisfying the violent transpiration and carbon-water exchange.Thus,leaf area increased(p < 0.01)and dry matter accumulation decreased(p < 0.01).This fully demonstrated the coordination between leaf traits and wood traits.In addition,both specific leaf area and Huber value were independent of wood density(p > 0.05),and there was a significant negative correlation between Huber value and twig wood density(p < 0.01),indicating the correlation between Huber value and woody tissue density were not as close as the leaf economics.(5)Through causal mediation analysis,it was found that vessel diameter of angiosperms can effectively mediate the coupling relationship between leaf size and woody tissue density,which was partial mediation.When the plant was equipped with large conduits to transport water,hydraulic conductivity increased,but hydraulic safety and mechanical support capacity decreased,resulting in a decrease in woody tissue density(p < 0.001).The larger the diameter of conduits,the more it can compensate for the increasing hydraulic resistance when transported upwards,and then effectively promoted leaf physiological activities and its area growth(p < 0.001).(6)Finally,it was found that in gymnosperms,the diameter of tracheid could not mediate the relationship between leaf size and woody tissue density.This is due to the fact that the correlation between leaf size and woody tissue density is very weak(p > 0.05),indicating that there was no synergistic change between both.In addition,tracheid diameter was independent of leaf size(p > 0.05)and only weakly correlated with wood density(p = 0.03),indicating that tracheid diameter of gymnosperms was not effectively associated with leaf and wood traits.The reason for the different results of the two functional groups may be the difference in the woody anatomical structure of them,then different ecological strategies will be adopted at the level of wood,leaf and whole plants.In summary,there existed coupling relationships between leaf and wood traits of woody plants in islands,and vessel diameter was of great importance in connecting leaf and stem function.Besides,the variability of vessel diameter mainly derived from the traits that constrained its hydraulic transport.The conclusions from this thesis provided a theoretical basis for a deep understanding of island plant growth and resource utilization strategies.