Responses Of Key Functional Traits And Growth Of Robinia Pseudoacacia On Environmental Changes And Stress In Loess Hilly-gully Region,China

Robinia pseudoacacia,with its fast growth rate,developed root system,high survival rate and drought tolerance,was widely planted in loess hilly-gully region as a pioneer tree species for vegetation restoration.However,due to improper selection of site conditions and restrictions on stand density,some artificial Robinia pseudoacacia forests existed severe problems such as dead treetops,stand degradation and low ecological benefits.We investigated the functional traits,growth patterns and biomass characteristics of Robinia pseudoacaciain different living conditions and water gradients,analyzed the adaptation strategies,and then revealed the mechanism of inefficient artificial Robinia pseudoacaciaforest.This study provided a theoretical basis for afforestation selection,management and vegetation restoration on the Loess Plateau.The results were as follows:(1)The functional traits of Robinia pseudoacacia showed changes with slope position and aspect: leaf nitrogen content,leaf phosphorus content,leaf organic carbon content,root phosphorus content,root organic carbon content and root tissue density of the sunny slope were higher than that of the shady slope;the root nitrogen content,leaf thickness,leaf tissue density,specific leaf area and specific root length were all smaller than the shady slope.With the increase of slope position,leaf nitrogen content,leaf phosphorus content,leaf organic carbon content,root phosphorus content,root nitrogen content,specific leaf area and leaf thickness showed a decreasing trend;root organic carbon content and leaf tissue density increased.Robinia pseudoacacia adapted to heterogeneous environment by the conversion of functional traits,adaptation strategies in different slopes and slopes: it costed more in growth on sunny slopes and lower slopes,and tends to grow rapidly;On shady slopes and upper slope focused on conservative defenses and was more adaptable to adverse environments.(2)Through the correlation analysis between the functional traits of Robinia pseudoacacia and environmental factors and analysis between functional traits: the relationship between the functional traits of Robinia pseudoacacia and environmental factors were significant,which was mainly affected by soil moisture,nutrient and aspect;the covariational relationship between traits existed not only in the same organ,but also in different organs.With the increase of slope position,plant height,DBH,ground diameter and crown width were significantly reduced.The plant height,DBH,ground diameter and crown width of the sunny slope were larger than that of the shady slope.According to the variation of functional traits,growth morphological characteristics and adaptation strategies of Robinia pseudoacacia under different site conditions,Robinia pseudoacacia grew better on sunny slopes and lower slopes,and exerted more ecological benefits.(3)With the deepening of water stress,the seedlings of Robinia pseudoacacia reduced the water loss by increasing the leaf thickness and reducing the specific leaf area;the increasing the root phosphorus content,root organic carbon content and root tissue density to improve the efficiency of roots absorbing water and nutrients;increased adaptation cost to maintain growth balance.This study indicated that under drought stress,the seedlings of Robinia pseudoacacia ensure the survival,increased the investment in defense organizations,reduced the utilization efficiency of water and nutrients,inhibit growth,and adapt the adaptation strategy from wasteful to intensive defense.(4)The plant height,ground diameter,crown width,total biomass,aboveground biomass,underground biomass and root-shoot ratio of Robinia pseudoacacia increased cumulatively followed the extension of drought time.With the deepening of water stress,the total biomass,aboveground biomass and underground biomass decreased,and the root-shoot ratio increased.Mild water stress had little effect on seedlings,and even promoted its growth to some extent.Under moderate and severe water stress,the growth of seedlings was significantly inhibited,the root length increased,and the biomass was transferred from above ground to underground.There was no significant difference between moderate and severe water stress,indicating that the water supply under moderate water stress was less than the minimum threshold for the normal growth of Robinia pseudoacacia seedlings.Therefore,living conditions of afforestation and the influence of soil moisture on the growth and adaptation strategies of Robinia pseudoacacia should be fully considered in loess hilly-gully region.In that case the growth of Robinia pseudoacacia can be carried out in a threshold environment capable of maintaining its own growth and fully adapting to stress,and gained the greatest ecological benefits.