Declined Characteristics And Individual Differentiation Mechanism Of Robinia Pseudoacacia Plantation In The Yellow River Delta

The Robinia pseudoacacia plantation is the largest plantation in the Yellow River Delta,which has become the main barrier to protect this area from wind and sand damages.Since th e 1990s,the growth of R.pseudoacacia in the Yellow River Delta has deteriorated and aged s everely.The individual differentiation of R.pseudoacacia stands was so obvious that withere d shoots and death has happened to a large number of trees,which has posed great challenges on forest management.The decline of stand productivity and the soil fertility have always bee n serious problems during forestry production in China.Compared to this,the causes of the de cline of stand productivity in coastal saline-alkali land seems much more complicated.Theref ore,it is of great significance for the restoration and reconstruction of R.pseudoacacia plantat ion and the re-establishment of a stable ecological system in the Yellow River Delta by clarify ing mechanism of the individual differentiation thus revealing causes leading to the growth de cline the of R.pseudoacacia plantations in coastal saline-alkali land.In this study,the 17-and 30-year old R.pseudoacacia stands in the coastal saline-alkali land of the Yellow River Delta were taken as the research object.And in combination with the pot experiment of the R.pseudoacacia seedlings,the physiological and ecological characteristics of the salt resistance of the R.pseudoacacia were systematically discussed.Based on different salinity gradient and two stand age,through the measurements of growth index,the interactions between tree and soil,in association with the stem analysis and trace of soil water-salt transport,we studied the rules of individual growth,characteristics of diameter distribution,photosynthetic physiology,root attributes,biomass allocation,the allocation of NSC,carbon and nitrogen,and the dynamics of sodium accumulation of R.pseudoacacia.And therefore mechanism of individual differentiation,community structure and productivity dynamics of R.pseudoacacia in coastal saline-alkali land of the Yellow River Delta can be further clarified.The major findings were shown as follows:(1)Under salt stress,the aboveground and underground parts of R.pseudoacacia showed obvious physiological and ecological adaptabilities.The growth of R.pseudoacacia seedlings was significantly inhibited by salt stress.With the increase of salt concentration,relative ground diameter growth rate,relative height growth rate,seedling biomass,maximum photosynthesis rate,chlorophyll content and root activity significantly decreased,and permeability of cytomembrane significantly increased.The uptake and utilization rate of nitrogen and the assimilation rate of ¹³C in leaves decreased significantly with salt concentration.As one kind of adaptability mechanism,adjustment of root biomass allocation plays an important role in mitigating the damage caused by salt stress.For example,under salt stress,R.pseudoacacia reduced leaf biomass allocation and increased root biomass allocation,the root mass ratio and root-shoot ratio therefore increased.Besides,the increase of specific root length and specific root surface area improved the ability of roots in uptaking water and nutrients.In terms of the allocation of carbon and nitrogen,a large amount of photosynthate was transported to the root system so that the non-structural carbohydrate content of the root was the largest congparing with other parts of the seedling.The regulations of root physiology and morphology under salt stress are beneficial for the maintenance of fine root activity,and thus in favor of the growth of aboveground part of the seedling,which seemed to be an effective strategy for R.pseudoacacia to cope with salt adversity.(2)The individual differentiation resulted from competition among trees in the R.pseudoacacia stand in coastal saline-alkali land varied significantly with stand age.During the first 6 years,the growth mode of trees was nearly linear.In the 9^th year,the the growth of trees almost reached a plateau,and then declined rapidly.In the 14^th year,height growth of the trees was nearly stopped.The growth of DBH also was rapid in the first 6 years,but still maintained a relatively high growth rate from the 7^th to 11^th year,and then decreased rapidly in the 14^thyear.In the beginning 25 years,the volume of wood was continuously increasing,and the decreaseing growth of diameter at breast height and volume obviously lagged behind tree height.Under favorable climate condition,especially in the years with frequent precipitation,the cambium activity of trees was more vigorous and the width of annual ring increasing accordingly.However,on the whole the general dynamics of tree growth showed certain regularity,and the change rules of healthy tree,medium tree and dying tree were almost consistent.The emergence of healthy,medium and dying trees was the result of long-term accumulation of individual differentiation and changes of stand structure.Soil salinity was the external factor of individual differentiation and tree decline in R.pseudoacacia stand,which was accelerated by higher salt stress.Under salt stress,the growth of height,diameter at breast height and volume of significantly decreased with the soil salt content.Despite the fact that growth of some healthy trees in higher-saline R.pseudoacacia stands was better than that in lower-saline stands,in the majority of cases,the growth of trees in lower-saline stands was significantly better.Therefore,soil salt content was acted as a key factor in restricting the growth of R.pseudoacacia.The increasing soil salt content accelerated the progress of individual differentiation and recession within R.pseudoacacia stands.For the medium tree for instance,decline of R.pseudoacacia in higher-saline stands was 3 years earlier than that in lower-saline stands.The effects of soil salt content on individual differentiation and tree decline in R.pseudo acacia stand were reflected in the quantitative distribution characteristics of trees with diamet er class.Among different soil salt content,compared with that in higher-saline stands,the pro portion of healthy trees was realatively smaller,the proportion of medium and dying trees real atively larger in lower-saline stands.Among different stand ages,compared with that in the 30a stand,the proportion of medium trees in the 17a stand was quite small(merely 2%).Comp ared with that in the 17a stand,the proportion of healthy trees was decreasing,the proportion of medium and dying trees increasing in the 31a stand.The Na⁺content in R.pseudoacacia is continuously accumulated with the growth proces s of the individual.Stand age and soil salt content both had significantly effects on the accum ulation,which is an important internal factor influencing individual differentiation and tree de cline.The order of Na⁺content in different organs was root>stem.The order of Na⁺content i n different heights of the tree was root>0m>2.6m>4.6m.The order of Na⁺content in the tr unk among the individuals with three degrees of decline was healthy tree>medium tree>dyi ng tree,while in the root was dying tree>medium tree>healthy tree.The hydrocytosis and c ystocytosis of the root cells,isolation of absorbing Na⁺in the vacuole,and the repellent ability of root,stem and leaf together constructed salt-resistance mechanism within R.pseudoacacia.The roots of healthy tree had better command of the salt absorbing.Through this way,trees c an prevent themselves from ion toxicity resulted from the excessive Na⁺absorption.The cell membrane permeability of the root of dying tree increased,the ability to control the absorptio n of salt ions decreased,and the concentration of salt ions was high.This confirms the conclus ion of previous studies and is also an important cause and key mechanism of individual differ entiation and tree decline.The activity and morphology of fine roots vary with forest age and forest salinity,and ar e closely related to individual differentiation and tree decline.With the increase of soil salinit y,the biomass of fine roots decreased.The fine root biomass of healthy tree was better than th at of medium tree and dying tree.The average diameter of fine roots increased and the ability of roots to absorb soil water and nutrients decreased.The average root diameter increased wit h the increase of soil depth.Root activity represents the ability of roots in plants to absorb and transform water and nutrients from the root environment,and is often used as an indicator of root senescence.The root activity decreased with the increase of salinity.The fine root vigor o f different degrees of decline was shown as healthy tree>medium tree>dying tree.The ratio of root length to root length reflects the root expansion ability and the concentration degree o f soil resource utilization,and its change affects the root absorption efficiency.Root length an d specific root surface area of R.pseudoacacia show a decreasing trend.The ratio of root lengt h decreased with the increase of soil salinity and the increase of soil depth.Root vigor,root nu mber and distribution were negatively correlated with individual differentiation and tree decli ne,but positively correlated with overground growth.With the increase of forest age,the biomass of fine roots of R.pseudoacacia increased si gnificantly and the specific surface area decreased significantly.The decline level had a signif icant effect on the biomass of fine roots,healthy tree>medium tree>dying tree,while the de cline level had no significant effect on other indexes of fine roots.R.pseudoacacia showed certain adaptability to salt and alkali(1.65‰-2.3‰),and still m aintained a high photosynthetic capacity in the face of adversity;The age effects and decline d egree on photosynthetic physiological indexes were also insignificant.The difference of synth etic nutrients in individual differentiation of R.pseudoacacia does not lie in photosynthetic rat e,but in photosynthetic leaf area.With the increase of soil salt concentration,the intercellular CO₂concentration and dark respiration rate of leaves decreased significantly,while water use efficiency increased significantly.However,the effects of photosynthetic and physiological in dexes between pure forests of different ages(17a,31a)and different salt gradients(1.65‰,2.3‰)were not significant.It shows that photosynthesis is the basic source of plant body to syn thesize nutrients and obtain energy.Under different salt concentrations,the content of soluble sugar and non-structural carbo hydrate in the individual of potted R.pseudoacacia seedlings and 31-year-old R.pseudoacaci a were consistent,which decreased with the increase of soil salinity.R.pseudoacacia could us e up non-structural carbohydrate to resist the harm of salt stress and to recover its growth.The variation of non-structural carbohydrate in individuals with different levels of decline was tha t of the dying tree>healthy tree>medium tree.Dying tree uses a large amount of non-structu ral carbohydrate for storage rather than consumption,resulting in high non-structural carbohy drate levels.Among different organs,the root system was the highest,and the R.pseudoacacia distributed more non-structural carbohydrates to the root system,enhancing its resistance to s alt stress.With the growth of forest age,stomatal conductance,transpiration rate and intercellular CO₂ concentration of R.pseudoacacia leaves increased significantly,while water use efficien cy and dark respiration rate of leaves decreased significantly.There were significant difference s in the effects of decline level on stomatal conductance,transpiration rate,intercellular CO₂ c oncentration and water use efficiency,and the changes of stomatal conductance and transpirat ion rate were as follows:healthy tree>medium tree>dying tree.(3)The soil water content and salt content of R.pseudoacacia plantation have distinct se asonal characteristics.Soil water content and salt content of high salt forest land were signific antly higher than that of low salt forest land.High soil water content caused low oxygen stress in roots,which inhibited root growth and aggravated root decline.The influence of sampling time on soil water content,salt content and p H was significant,showing obvious water-salt dy namics.The water content was the highest in August in summer,which was in the stage of de salination.In November,when the water table is low,salt accumulates again.Water salt is cha racterized by"salt comes with water and salt goes with water".The contents of total nitrogen,hydrolytic nitrogen and available phosphorus in the rhizos phere soil of trees in the R.pseudoacacia forest all decreased significantly with the increase o f soil salt concentration.As a result,soil salinization resulted in soil impoverishment,which r esulted in nutrient deficiency and aggravated decline of R.pseudoacacia.Different recession acacia individual rhizosphere soil total N,hydrolytic N,available P,available K content is sig nificant difference,total N,hydrolytic N,the change of effective P content as dying tree<me dium tree<healthy tree,available K content changes as medium tree<dying tree<healthy tr ee,which indicates that the individual differentiation leads to the change of rhizosphere micro environment,declined tree rhizosphere soil nutrient barren.The content of available K in rhizosphere soil increased significantly with the growth of forest age,and the content of available P decreased significantly with the growth of forest age,indicating that the saline-alkali conditions affected the availability of P and were the key limit ing factors for the growth of R.pseudoacacia in coastal saline-alkali land.The effects of diffe rent degrees of decline on the content of total nitrogen and available potassium in individual r hizosphere soil of R.pseudoacacia were significant,and the results showed that the healthy tr ee<dying tree<medium tree.(4)The productivity dynamics of R.pseudoacacia plantation in the Yellow River Delta u nder salt stress varied with the salinity and the forest age.The proportion of healthy trees in hi gh salt forest decreased,medium and dying trees increased,and the stand productivity decreas ed.With the increase of forest age,the volume growth and aboveground biomass increased fir st and then decreased.The dynamic change of stand productivity is the result of salt stress,for est age,photosynthetic capacity,diameter distribution,decay process,forest-soil interaction a nd the comprehensive effect of the external environment.