Changes Of Community Characteristics Of Robinia Pseudoacacia Forest Along Rainfall Gradient And Management In Loess Plateau

Robinia pseudoacacia forest is the most widely distributed and the largest artificial forest in the Loess Plateau.Under the influence of climate,soil,landform and other environmental factors,a large area of low efficiency robinia pseudoacacia forest appeared in forest grassland area and grassland area.To understand the status quo and cause of low efficiency Robinia pseudoacacia forest and explore the control measures of low efficiency Robinia pseudoacacia forest management is an urgent problem.In this paper,the typical robinia pseudoacacia forest communities in the Loess Plateau of northern Shaanxi were selected from north to south along the environmental gradient,including Shenmu,Suide,Yanan Yangzhuangou and Yongshou.Soil physicochemical properties（bulk density,water content,p H,organic carbon,total nitrogen,total phosphorus,ammonium nitrogen,nitrate nitrogen,available phosphorus content）,soil enzyme activity characteristics（BG,LAP,NAG,AP）,plant composition（composition,structure,plant diversity）of robinia pseudoacacia forest and understory were systematically studied.The physiological characteristics of dominant species Robinia pseudoacacia（xylem vessel characteristics,water conductivity,non-structural carbohydrate）along the environmental gradient and the influencing factors.The management strategy of Robinia pseudoacacia forest in this area was proposed.The main research results are as follows:1.From north to south,soil physical and chemical properties and soil enzyme activities of Robinia pseudoacacia forest showed significant trends.The main results were as follows:（1）the forest soil bulk density decreased first and then increased,while the water content of loose soil and rhizosphere soil increased.（2）The p H of loose soil decreased in the range of8.14-8.61,while the contents of total phosphorus（TP）,available phosphorus（A-P）and organic carbon（SOC）increased in the range of 0.30-0.42 g·kg^-1,1.12-2.24 mg·kg^-1 and3.00-7.60 g·kg^-1,respectively.NO_3--N increased first and then decreased in the range of4.95～14.79mg·kg^-1.Soil total nitrogen（TN）,C/N and C/P increased first,then decreased and then increased in the range of 0.24-0.70 g·kg^-1,10.81-15.04 and 8.95-17.95,respectively.Ammonium nitrogen(NH₄₊-N)firstly decreased,then increased and then decreased in the range of 0.63～1.05mg·kg^-1.N/P decreased first and then increased in the range of 0.64～1.66.（3）p H of rhizosphere soil decreased in the range of 8.10～8.61;Soil SOC,TP,A-P,NO_3--N and C/P increased in the range of 6.89～22.76g·kg^-1,0.57～0.68g·kg^-1,2.15～7.69mg·kg^-1,5.39～17.67mg·kg^-1 and 12.17～33.65,respectively.TN and N/P decreased first and then increased in the range of 0.80～2.01g·kg^-1 and 1.33～2.98,respectively.NH₄₊-N decreases first,then increases and then decreases in the range of 1.07～1.36mg·kg^-1.C/N increases first and then decreases in the range of 8.34 to 12.94.（4）The enzyme activity of LAP soil in loose soil increased continuously in the range of 1.27～3.93nmol·g^-1·h^-1;BG and AP soil enzyme activities decreased first and then increased in the range of 46.79-119.11 nmol·g^-1·h^-1 and 10.05-40.97 nmol·g^-1·h^-1,respectively.Soil enzyme activity of NAG increased first and then decreased in the range of 7.82-8.76 nmol·g^-1·h^-1.Among them,BG/（LAP+NAG）firstly decreases and then increases in the range of 4.12～9.61;BG/AP increases first,then decreases and then increases in the range of 2.30～4.66.（LAP+NAG）/AP ranges from 0.30 to 1.09,increasing first and then decreasing.（5）The enzyme activities of LAP and AP in rhizosphere soil increased in the range of 3.33～8.40nmol·g^-1·h^-1 and 12.74～30.14nmol·g^-1·h^-1,respectively.In the range of 50.97～117.54nmol·g^-1·h^-1,the BG soil enzyme activity firstly increased,then decreased and then increased.In the range of 9.06～28.07nmol·g^-1·h^-1,soil enzyme activity of NAG increased first,then decreased and then increased.The BG/（LAP+NAG）and BG/AP ranges from 2.77 to 4.31 and from 2.47 to 4.15,respectively,decreasing first and then increasing.（LAP+NAG）/AP ranges from 0.97 to 1.25,increasing first,decreasing then increasing again.（6）Redundancy analysis（RDA）and correlation analysis showed that p H and N/P were the main factors affecting the enzyme activity and stoichiometric ratio of loose soil.C/N and p H are the main factors affecting soil enzyme activity and stoichiometric ratio in rhizosphere soil.2.From north to south,the biomass and diversity index of robinia pseudoacacia forest showed an increasing trend,which was as follows:（1）Robinia pseudoacacia biomass increased in the range of 92.31～163.58kg?m^-2.（2）The understory species of Robinia pseudoacacia mainly consist of four families:Rosaceae,Compositae,gramineae and legumes.The important values of gramineae and Compositae increased first and then decreased in the range of 13.32-46.52 and 19.79-43.45,respectively.The importance values of legumes decreased first and then increased in the range of 2.70-21.23.Rosaceae did not appear under robinia pseudoacacia trees in Shenmu and Suide,and the important values of Yan’an and Yongshou were 31.60 and 28.21,respectively.（3）From north to south,the species richness Margalef index,Gleason index,Simpson index,Shannon index,evenness Pielou index were 0.93-2.05,1.00-2.28,0.70-0.90,1.46-2.43 and 0.82,respectively They all showed an increasing trend in the range of～0.94.The diversity index of Whittakerβand Wilsonβincreased first and then decreased in the range of 1.93～4.91 and 0.35～0.86 from north to south,respectively.（4）Community species richness index,diversity index and evenness index were affected by soil physical and chemical properties,soil enzyme activity and stoichiometric ratio.CCA analysis and correlation analysis showed that LAP,NAG,SWC and TP were the main factors affecting plant diversity.3.From north to south,the root duct characteristics,water conductivity and NSC of Robinia pseudoacacia community had a significant trend of change,and the changes were mainly affected by soil water content and other soil factors.（1）The inner diameter of Robinia pseudoacacia root catheter increased gradually in the range of 80.42～169.77μm;The catheter density decreased gradually in the range of 41.43～61.73ind·mm^-2.The wall thickness of the guide tube gradually increased in the range of 4.96～14.21μm.Correlation analysis showed that SWC,p H,SOC,TP,A-P,NO_3--N,NH₄₊-N,C/N,C/P,N/P affected the inner diameter of the catheter,the wall thickness of the catheter and the density of the catheter.CCA analysis and correlation analysis showed that SWC and NO_3--N were the main factors affecting the characteristics of Robinia pseudoacacia root ducts.（2）From north to south,the root water conductivity and specific water conductivity increased in the range of1.44～4.82g·m·s^-1 and 1.78～6.78g·m^-1·s^-1,respectively,and there was a good exponential relationship between the root water conductivity and specific water conductivity and soil water content.Correlation analysis showed that water conductivity and specific water conductivity were affected by catheter diameter,but had no significant relationship with catheter density.（3）From north to south,the soluble sugar concentration of Robinia pseudoacacia root decreased first and then increased in the range of 6.02-11.24 mg/100mg.In the range of 5.36～8.54mg/100mg,starch concentration firstly increased and then decreased.The concentration of total NSC decreased in the range of 12.87-15.22 mg/100mg.The starch/soluble sugar increased first and then decreased.With the increase of root order,soluble sugar concentration,starch concentration and total NSC concentration showed an overall trend of increasing or first increasing and then decreasing.Correlation analysis showed that soil water content was negatively correlated with soluble sugar concentration and total NSC concentration.There was a significant negative correlation between soluble sugar concentration and starch concentration.The results showed that the NSC distribution mechanism of Robinia pseudoacacia root system under drought stress.4.On the basis of the above studies,a comprehensive evaluation of Robinia pseudoacacia forest with different environmental gradients was carried out with the entropy weight TOPSIS model.The results showed that the growth of robinia pseudoacacia forest was the worst in Shenmu and Suide in the northernmost part,which were characterized by poor understory soil nutrient and low enzyme activity,low total biomass of Robinia pseudoacacia forest,single understory species and poor diversity,and poor root water conductivity（vessel characteristics and water conductivity）.From north to south,the above problems gradually become better,and the artificial robinia pseudoacacia forest grows best in Yongshou area in the south.（2）At present,the density of robinia pseudoacacia forest in the four regions is generally high,and the best stand density can be achieved by thinning.The retained density of grassland area,forest grassland area and forest area after thinning is500～800 plants/hm²,900～1650 plants/hm²,and 1500～1775 plants/hm²,respectively.（3）The planting of mixed forest is beneficial to protect and restore the ecosystem.It is suggested that the mixed forest in grassland area,forest grassland area and forest area should be grassland or sea buckthorn（sand willow,Caragana Korshinskii）shrub,robinia-Pinus tabulaeformis mixed forest,Robinia-Liaodongensis mixed forest,respectively.In conclusion,from north to south,community characteristics of Robinia pseudoacacia forest on the Loess Plateau changed significantly with environmental gradients such as rainfall,temperature and soil.The growth of robinia pseudoacacia forest in the northern grassland area was the worst,the plant diversity index was the lowest,the root duct was the smallest and the water conductivity was the lowest.Decline and even death are more likely to occur during drought stress.The growth of robinia pseudoacacia forest is the best in the southern forest area,but the single structure and low plant diversity restrict the play of ecological functions such as biodiversity conservation.In view of the problems existing in robinia pseudoacacia forest in different regions,thinning and mixed afforestation are the approaches to improve the structure,ecological function and stability of robinia pseudoacacia forest.