The Adaptation Strategy For Typical Artificial Forests To Different Rainfall Conditions On The Abandoned Farmland In The Loess Plateau

Converting farmland into forest is the main measure for ecological restoration and reconstruction of degraded ecosystem in the Loess Plateau.However,with the warming and drying of the climatein recent years,artificial vegetation grows slowly with problems of soil desiccation,lower productivity,and higher mortality.Therefore,the adaptation strategy of artificial vegetation under drought stress has practical guiding significance for vegetation construction on the Loess Plateau.In the background of Grain for Green Project,the study took the plantation of Robinia pseudoacacia and Pinus tabulaeformis under different rainfall conditions on the Loess Plateau as examples,which are predominately revegetation measures in the Loess Plateau.The combination of field experiment and laboratory analysis was used to analyze the community structure of Robinia pseudoacacia and Pinus tabulaeformis under different drought conditions,leaf morphological traits,leaf anatomical traits and physiological traits.We also want to find driving factors affecting the community structure,leaf morphological traits,leaf anatomical traits and physiological traits of Robinia pseudoacaciaand Pinus tabulaeformis,and revealadaptation strategies to drought of of Robinia pseudoacacia and Pinus tabulaeformis.The main conclusions of this study are as follows:(1)The composition and structure of the undergrowth community are affected by drought and afforestation species.In all sampling ranges,there are 2548 plants,belonging to25 families,51 genera and 56 species in the artificial vegetation of Robinia pseudoacacia.while there are 62 species with a total of 1,281 plants,belonging to 27 families and 57genera in the artificial vegetation of Pinus tabulaeformis.With the decrease of rainfall,the quantity of species and plants decreased graduallyin the artificial vegetation of Robinia pseudoacacia and Pinus tabulaeformis.The dominant species under Robinia pseudoacacia forests were transformed from Rosaceae with relatively poor drought tolerance to Gramineaeaceae with better drought tolerance.The species under the pine forest are mainly composed of Compositae,Gramineae and Leguminosae with good drought tolerance and resistance to infertility.(2)Drought affects the leaf morphological traits and anatomical structure of afforestation tree species,and Robinia pseudoacacia and Pinus tabulaeformis had the different changes in themorphologicaltraits and anatomical structure to the drought.With the decrease of rainfall,the leaves of Robinia pseudoacacia mainly resist the damage caused by drought stress through increasing dry matter content,leaf thickness,upper epidermis and epidermis thickness,palisade tissue thickness and tissue tightness,reducing specific leaf area and tissue looseness.The leaves of Pinus tabulaeformis mainly resist the damage caused by drought stress through increasing the dry matter content,leaf cross-sectional area,skin layer area,resin channel area and transport tissue area,reducing specific leaf area.(3)Drought significantly affected the leaf physiological characteristics of afforestation tree species,and different tree species responded to environmental changes through changes in different physiological characteristics.With the decrease of rainfall,the leaves relative water content and chlorophyll content of Robinia pseudoacacia and Pinus tabulaeformis decreased,and the various of chlorophyll content of Robinia pseudoacacia was significantly greater than that of Pinus tabulaeformis;the proline and soluble protein showed an overall upward trend,the proline content of Robinia pseudoacacia was significantly higher than that of Pinus tabulaeformis,and the soluble protein content of Robinia pseudoacacia was significantly lower than that of Pinus tabulaeformis under the same environmental conditions;the activities of superoxide dismutase,peroxidase and catalase in the leaves of Robinia pseudoacacia and the activity of peroxidase in the leaves of Pinus tabulaeformis were firstly increased and then decreased,and reached maximum peak at 452mm mean annual precipitation(MAP).While the superoxide dismutase and catalase activities of Pinus tabulaeformis showed an increased trend;the content of malondialdehyde increased first and then decreased,while the content of superoxide anion increased.With the changes of rainfall,there were no significant changes for leaves carbon content,while significant changes for leaves nitrogen(N),phosphorus(P)and N/P ratio of Robinia pseudoacacia and Pinus tabulaeformis.In the selected area,the LN/LP value of the Robinia pseudoacacia is greater than 16,and the LN/LP value of the Pinus tabulaeformis is less than 14,indicating that Robinia pseudoacacia may be restricted by P and Pinus tabulaeformis may be restricted by N.(4)Soil characteristics and climatic factors significantly affect plant community characteristics,leaf morphological anatomy traits and physiological characteristics.The dominant factors affecting the plant community characteristics,leaf morphological anatomy and plant physiological characteristics of Robinia pseudoacacia were bulk density(BD),MAP and mean annual temperature(MAT),respectively.The dominant factors affecting plant community characteristics,leaf morphological anatomy and plant physiological characteristics of Pinus tabulaeformis were MAT,BD and TN,respectively.At the same time,the Robinia pseudoacacia responds to environmental changes mainly through the various of genus and species,palisade tissue,and O~-.Pinus tabulaeformis responds to environmental changes mainly through the changes infamilies,genera and species,cross-sectional area and O~-.Through the comprehensive membership function analysis values of plant anatomical features and physiological characteristics,it is found that the Robinia pseudoacacia enhances its resistance mainly by regulating the permeating substances,adjusting morphological traits and protective tissue structure,while the Pinus tabulaeformis enhances its resistance mainly by adjusting morphological traits,protective tissue structure,and vascular organization.(5)Drought has significant impacts on the communities stability of artificial vegetation,and different afforestation species respond to environmental changes through different adaptation strategies.The communities stability of Robinia pseudoacacia and Pinus tabulaeformis increased first and then decreased with the decrease of rainfall.With the decrease of rainfall,the stress resistance of Robinia pseudoacacia showed a trend of decreasing first and then increasing,and the minimum and maximum values were reached when MAP was 507 mm and 365 mm,respectively.With the decrease of rainfall,the stress resistance of Pinus tabulaeformis showed a trend of increasing first and then decreasing and the minimum and maximum values are reached at 600 mm and 452 mm.When the MAP is365 mm,the comprehensive membership function value of the Robinia pseudoacacia is greater than that of the Pinus tabulaeformis,while the results were opposite under the other condition,which indicates that under the MAP of 452 mm,Robinia pseudoacacia is more affected by drought than Pinus tabulaeformis.Comprehensive plant system stability and plant adaptability analysis showed that compared with Robinia pseudoacacia,Pinus tabulaeformis was more conducive to the growth of MAP less than 452 mm,and Robinia pseudoacacia was more suitable for higher MAP conditions.