| Artificial afforestation is an important measure for the restoration and reconstruction of degraded ecosystem in the Loess Plateau,which plays an important role in ecological restoration and soil and water conservation in this area.After implementing the project of returning farmland to forest,the afforestation has made the vegetation area in the Loess Plateau greatly increase,while the artificial forest grows slowly with high mortality,difficult regeneration and poor community stability,which results in the decreasing of ecological service function and unsustainability of the artificial forest.Therefore,the evolution characteristics of afforestation community in arid areas are understood from the perspective of biology,to reveal the reasons for difficult natural regeneration of artificial community,and to find out the key factors affecting the stability of artificial forest community,which is an important premise to solve the above problems,as well as the basis of sustainable development of artificial forest community,and has the important practical significance for the effective control of desertification and the stable operation of artificial ecosystems.The study takes Robinia pseudoacacia as an example,which is one of the tree species with the largest area in the Loess Plateau,and the community composition structure and soil restoration are studied by substitution of space for time;then the regeneration ability of Robinia pseudoacacia is analyzed,of which the main influencing factors are discussed according to community,soil and environmental indicators;finally considering the related actual results of community composition structure,soil,regeneration and environment as the basis,the evaluation index system is selected and constructed,for the comprehensive evaluation of the stability of the artificial Robinia pseudoacacia community.At the same time,the effectiveness of 3 kinds of commonly used evaluation method for forest stability on the artificial Robinia pseudoacaciais compared,to provide the basis for the quantitative study on the stability of artificial grassland in the future.The main results of this study are as follows:1?There are abundant species in the artificial Robinia pseudoacacia forest of the Loess Plateau,and 96 species of angiosperms are found in the study,which respectively belong to31 families and 73 genera,with Gramineae having the largest number of plants,and Compositae as the second rank followed by Rosaceae,Valerianaceae,Bignoniaceae,and Leguminosae.The numbers of families,genera and species show the upward trend with the growth of forest age;as for the variation degree of families,genera and species,it shows that the variation degree of space is greater than that of time,and whether of space or of time,the variation degree shows family<genera<species.The woody plant with the largest important value is Rubu,corchorifolius,followed by Ros,hugonis and Ulmus pumila.The herbaceous plant with the largest important value is Setaria viridis,followed by Melica scabrosa.The plant diversity of Robinia pseudoacacia forest has increased after afforestation,of which the various indices have significant declining trend along with the increasing of altitude,and have no significant response relationship with latitude,with the higher index of shady slope than that of sunny slope.All of the 6 kinds of diversity indices have large variation,among which the whole richness index has the largest variation.2?The vertical structure of artificial forest community of Robinia pseudoacacia is simple,without obvious multi-layer phenomenon.The arbor trees form a single layer,and the shrub is so rare that cannot form a layer,with uneven distribution of herbs;the heterogeneity of horizontal structure is strong,of which the pattern depends on the distribution pattern of upper arbor layer.The undergrowth shrubs and herbs are scarce and distribute discontinuously,with the heterogeneous community level and obvious patchiness.There is a difference of niche overlap and interspecific linkage among species of the forest.3?Afforestation of Robinia pseudoacacia affects the physical properties of soil,has the function of reducing soil bulk density,and can improve water content of the 0-40cm layer of soil;After afforestation,the proportion of large particle size aggregate has increased and the proportion of micro aggregate significantly reduced with the increasing of forest age;the content of organic carbon and total nitrogen in the 0-40cm soil layer show an upward trend with the increase of forest age.Afforestation can increase soil nutrient and increase available nutrient content and enzyme activity level in the soil,and also can significantly increase the available soil nutrients,especially DOC and DON.In the period of returning farmland to forest,most of the variation of DOC and DON can be explained by the month and depth,among which the interpretation rate is the highest.It can also significantly increase the activity of soil enzyme,which shows the significant seasonal changes.The activity peaks of 4enzymes focused in this study appear in June.Invertase plays an important role in restoration of soil carbon and nitrogen coordination in the afforestation of Robinia pseudoacacia.4?The sexual reproduction or asexual reproduction of artificial Robinia pseudoacacia forest in the Loess Plateau,show poor status near natural conditions in the absence of artificial assistance,with the seedling density of only 0.048 strains/m~2;the seed germination rate is low at 79.66%(±12.61,and the thousand kernel weight is only 16.97g(±1.19),which is significantly lower than that in other parts of the country and the world that has been reported.The reason for low seed quality is drought and water shortage of the soil.With the age increasing of Robinia pseudoacacia forest,the seed weight continues to decrease,which means the natural regeneration of artificial locust will become more and more difficult.5?The seedling of artificial Robinia pseudoacacia forest in the Loess Plateau has a greater number than that of sprouts,while the number of seedling has serious losses with the growth of branch diameter,and the amount of sprouts is stable,which indicates that the seedlings are easier to grow and survive in the Loess Plateau.The study find that the main reasons for the seldom regeneration of artificial Robinia pseudoacacia in the Loess Plateau are the lack of soil water and available nutrients and the competition with other plants on the surface.The spatial distribution pattern of Robinia pseudoacacia seedlings shows that both sexual reproduction and asexual reproduction are clustered in small scales.6?The artificial Robinia pseudoacacia community in the Loess Plateau has poor stability;the method of membership function and the method of comprehensive evaluation based on analysis of principal component can get good result in stability evaluation of locust community,of which the results are consistent and can be checked by each other;The main factors affecting the stability of the artificial Robinia pseudoacacia community in the Loess plateau are:species diversity,species richness,canopy density of forest canopy,understory herbaceous coverage,soil water,accumulation of soil organic carbon,soil available P,litter storage and natural regeneration ability. |
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