| In this paper,the pure forests of 10-year-old Robinia pseudoacacia(C),Salix matsudana(L),Populus ×Euramercana ’Neva’(Y)and Tamarix chinensis(G)and the mixed forests of their different combinations,including the mixings of R.pseudoacacia× T.chinensis × S.matsudana(CGL),R.pseudoacacia × T.chinensis(CG),R.pseudoacacia × S.matsudana(CL),T.chinensis × S.matsudana(GL),P.×Euramercana ’Neva’ × T.chinensis × S.matsudana(YGL),P.×Euramercana ’Neva’ × T.chinensis(YG),P.×Euramercana ’Neva’ × S.matsudana(YL),as well as the non-forest land(CK),were taken as the research objects.The soil samples from 6 soil layers(0-10 cm,10-20 cm,20-30 cm,30-40 cm,40-50 cm,50-60cm)of the pure forests and their mixed forests were collected in March,May,July,September and November in 2018 and May,July and September in 2019,the soil nutrient content and enzyme activity were determined,and the seasonal dynamic change law of fine root growth of various tree species was studied to explore the dynamic relationship between root distribution and soil nutrient transformation.From May to October in 2018,the monthly dynamic change of undergrowth herbaceous diversity was investigated.In July 2019,the rhizosphere microbial diversity of each forest land was sampled and determined,the chlorophyll content and photosynthetic respiration of R.pseudoacacia,S.matsudana and P.×Euramercana ’Neva’ were determined.The stand productivity characteristics of the mixed forests and pure forests in the coastal saline-alkali area of the Yellow River Delta were explored,such as tree growth conditions,fine root growth and photosynthetic respiration;The forestry environment and biodiversity characteristics were investigated,such as soil water and salt movement and nutrient content,seasonal change law of soil enzyme activity,seasonal change of undergrowth herbaceous plants and soil rhizosphere microbial diversity;The direct influence of environmental factors of artificial forests in coastal saline-alkali area on productivity and the indirect effect on productivity through biodiversity were studied by using the Structural Equation Model.The results are as follows:(1)The mixing influenced the stand productivity by influencing the growth of the aboveground and underground parts of the tree species.In terms of the aboveground biomass of trees,R.pseudoacacia was ranked as CG > CGL > C > CL.S.matsudana was ranked as CGL > CL > GL > YL > L.P.×Euramercana ’Neva’ was ranked as YGL > YG > YL > Y.The aboveground biomass of T.chinensis in mixed forests was smaller than that in pure forests.From the perspective of the growth of underground fine roots,root length density,surface area density,volume density and biomass density of each tree species decreased with the increase of soil depth,while the specific root length gradually increased with the increase of soil depth.In terms of seasons,root length density,surface area density,volume density and biomass density showed the trend of being higher in summer and autumn,lower in winter and spring,and higher in 2019 than in 2018,and the specific root length was the opposite.The law of root growth was similar to that of aboveground biomass.Generally speaking,the overall growth condition of the mixed forest was better than that of the pure forest,but the growth of T.chinensis in the mixed forest was restricted.ANOVA found that the net photosynthetic rate of tree species was significantly influenced by the composition of mixed tree species,while the respiration rate was not significantly influenced by the composition of mixed tree species.In terms of comprehensive productivity,the mixed effect of R.pseudoacacia in CGL and CG was superior to C;The mixed effect of S.matsudana in YGL,CL,GL and YL was superior to L;The mixed effect of P.×Euramercana ’Neva’ in YG and YGL was superior to Y;As the pioneer and companion tree species in mixed forests,T.chinensis can promote forward succession of mixed forests.(2)The relationship between water and salt dynamics and stand growth in a mixed forest is reciprocal causation,which is an important factor in the change of stand productivity.According to the survey,the mixed forest improved the soil water and salt conditions,and increased soil nutrient content and enzyme activity.Seasonal changes in water and salt movements of mixed forests were roughly divided into three periods: From March to September,the water content gradually increased,reaching the maximum in September;from September to November,the water content gradually decreased.Along with the seasonal changes of soil water content,the soil salt content and p H in the mixed forest also showed the law of being low in rainy seasons of summer and autumn,high in the dry seasons of winter and spring,and gradually decreased with the years.The soil depth showed a significant upward trend with increasing soil depth.From the perspective of soil layer distribution,the soil salt content and p H within 0-30 cm of the surface layer were lower,but higher below30 cm,especially for pure forests and bleaching ground.R.pseudoacacia and T.chinensis were typical shallow-rooted tree species,and their root system distribution layers were at 0-30cm.After the two species were mixed,the soil water content increased and the salt content decreased within in the soil layers where roots were distributed;.P.×Euramercana’Neva’and S.matsudana were deep-rooted tree species,and the mixed forests of shallow-rooted and deep-rooted tree species had differences in vertical spatial distribution of root system,compared with the pure forests,the mixed forests had a certain degree of complementarity in soil water.Therefore,in consideration of soil improvement,the mixed benefit of shallowrooted tree species and deep-rooted tree species is the best.(3)Nutrient complementation was achieved through reasonable mixing and nutrient conditions of coastal saline-alkali difficult sites were significantly improved.The nutrient content of coastal saline-alkali area was low,but the soil nutrient content of mixed forests was higher than that of pure forests and non-forest land.Specifically,the physical and chemical properties of soil and the content of soil enzyme activity in each layer and whole layer of mixed forests were higher than those in pure forests,and the content of soil organic matter,total nitrogen and available nutrients in different mixed forests were greater than those of the pure forests.The increasing condition was that CGL was greater than the mixing of CG and GL,CL was superior to C,but the difference was not large.YGL and YG were superior to YL,while Y and CK were relatively low.The mixed forest increased canopy density and decreased relative light intensity.(4)The mixing influenced the composition and diversity of the undergrowth herbaceous plants and soil microbial communities,and contributed to the establishment of a stable ecosystem.Through the survey on undergrowth vegetation of mixed forests of different combinations in the Yellow River Delta,in terms of the diversity of undergrowth herbaceous plants,the quantity of undergrowth herbaceous plants in some mixed forests of three tree species was greater than that in the mixed forests of two tree species,and was greater than their pure forests.By dividing the functional groups,the representative functional groups FG4,FG8,and FG9 were selected,which can be used as indicative plants to visually evaluate the advantages and disadvantages of afforestation.The soil microbial community structure among the forest lands also showed certain differences,similar to the differences in the diversity of undergrowth herbaceous plants,the mixing increased the soil microbial diversity in the forest lands.In terms of soil bacterial richness,diversity and evenness,the mixed forest of three tree species was better than the mixed forest of two tree species,and pure forests had a lower soil bacterial diversity.In terms of coverage,pure forests were higher than mixed forests,indicating that the stability of soil bacterial communities in mixed forests was higher.The soil florae of mixed forests with good forestation effect also had obvious indicative florae,for example,the abundance of Proteobacteria and Acidobacteria can be used as a criterion for judging the degree of improvement of the soil environment.(5)Through the establishment of a structural model,the relationship between stand productivity,biodiversity,and environmental factors was explored,and it is found that biodiversity plays a mediating effect between environmental factors and productivity,and it is a part of the mediating effect.That is to say,in addition to directly influencing the productivity of artificial forests in the coastal saline-alkali area,the site environment can also indirectly influence the productivity of artificial forests by influencing the biodiversity of the forest lands.The environmental factor and its influence on productivity through species diversity effect was 0.55,the effect value environmental factor on species diversity was 0.402;the mediating effect value was 0.14,i.e.the influence of environmental factor on productivity through species diversity was 0.14,the influence of the environmental factor on productivity was 0.409. |
