| Forests are the main body of terrestrial ecosystems,the cornerstone of human survival and sustainable development,and vital to human well-being and the health of the planet.Planted forest is an important part of the world’s forest resources,and the development of planted forest has become a global consensus.However,with the continuous increase of global plantation area,the ecological problems and sustainable development of plantations have attracted much attention,and gradually become the frontier and hot spot of international ecology and forestry research.Eucalyptus plantation is an important artificial timber forest in southern China,with an area of 5.46 million hectares,ranking second in the world.The development of Eucalyptus plantation is critical for ensuring national timber security,ecological security and achieving the "dual carbon target".Yet,the current management of Eucalyptus plantations follows a traditional silviculture regime,which have exacerbated a suite of ecological problems.Hence,it is imperative the traditional silviculture regime of Eucalyptus plantations in China be reformed.Recent studies have shown that adopting the eco-silviculture regime can effectively overcome many disadvantages existing in the traditional eucalypt forestry,and can significantly improve the biomass,carbon storage and productivity of the ecosystem.However,the roles and mechanisms that arise in mixed species stands under an eco-silviculture regime are not yet fully understood,and need to be further studied.Here,through field investigation,systematic sampling,biochemical analysis and comprehensive comparison,we evaluated the characteristics and differences of soil properties,understory vegetation composition and plant diversity,as well as ecosystem biomass,carbon storage and productivity in seven-year-old pure E.urophylla × E.grandis plantations and pure Castanopsis hystrix plantations,as well as mixed plantations of these two trees under an eco-silviculture regime in the Experimental Center of Tropical Forestry,Chinese Academy of Forestry,in Pingxiang City of the Guangxi Zhuang Autonomous Region in southern China.We also evaluated the non-additive effects of mixing species on ecosystem biomass,carbon storage and productivity.Variance partitioning analysis and structural equation model were used to quantitatively examine and evaluate the main controlling factors,critical pathways and impacts of biological and abiotic factors on ecosystem biomass,carbon storage and productivity.Furthermore,we investigated the effects and mechanisms of the effects of mixing Eucalyptus and C.hystrix trees on understory vegetation,ecosystem biomass,carbon storage and productivity under the eco-silviculture regime.The main results are as follows:(1)The mixture of Eucalyptus and C.hystrix trees significantly improved soil physical properties.In the mixed plantations,soil bulk density was decreased,soil porosity was increased and soil water retention was improved,especially in surface layer(0～20cm).Compared with the pure forests,the mixed plantation increased the quantity and quality of litter,promoted the return of nutrients,improved the availability of nutrients,and increased soil organic carbon(SOC),total nitrogen,total potassium,and soil available phosphorus to a certain extent.(2)The mixture effects on soil microbial community were inconsistent.The contents of phospholipid fatty acids of soil microbial community,bacteria and actinomycetes in 0～10cm and 10～20cm layers of MEC were significantly higher than those of PCH,but there was no significant difference between MEC and PEU.The contents of soil microbial biomass carbon,microbial biomass nitrogen and microbial biomass phosphorus in MEC were significantly higher than those in PEU and PCH.Tree species mixture significantly increased the activities of leucine aminopeptidase,but significantly decreased the activities ofβ-1,4-glucosidase,β-1,4-n-acetylglucosidase and acid phosphatase in soil.The results indicated that the mixture of Eucalyptus and C.hystrix trees changed the physical and chemical properties of litter and soil,and then affected the characteristics of soil microbial community.(3)Under eco-silviculture regime,mixed plantation had higher understory plant diversity than pure plantations,but different community layers and diversity indexes were inconsistent.Species richness(S),Shannon-Weiner index and Simpson index of shrub layers in mixed plantation were significantly higher than those in the two pure plantations.The S of understory community in mixed plantation was significantly higher than that in the two pure plantations.Redundancy analysis showed that soil C/P acquiring enzyme activity ratio(ECP),and gram positive/negative bacteria PLFA ratio(GP∶GN)accounted for more than 45% of the total explanation rate,indicating that soil ECP and GP∶GN were the most important driving factors affecting species composition and diversity of understory plant community.(4)Mixed Eucalyptus and C.hystrix trees significantly increased DBH and height growth of the two species,resulting in significant differences in productivity of different stands.The tree layer and ecosystem productivity of PEU and MEC were significantly higher than that of PCH,but the difference between the former two plantations was not significant.Non-additive effect analysis showed that the tree layer and ecosystem productivity of MEC increased by 55.34% and 46.91% compared with pure plantations,respectively.On the contrary,the productivity of understory shrub layer and herb layer in MEC decreased by 54.16% and 58.78% compared with that in pure plantations,respectively.These results showed that the mixed tree species with ecological complementarity could make full use of forest resources and improve the productivity of forest ecosystem under eco-silviculture regime.(5)Structural equation models showed that tree species mixing indirectly affected ecosystem productivity by affecting plant community characteristics,soil physicochemical properties and soil microbial characteristics.These results showed that mixing Eucalyptus and C.hystrix increased tree biomass,community species richness and Shannon-Weiner index of shrub layer.Although mixed tree species decreased soil C∶N ratio,C∶P ratio and soil bulk density(SBD),the decrease of C∶N and C∶P actually increased soil nutrients,and the decrease of SBD was also beneficial to plant growth,biomass and productivity.(6)Mixing Eucalyptus and C.hystrix had significant effects on biomass and carbon storage in the ecosystem.The total biomass of PEU and MEC was significantly higher than that of PCH,but the difference between the former two plantations was not significant.The ecosystem carbon storage of MEC was significantly higher than that of the two pure plantations,and that of PCH was significantly higher than that of PEU.Non-additive effect analysis showed that the carbon storage of tree layer,soil layer and ecosystem in MEC increased by64.66%,32.36% and 37.46%,respectively,compared with that of pure plantations.On the contrary,the carbon storage of understory vegetation in MEC decreased by 49.30% compared with that of pure plantations.These results indicated that mixing Eucalyptus and C.hystrix had significant synergistic effects on ecosystem biomass and carbon storage,and had significant antagonistic effects on understory vegetation biomass and carbon storage.(7)Structural equation model showed that mixing Eucalyptus and C.hystrix indirectly affected ecosystem carbon storage through soil physicochemical properties.Tree species mixing decreased Soil C∶N,C∶P and SBD,resulting in significant negative effects on the first principal component axis of soil physicochemical properties(Soil PC1).Soil PC1’s negative effects on ecosystem carbon storage ultimately led to tree species mixing increasing ecosystem carbon storage.This was related to the significant increase of tree biomass and higher SOC content by tree species mixing.The structural equation model also proved that tree species mixture had a significant positive effect on ecosystem carbon storage. |
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