Community Structure And Carbon Dynamics During Close-to-Nature Transformation In South Subtropical Conifer Plantation

Global climate change is an indisputable fact.As an important way totackle global climate change,increasing forest carbon sinks through afforestation and forest management plays important role in the multi-target cultivation and sustainable management of plantations.The area of plantation in China ranks first in the world,and the southern subtropical region is dominated by pure stand of coniferous plantations.The simple forest structure and unreasonable management schemes have resulted in a series of ecological problems,including species diversity reduction,and carbon sequestrationsuppression.Therefore,it is a common concern for the researchers in forestry and ecology to enhance stand productivity and increase forest carbon sequestration through improving management measures and adjusting stand community structure,achieving the multi-objectives of forest,such as wood production,biodiversity conservation,carbon sequestration and emission reduction,and finally to mitigate and adapt global climate change and establish healthy and stable multifunctional plantation ecosystems.Recently,the close-to-nature transformation as one of the most promising ways for carbon sink and multi-objective operation has attracted extensive attention.However,many problems are not yet clear,such as the effects of close-to-nature transformation on community structure,biomass and carbon cycle in coniferous plantation,the relations between the change of stand structure,carbon dynamic and the litter production and quality,soil microbial community composition,and soil respiration during the close-to-nature transformation.In particular,the researches on the continuous positioning of observations are still not enough.In this study,four types of plantations with similar location and site conditions in the Experimental Center of Tropical Forestry,Chinese Academy Of Forestry,Pingxiang,Guangxi,China have been selected as research objects,where is also pertained in Guangxi Youyiguan Forest Ecosystem Research Station.The four plantations are the close to nature transformation plantation of Pinus massoniana,the unimproved pure stand of P.massoniana,the close to natural transformation stand of Cunninghamia lanceolata,and the unimproved pure stand of C.lanceolata.Through consecutive field permanent plot observation,indoor routine physical and chemical analysis,phospholipid fatty acid method,and static chamber gas chromatography,the community structure of the forest and the change of carbon storage of the ecosystems were studied.Specifically,the study including:?1?the dynamics of community structure and species diversity in different plantations during the close-to-nature transformation;?2?the dynamics of biomass and productivity in different plantations during the close-to-nature transformation;?3?the vegetation and the carbon dynamics in different plantations;?4?the dynamics of soil carbon storage in the plantations;?5?the soil microbial biomass and community structure and their main influencing factors during close-to-nature transformation;and?6?the effect of close-to-nature transformation on soil greenhouse gas flux and its main influencing factors.This study is aiming at elucidating the forest species diversity and carbon storage dynamics as well as the mechanism of the changes in soil microbial community composition and soil greenhouse gas emission flux under different management modes,revealing the response mechanism of plantation community structure and carbon cycle to the close-to-nature transformation,and providing scientific basis for plantation management under the context of global change.The main findings are as follows:?1?Compared to the pure stand,the close-to-nature transformation has increased the richness of tree species,altered the species composition and the dominant species of the community and community layers,reduced the dominance and importance value of P.massoniana and C.lanceolata,and weakened the dominance of single tree species in the community.Therefore,the composition of the community species is diversified and evenly distributed.The close-to-nature transformation has changed the dynamics of plant diversity of the shrubs and herbaceous layers in the two plantations.These indexes were change as "S"-shape during the first ten years of the transformation,which were decreased first followed by an increase,and then decreased and finally stabilized.These changes were mainly affected by canopy height and basal area of the forest stands.?2?The biomass and productivity of P.massoniana and C.lanceolata plantations were significantly increased by the close to nature transformation,and the biomass of the two species was increased by 46.71% and 37.24% respectively,which is mainly caused by the change of the community structure.The tree biomass of the four plantations increased with the increase of stand age,and the close to nature transformation significantly enhanced forest annual net primary productivity.These results indicated that reasonable management measures can not only improve the forest stand structure and productivity,but also create the favorable conditions for the enhancement of vegetation carbon fixation capacity and potential.?3?The close-to-nature transformation has no significant effect on the carbon content in different organs of P.massoniana and C.lanceolata,while significantly increased the vegetation carbon storage.After eight years of the transformation,the vegetation carbon storage in P.massoniana and C.lanceolata plantations has been increased by 46.7% and 37.2% respectively.The increased vegetation carbon storage in the close-to-nature forest as compared to the control is mainly due to the rapid increase of biomass of those interplanted species as well as P.massoniana and C.lanceolata.?4?The close-to-nature transformation significantly increased the soil carbon content at 0-40 cm and soil carbon storage at 0-100 cm.After eight years of the transformation,the carbon content at the 0-40 cm layer in P.massoniana and C.lanceolata plantations were increased by 13.2% and 9.1% respectively,while the carbon storage at 0-100 cm soil layer were increased by 10.8% and 8.7% respectively.The soil carbon storage increased with the stand age.As compared to the control plantation,the soil annual net carbon sequestration rate was significantly higher in the transforming forests.There was a significantly positive correlation between soil organic carbon content and plant diversity.?5?The close-to-nature transformation significantly increased the biomass of soil bacteria,fungi,arbuscular mycorrhizal fungi and total microbial biomass while reduced the biomass of actinomycetes in the soils of P.massoniana and C.lanceolata as compared to the control.In the dry and cold season,the total amount of PLFAs of P.massoniana and C.lanceolata was 21.0% and 20.7% higher than that of their corresponding control,while 18.3% and 16.3% higher in the hot and humid season respectively.The close-to-nature transformation has significantly changed the soil microbial community composition of the two plantations,increasing the relative percentage of soil bacteria,inhibiting the growth of actinomycetes,while no significant effect was detected on the relative content of soil fungi.RDA analysis indicated that soil pH value,fine root biomass,C:N ratio of plant litter,soil total phosphorus content and soil porosity were the main environmental factors that significantly affecting the soil microbes of P.massoniana and C.lanceolata plantations.?6?The average soil CO₂ and N₂O emission rates of in the transformed plantation were higher than that of the control,while the average soil CH4 uptake rates were lower in the transforming forests.The difference of CO₂ emission among forest stands was mainly due to the C:N ratio of plant fine root and soil temperature,while the difference of N₂O emission was mainly caused by the C:N ratio of plant litter leaves and soil available nitrogen content.The difference among the C:N ratio of litter leaves could result in the difference in soil CH4 uptake rate.These results showed that the close to nature transformation of P.massoniana and C.lanceolata plantations have negative effects on reducing greenhouse soil emissions.However,the close-to-nature transformation significantly increased forest net carbon stocks as a whole,since it was dominant for the carbon absorption and sequestration.