Spatial Variation Characteristics Of Carbon Density And Storage In Forest Ecosystems In Zhejiang Province

The increasing concentration of atmospheric CO₂ and its contribution to global warming has become a well-known phenomenon and long-term large-scale research topic.Mitigation methods mainly include reducing anthropogenic CO₂ emission and enhancing the potentials of carbon（C）sequestration by terrestrial ecosystems.Forest ecosystems are the most important and active component of terrestrial ecosystems as they store approx.80% of all above-ground C and 40% of all below-ground terrestrial C,and thus forest ecosystems play an irreplaceable role in maintaining the global C balance and mitigating climate change,which mainly includes the four C types of vegetation,soil,forest litter,and dead wood.Therefore,it is necessary to carry out research on the spatial distribution of C density and estimation of C stocks in forest ecosystems in order to provide a scientific basis for forest management practices and prediction of potentials for C sequestration in the future.Geostatistics is the most effective tool for spatial variability analysis at present.It can be combined with other spatial analysis techniques（Moran’s I,Neighborhood statistics,etc.）to study the spatial variation and distribution pattern of environmental variables,and can provide statistical information of spatial variability,which has been widely applied to the fields of ecology,geoscience and environmental science.In this study,based on the spatial distribution of GIS grid points in the whole forest areas in Zhejiang Province,a total of 838 forest samples plots（soil,litter,etc.）were selected,combined with the relevant data from the Forest Resources Monitoring Center in Zhejiang Province,the spatial variability of forest ecosystems C density in Zhejiang Province was studied using Moran’s I combined with geostatistics and GIS,and in order to accurately estimate the C storage in forest ecosystems in Zhejiang Province.The main results are following:（1）The average C density of forest ecosystems in Zhejiang Province was 145.22Mg/ha,which was much lower than the average level of China of 258.83 Mg/ha,likely because of young and middle-aged forests with low C densities dominated in Zhejiang Province.The average C density values of forest vegetation,soil,litter,and dead wood layer were 27.34 Mg/ha,108.89 Mg/ha,1.79 Mg/ha,and 1.38 Mg/ha,respectively.The CVs of forest vegetation,soil,and litter layer ranged from 66.48%to 73.69%,indicating moderate variability in Zhejiang Province.（2）Forest vegetation C,soil C,and forest litter C was significantly and positively correlated with forest age and altitude.Forest vegetation C exhibited positive and significant correlations with most influential factors expect for slope aspect,especially the influential factor of forest age（r=0.469,P<0.01）,indicating that forest age played a vital role in the C storage of forest vegetation biomass,and the biomass C accumulation increases with forest age.Meanwhile,positive and significant relationships between forest soil C and altitude,forest age,and forest vegetation C were observed.In addition,forest litter C displayed significantly positive correlations with forest soil C,altitude,forest age,and forest vegetation C;while significantly negative correlations between forest litter C and slope aspect,slope position,tree species,and forest type.（3）The global Moran’s I values of forest vegetation,soil,and litter layer were0.165,0.181,and 0.123,respectively,indicating that significant positive spatial autocorrelations were observed across the province.Local Moran’s I hotspot analysis showed that the high-high spatial cluster areas were clearly identified in the southwest and northwest mountainous areas of Zhejiang Province,the low-low spatial clusters were mainly distributed in the central part of Zhejiang Province,and relatively small low-low spatial clusters were found in the southeast coastal areas of the province.While most of the high-low outliers were close to the low-low areas,on the other hand,the low-high outliers were mainly located closely to the high-high spatial cluster areas,probably owing to natural disasters and human disturbances.（4）The semivariograms of forest vegetation and soil layer fitted well to the Exponential model,and forest litter layer was best fitted by a Gaussian model.Forest vegetation,soil,and litter layer all had moderate spatial dependence based on the“nugget-to-sill”values of 74.12%,65.79%and 54.24%.In addition,the ranges of forest vegetation,soil,and litter layer were 14.53 km,15.70 km,and 137.28 km,respectively.All ranges were much greater than the sample interval in this study,implying that current sampling design was appropriated to study the spatial structure of forest ecosystems C density in Zhejiang Province.（5）The spatial distribution of forest ecosystems C density showed a decreasing trend from the southwest to northeast area,roughly in line with Zhejiang Province’s topographic feature,as well as differences in land use,forest management,economic and social development.In sum,high C densities mainly located in the southwest and northwest parts of Zhejiang Province,and relatively low C densities in the northern plain,center hilly basin and coastal part of southeast.These high and low value spatial patterns were in line with the above spatial clusters revealed by local Moran’s I.（6）The total C storage in forest ecosystems in Zhejiang Province was approx.877.19 Tg C（1 Tg C=10¹22 g C）,of which 203.88 Tg C was in the vegetation layer,656.20 Tg C in the soil layer,10.84 Tg C in the litter layer,and 6.27 Tg C in the dead wood layer,accounting for 23.24%,74.81%,1.24%,and 0.71%of the total C storage,respectively.The soil layer was the largest C pool in forest ecosystems,and which was the most important contributor to the C storage of forest ecosystems in Zhejiang Province.