Analysis Of Spatial And Temporal Variation Characteristics Of Forest Carbon Sink In Zhejiang Province And Its Driving Mechanism

Global warming has become an inevitable and urgent challenge due to increased human activities and massive burning of fossil fuels.Forest ecosystems play a leading role in maintaining ecological services and are also an important carbon sink for atmospheric carbon dioxide（CO₂）.Zhejiang Province,located in the southeastern region of China,is a typical subtropical monsoon region with large net ecosystem productivity,which plays a crucial role in regional ecological improvement and carbon balance.Accurate estimation of forest vegetation carbon stocks,carbon density and their distribution characteristics and drivers has become a core research component of the regional carbon cycle.However,so far,there are few reports on the spatial and temporal changes of long-term forest ecosystem carbon stocks in Zhejiang Province,let alone the analysis of the driving mechanisms of the carbon sink（or source）function of forest vegetation.Therefore,this paper systematically analyzes the status and changes of forest resources,vegetation carbon stock,carbon density and carbon sink（or source）function in Zhejiang Province in each period by using the data of six first-class inventories of forest resources and their reporting results（1989,1994,1999,2004,2009,2014）,combined with meteorological station data and statistical yearbook data,and also uses structural equation modeling and variance decomposition The analysis also explores the influence of different factors on vegetation carbon accumulation and regional carbon sequestration at the sample site and regional scales respectively,so as to comprehensively understand the spatial and temporal distribution patterns of forest vegetation carbon stock and future development potential in Zhejiang Province,clarify the influence of topographic conditions,climate change,economic development and human activities on forest carbon sinks,and provide a scientific basis for accurate estimation of forest vegetation carbon stock and carbon cycle research in subtropical forests.The results show that:（1）The forest structure of Zhejiang Province has changed significantly in the past25 years,mainly the growth of broadleaf forests and the decrease of coniferous forests;the carbon density and total carbon stock of forest vegetation have been increased significantly,and the total carbon stock of vegetation increased from 51.91 Tg C to 212.01Tg C,an increase of nearly 300%,while the average annual rate of vegetation carbon accumulation reached 0.95 Mg C/（hm²·a）.（2）The carbon density of forest vegetation in Zhejiang Province has different distribution characteristics:Ⅰ.It increases with the increase of elevation;Ⅱ.With the increase of slope,it increases first and then decreases,and is maximum in the sharp slope（35°～45°）;Ⅲ.The high vegetation carbon density area is mainly distributed in southwest Zhejiang,northwest Zhejiang and east Zhejiang,while the low vegetation carbon density is mainly distributed in the southern,northeastern and central parts of the country.（3）Regional vegetation carbon sequestration rates are affected by temperature,precipitation,elevation,slope,population density,and GDP per capita:Ⅰ.Increases significantly with increasing temperature;Ⅱ.Significant unimodal relationship with precipitation,peaking at precipitation near 1650mm;Ⅲ.Increases with the increase of elevation and slope;Ⅳ.Negative logarithmic relationship with population density;Ⅴ.showed a trend of increasing and then decreasing with the growth of GDP per capita.Variance Partitioning Analysis found that the three groups of variables together explained64.3%of the total variance of regional vegetation carbon sequestration rate,and their contribution rates were ranked as follows:climate variable group>anthropogenic disturbance variable group>topography variable group.（4）Stand characteristic variables（changes in stand density and mean diameter at breast height）and climatic factors（e.g.,average annual precipitation,temperature,sunshine duration,and monthly precipitation fluctuations）were the main drivers of changes in vegetation carbon accumulation rate.All factors had positive effects,except for precipitation fluctuations,which had a negative effect on the vegetation carbon accumulation rate.Meanwhile,structural equation modeling（SEM）revealed an intrinsic link between vegetation carbon accumulation rate and climate factors and stand characteristics variables,and explained 71.9%～80.2%of the total variance of vegetation carbon accumulation rate:stand factors significantly affected vegetation carbon accumulation rate in a direct way,while climate factors mainly affected vegetation carbon accumulation rate in an indirect way by influencing stand factors.The positive effect of average annual precipitation decreases with increasing average annual precipitation,but the negative effect of precipitation fluctuation increases with increasing precipitation fluctuation.Compared with coniferous forests,broadleaf forests are better able to cope with the negative effects due to increased precipitation fluctuations.（5）Forestation in subtropical areas should choose broad-leaved species and sites with high elevation,steep slope,high temperature,suitable annual precipitation（about1650mm）,sufficient sunshine hours,less precipitation fluctuation and low population density.