Study On The Coupling Relationship Between Carbon And Water In Yiyang Forest Ecosystem Based On Optimized Biomass Model

The process of carbon-water in forest ecosystem exhibits a strong coupling relationship.Optimizing the construction of biomass equation and systematically revealing the index of carbon uptake and water circulation are the key to exploring the coupling relationship between forest carbon and water and its mechanism,which can comprehensively improve the accuracy of forest carbon storage assessment,guide soil and water conservation related engineering measures,and scientifically promote the forest soil and water conservation sink increasing ability.The biomass models of trunk,branch,leaf and root biomass of 10 dominant tree species(DTS)in Yiyang region were constructed and optimized,and the spatio-temporal characteristics and hierarchical driving mechanism of the coupling between forest carbon storage and water conservation were systematically revealed using forest field survey data,remote sensing technology,Integrated Valuation of Ecosystem Services and Tradeoffs(InVEST)model,random forest and other multiple machine learning methods.This study focuses on the key roles of forest carbon uptake and allocation(carbon uptake by trunk,branch,leaf and root)and water conservation indicators(rainfall,vegetation transpiration,evaporation and soil water)on the carbon-water coupling at regional scales.The results show that:(1)The integrated Random Forest-Least Squares(RF-LS)method can accurately invert the variances of 87.48%,76.54%,91.94%and 92.84%of the biomass of trunk,branch,leaf and root,respectively.The predicted value was significantly correlated with the measured value(P<0.001).The R2 of aboveground biomass increased by 12.01%and the RMSE decreased by 7.50 Mg·hm-2.In addition,the coefficients a and b of 10 kinds of DTS were fitted and optimized according to the measured diameter at breast height(D),height(H)and allometric growth equation W=a(D2H)b,and the R2 of bamboo(BG)increased by 74%.(2)The aboveground biomass carbon density of Yiyang forest was 65.1±6.37 Mg·hm-2,trunk(47.6±3.86)>root(13.8±1.14)>branch(9.5±0.81)>leaf(8.1±0.6).The average carbon uptake capacity of Euramerican poplar(EP)and Masson pine(MP)was 65.45 kg·a-1 and 56.32 kg·a-1,respectively,while the total carbon uptake capacity of Bamboo(BG)and Chinese fir(CF)was 778.48×104 Mg and 180.64×104 Mg,respectively.The average depth and total yield of Yiyang forest ecosystem were 991.28±224.62 mm and 6.22±1.51 km3 respectively based on the In VEST model.In addition,precipitation(PRE)was highly correlated with the interannual variation trend of WY,and showed a positive promoting effect on water conservation function.(3)Based on the principle of "coupling coordination degree" and Spearman coefficient,the carbon and water in the forest ecosystem of Yiyang City showed a good coupling coordination relationship.From 1980 to 2020,the average carbon and water coupling degree(C)was 0.91±0.05,the coordination degree(T)was 0.7±0.04,and the coupling-coordination degree(D)was 0.79±0.05.The carbon-water coupling benefits of the forest ecosystem in the study area tended to be balanced,the positive coupling effect(PE)and negative coupling effect(NE)of carbon and water were 0.59 and 0.57,respectively,and PE increased by 2%on average every 10 years.(4)Based on the random forest method,the contribution of forest(FL)change to the positive coupling effect of carbon-water(PE)was 47.01±8.92%,followed by 17.91±0.68%of leaf carbon(CWL),17.11±2.31%of root depth(RDEP)and 16.57±2.06%of vegetation transpiration(ET0).In addition,atmospheric dryness(FRA),vegetation evaporation index(KC),soil carbon(CSOL),trunk carbon(CWT)and root carbon(CWR)showed significant potential effects on PE.The contribution of forest(FL)changes to the negative coupling effect of carbon-water(NE)was 72.15±25.03%,followed by branch carbon(CWB)’s 16.09±1.15%,temperature(TEM)’s 15.23±1.92%and potential evaporation(PET)’s 15.03±2.64%.Moreover,shrub(SL),atmospheric dryness(FRA)and trunk carbon(CWT)exhibited significant potential effects on NE.(5)Based on the random forest model,Pearson correlation and significance test,PCA principal component clustering,precipitation(PRE),forest water conservation(WC)and soil water content(SWC)were directly and positively coupled with the carbon storage of each part of the forest,indicating that the enhancement of forest water holding capacity could directly promote forest carbon uptake.However,the increase of relative humidity(RHU)and atmospheric dryness(FRA)would inhibit forest carbon uptake,indicating that atmospheric water stress has a direct negative coupling relationship with forest carbon and water.Indirectly,the enhancement of solar radiation(RAD)and temperature(TEM)can promote forest carbon uptake(CU)and water conservation(WC),but inhibit vegetation evaporation(KC)and transpiration(ET0),which to a certain extent is not conducive to the photosynthesis and respiration rate of leaf surface,and indirectly affects the carbon and water interaction of forest vegetation.This study improved the prediction accuracy of biomass in forest,constructed a forest biomass estimation database based on the classification system of dominant tree species,optimized the allometric growth equation coefficients of 10 dominant tree species in subtropical regions,and revealed the relationship between forest carbon uptake and water conservation indexes,as well as the driving mechanism of carbon and water coupling effects.The results are helpful to further understand the effects of forest carbon sequestration on water conservation and the response of forest water retention to carbon uptake,and help guide the measures of carbon storage and water yield in forest ecosystem,and further improve the regional forest carbon-water coupling effects and its water conservation function.