Controls Of Carbon Accumulation And Sequestration In Acacia Mangium Wild. Plantation Ecosystems

Acacia mangium Willd.was first introduced into Vietnam in 1980 and it's now one of the most important fast-growing afforestation tree species in some of the eight important forestry-ecological regions and Southeastern region in Vietnam,providing significant ecological,economic,environmental,and social benefits.In the present study,a total of 36 A.mangium trees with diameter at breast height ranging from 13.38 to 22.87 cm were harvested from the three different aged stands in order to assess the biomass of different tree components in Chang Riec Historical-Cultural Forest,Southeastern region,Vietnam.Biomasses of understory(shrubs and herbs),and litter were also determined.Carbon(C)storage in the trees and under story biomass,litter,and mineral soil(0-50 cm)were estimated by analyzing the C content of each compartment.Furthermore,in order to provide the underlying mechanisms that lead to better understanding of C cycle in the region using a sample survey data,the dominant driving factors for ecosystem C storage of A.mangium forest in the region was explored.The major research results obtained are as follows.1)The biomass of each component of the tree could be predicted from an allometric equation using DBH as the independent variable regardless of stand age.The allometric equations for stem,branches,roots,aboveground trees,and whole trees were power functions in the form of Yi=a×DBHb,while was exponential function in the form of Yi=a×eb×DBH for leaves.The allometric equations that were developed interpreted over 92%of the variability for the tree compartments.The equations were most fitted for the aboveground tree biomass and total tree biomass among the various components.The biomass in trees,understory vegetation,litter,and ecosystem increased with stand age.Aboveground tree biomass C represented 69.63%of the total,belowground tree biomass C made up 18.71%,and litter comprised 8.10%,whereas only a small amount(3.56%)was associated with understory vegetation.The highest C concentration was observed in tree stem while the lowest was recorded in the tree roots.The average C content of total tree(47.97%)was higher than those of understory(44.15%)and litter(44.45%).The use of component-specific C content values other than a fixed factor of 0.5 to convert biomass to C storage is required in order to more precisely evaluate the forest C storage.Total biomass C storage of three planted forests ranged from 41.61 to 110.33 Mg C ha-1,of which 82.98-92.01%of C storage was in the trees.These results suggest that A.mangium is a promising afforestation tree species with fast growing,high biomass accumulation and high C sequestration potential.2)Soil C concentration(SOC)decreased significantly with increasing soil depth irrespective of stand age.The SOC content of the upper 0-10 cm soil layer was the highest in all three stands,and the values for each stand were 12.70,20.08,and 21.90 g·kg-1,respectively.Soil organic C concentration at all soil depths increased significantly with stand age.Soil organic C stock in the top 50 cm depth in 4-,7-and 11-year-old stands of A.mangium were 86.86,126.88 and 140.94 Mg C ha-1 respectively.More than 60%of soil C was stored within 0-30 cm soil layer of each stand age.These findings emphasized that afforestation with A.mangium can promote the potential capability of SOC and supply needed data for assessing ecosystem C storage of A.mangium plantations,thereby contributing to the development of forest management and payment for forest environmental services in Vietnam.3)The soil BD presented the pattern of 4->7->11-year-old stands,and increased significantly with soil depth.The average contents of clay,silt,and sand in the entire soil profile for the three stand ages were 9.83%,12.26%,and 16.17%;29.42%,45.56%,and 63.19%,and 60.75%,42.18%,and 20.64%,respectively.The soil pH showed the pattern of 4->7->11-year-old stands,while pH appeared an increasing trend with soil depth.The soil total nitrogen(TN),total phosphorus(TP),total potassium(TK),available nitrogen(AA),available phosphorus(AP),and available potassium(AK)concentrations demonstrated a significant increase with stand development.In the different stand ages,the contents of soil TN,TP,TK,AN,AP,and AK showed a trend of significant decreasing with increase in soil depth.4)(i)The soil texture(i.e.,soil clay and silt contents)and soil nutrients(i.e.,soil TN and AN contents)were the major factor regulating the plant biomass C(PBC).The stand characteristics(i.e.,stand density,DBH,Hvn,and canopy density),and other soil physical(i.e.,soil BD)and chemical(i.e.,soil pH,AN,TP,AP,TK,and AK)properties also significantly influence the distribution of PBC;(ii)The soil nutrient(i.e.,soil TN content)and soil texture(i.e.,soil silt content)were identified as the primary factors driving the SOC.Furthermore,the other parameters(i.e.,plant biomass,soil pH,BD,AN,TP,AP,TK.and AK)could influence SOC by directly or indirectly affecting soil TN and silt.These results suggest that afforestation with A.mangium can facilitate PBC and SOC accumulation and improve soil nutrient regimes,and provide a new insight in understanding the factors controlling PBC and SOC in the Southeastern region of Vietnam.