Annual Carbon Sequestration Per Crown Cover Area of Urban Green Spaces in South Korea: Comparative Analysis Using Different Sampling Methodologies

The quantification of carbon sequestration in Urban Green Spaces (UGSs) is challenging due to their unique characteristics such as the fragmentation of green spaces, human-influenced species selection, and varying management practices. IPCC guidelines recommend calculating carbon sequestration in UGSs per individual tree or crown cover area. Although crown cover area-based estimation is commonly used in national greenhouse gas inventories, research on the growth rate based on crown cover area (CRW) and the effects of varying sampling methodologies is limited. This research aimed to calculate CRW [ton C (ha crown cover)^-1 yr^-1] in South Korea using two distinct sampling methods. Method_SS (M_SS; Systematic Sampling Method) divided urban areas into 500m x 500m grids and selected a 5% sample (1,603 grids) randomly. Within each grid, three sites were chosen and vegetation at three points was surveyed. For analysis, only points with trees excluding shrubs were included. Method_CS (M_CS; Categorized Sampling Method) divided UGSs into three categories: street trees, urban parks, and others. For each category, 48 sites with three plots each were selected. In the street trees category, a plot consisted of 20 trees, while in urban parks and others, a plot was defined as a 20 x 20 m area. At each plot, species, diameter at breast height (DBH), height, and crown width of all trees were measured. For the derivation of CRW, a total of 8,037 and 5,733 trees were used in M_SS and M_CS, respectively. CRW was calculated in four steps: 1) The carbon storage of individual trees (C_t1) was calculated using allometric equations and the carbon fraction. 2) The carbon storage from one year prior (C_t2) was estimated based on the annual DBH growth rate (cm yr^-1). 3) The annual carbon accumulation (kg C tree^-1 yr^-1) was calculated as the difference between C_t1 and C_t2. 4) CRW was calculated by dividing the total annual carbon accumulation by the total crown cover area of the surveyed trees. As a result, The study revealed a notable difference in CRW between M_SS and M_CS. M_SS reported CRW of 0.23 ton C ha^-1 yr^-1, while M_CS presented 0.30 ton C ha^-1 yr^-1. When categorized by land use, CRW was found to be highest in urban parks followed by others and street trees. In M_SS, the diverse sample locations resulted in a wider range of DBH values, including many large-sized trees (DBH ≥ 70 cm). The lower CRW estimates in M_SS were primarily due to the assumption that large-sized trees had zero annual carbon sequestration following the IPCC guidelines. This led to a higher inclusion of large-sized trees in M_SS, resulting in lower CRW values. Also, there were significant differences in species distribution, tree sizes (DBH), and CRW, depending on the sampling methodology. These variances are primarily due to the unique characteristics of UGSs. The study highlights the necessity for further research into more representative sampling methodologies for estimating carbon sequestration in UGSs.