The National Forest Inventory (NFI) in South Korea has been carried out for five year periods beginning in 1972 (Yim et al., 2015). The purpose of the NFI application is to provide basic information obtained by scientific methods of identifying the ecological dynamics of South Korean forests (KFRI, 2011). The CWD data used in this study was acquired from the 5th NFI, which had been collected utilizing fuel-sampling methods during 2006∼2010 (sample plots size: 20 m × 20 m, radius: 11.3 m, targeted-over middle diameter of CWD: ≥ 6 cm). NFI CWD data includes middle diameter, length, tree species, decay classes and cause of death (KFRI, 2011). We selected 44,772 sample plots for CWD, excluding denuded and bamboo forests. While the original NFI data does not contain the GIS coordinates of CWD, we could estimate the GIS coordinate of CWD from the number of settlements in the original NFI data set; and through the estimated GIS coordinate, we deduced regional segment of CWD data as a result.

Conversion factors were used to convert CWD density data to CWD mass data. CWD density differs among decay class (Sandström et al., 2007) and the appropriate CWD density factors therefore need to be applied to each decay class. There have been previous studies which examined changes in conversion factors by decay class through sampling and analysis method (Cheung and Brown, 1995; Harmon et al., 2000; Sandström et al., 2007; Yoon et al., 2011). We used the conversion factors by decay class developed by Yoon et al. (2011) because of its high representativeness of the typical South Korean forest species Pinus densiflora and Quercus mongolica. We assumed that the conversion factors for Pinus densiflora would represent coniferous forests and the ones for Quercus mongolica would represent deciduous forests (Table 2). In cases of mixed forests, we used averages of coniferous and deciduous forest conversion factors, which were 0.37 for decay class Ⅰ, 0.30 for decay class Ⅱ, 0.25 for decay class Ⅲ and 0.18 for decay class Ⅳ, respectively (Yoon et al., 2011).

Recent studies have frequently presented energy units by using TOE (Pimentel and Pimentel, 2007); 1 TOE is equivalent to energy of 10 million kcal (Han and Seo, 2010). According to South Korea Energy Basic Law Enforcement Regulations Article 5, 1 ton of firewood and charcoal can be converted to 0.45 TOE (KFRI, 2014).

The total wildfire fuel load of CWD was estimated as 8.9 million TOE, which is comparable to the calorific value of 8.1 million tons of crude oil. The total wildfire fuel load of CWD was 2.46% of that of living trees previously reported, and those of coniferous, deciduous and mixed forests accounted for 2.91%, 2.24% and 2.23%, respectively (Son et al., 2014). By forest type, coniferous, deciduous and mixed forests amounted to 3.5 million TOE, 2.8 million TOE and 2.6 million TOE, respectively. Proportionate to regional growing stock, Gangwon Province had the highest CWD wildfire fuel load (2.3 million TOE), followed by Gyeongbuk Province (1.4 million TOE) and Gyeongnam Province (1.1 million TOE), whereas Seoul exhibited the lowest CWD wildfire fuel load (0.02 million TOE) (Table 3). Wildfire fuel loads of CWD by decay classes were estimated to be 2.9 million TOE, 1.9 million TOE, 2.4 million TOE and 1.7 million TOE for decay classes Ⅰ, Ⅱ, Ⅲ and Ⅳ, respectively.

The result of total wildfire fuel load of CWD per hectare by forest areas turned out to be 1.40 TOE ha^-1 coniferous forests, deciduous forests and mixed forests were 1.34 TOE ha^-1, 1.65 TOE ha^-1 and 1.39 TOE ha^-1, respectively. Gwangju had the highest wildfire fuel load of CWD (2.59 TOE ha^-1), followed by Gangwon (1.66 TOE ha^-1) and Gyeongnam Province (1.62 TOE ha^-1), while Gyeongbuk Province exhibited the lowest wildfire fuel load of CWD (1.02 TOE ha^-1) (Table 3).

While coniferous forests had the highest wildfire fuel load among forest type, coniferous forests have the lowest conversion factors of woody density by decay class. The reason for this result is presumed that in 2010 the coniferous forested area was larger than other types of forested area (coniferous forests: 2,580,629 ha, deciduous forests: 1,718,916 ha, mixed forests: 1,864,925 ha) (KFS, 2010). We therefore concluded that wildfire fuel load of CWD in same size and area would have the highest value in deciduous forests, followed by mixed and coniferous forests.

Wildfire fuel load of CWD was positively correlated with growing stock quantity in that Gangwon Province had the highest CWD wildfire fuel load, whereas Seoul exhibited the lowest. The result shows the more growing stock the province has, a higher CWD wildfire fuel load would be. However, the correlations between wildfire fuel load of CWD and growing stock should be supported by further studies on the regional CWD input rate based on the development of NFI data.

Decay class Ⅰ had the highest wildfire fuel load of CWD due to its highest CWD mass and conversion factors of woody density among the four decay classes. We reasoned that wildfire fuel load of CWD is reflected in different conversion factors by forest type and decay class, however, certain degree of uncertainties remains in the results in that calorific values differ by tree species (Friedl et al., 2005; Hwang et al., 2011a; Hwang et al., 2011b; Zeng et al., 2014). Therefore, it is recommended that further studies develop the calorific value factors of CWD by tree species in order to deduce more precisely estimate of wildfire fuel load.

Proportionate to the growing stock, total wildfire fuel load of CWD was in a broad distinction by region, forest type and decay class, but that fuel load per hectare was not in a broad distinction by region. This result implies that different guidelines by region are not necessarily established for the management of CWD. Our results may serve as a foundation for scientific policy guidelines on preventing wildfires by proposing CWD as an indication of wildfire fuel load.