The magnitude of forest biomass plays a key role influencing the global carbon cycle and assisting in meeting greenhouse gas emission targets (Ciais et al. 2008, Li et al. 2010b, Nabuurs et al. 2010). With a large number of statistically valid plots, forest inventories have been recognized as appropriate data for identifying the size and spatial patterns of forest biomass at a regional or national scale (Schroeder et al. 1997, Choi et al. 2002, Son et al. 2007b, Guo et al. 2010, Li et al. 2010b). Most forest inventories only record the merchantable timber volume and exclude non-commercial components, such as branches, foliage, and twigs (Fang and Wang 2001). Biomass expansion factors (BEF) that convert, expand, or reduce the volume (or dry weight) of the biomass estimate are normally used for predicting tree biomass as a function of stand structural variables to estimate forest biomass and account for the non-commercial components (Fang et al. 2001, Lehtonen et al. 2004, Somogyi et al. 2007, Son et al. 2007b).
BEF have been introduced in recent decades to estimate forest biomass on a landscape or regional level (Fang et al. 1998, Van Camp et al. 2004, Somogyi et al. 2007). For example, BEF were obtained by compiling data from the literature to estimate country-level biomass (Fang et al. 1998, Van Camp et al. 2004). Reliable estimation and representativeness of BEF are crucial for accurately estimating changes in forest biomass and carbon stock. BEF have also been strongly recommended by the Intergovernmental Panel on Climate Change (IPCC) guidelines (IPCC 2003, 2006). The IPCC (2006) revised the volume-based BEF and weight-based BEF into the biomass conversion and expansion factors (BCEF) and BEF, respectively.
Q. mongolica and Q. variabilis are two of the most common oak species that are widely found in natural deciduous and mixed forests throughout Korea (Yi 2003, Son et al. 2004a, 2007a, Park et al. 2005a, Kwon and Lee 2006b) and play an important role in ecological, social and economic aspects in terms of increased biodiversity, cultural significance, and wood production across the country. Over the past several decades, data sets on biomass and productivity of Q. mongolica and Q. variabilis forests have been published with the large accumulation of field survey data (Song et al. 1997, Park et al. 2003, 2005a, Son et al. 2004b). Son et al. (2005) first calculated and tested the values of root to shoot ratio (R) and BEF for Q. mongolica and Q. variabilis by age classes using e destructive method. However, there is still a lack of information on the BEF for the two tree species.
This study estimated the biomass expansion factors (BCEF, BEF, R, and ecosystem biomass expansion factor [EBEF]) for Q .mongolica and Q. variabilis forests based on Korean data. The definition and equation for calculating the BEF were determined based on the IPCC guidelines (2003, 2006) and one publication (Li et al. 2010a).
Studies on biomass were reviewed in the literature by tree species in Korea, and biomass data sets reported for Q. mongolica and Q. variabilis with diameter at breast height (DBH) < 6 cm were omitted, because small DBH trees are considered saplings (Korea Forest Service 2009). In total, 39 data sets (N = 39) for Q. mongolica (Table 1) and 17 data sets (N = 17) for Q. variabilis were obtained (Table 2). Notably, not all publications reported stand volume, tree root biomass, and ground vegetation (including herbs and shrubs) biomass for the two tree species. For those studies, stand volume was estimated based on the reported mean DBH and height by multiplying the mean stem volume by the stand density using a revised volume table (Korea Forest Service 2009). The data of stem volume over the bark in the studies was usually determined using Smalian’s formula (Avery and Burkhart 1983, Park et al. 2005b, Li et al. 2011).
The general characteristics of Q. mongolica or Q. variabilis forest stands from most of the previous studies indicated the overstory vegetation was relatively or purely dominated by Q. mongolica or Q. variabilis, and little biomass information on other overstory species was reported (Song et al. 1997, Park et al. 2003, 2005a, Son et al. 2004b). Thus, we assumed that the overstory vegetation was purely dominated by Q. mongolica or Q. variabilis in the reported oak forests. According to the IPCC (2003, 2006), total tree biomass in Q. mongolica or Q. variabilis forests was calculated based on Eqs. (1) and (2).
where B is the total tree biomass (Mg/ha), V is the merchantable volume (m3/ha), R is the root to shoot ratio, which is dimensionless, WD is the basic wood density, BCEF is the aboveground tree biomass to stand volume ratio (Mg/m3), and BEF is the aboveground tree biomass to tree stem (over bark) biomass ratio, which is dimensionless.
In addition, EBEF, the ratio of forest ecosystem biomass (total tree biomass plus ground vegetation biomass) to total tree biomass, is used to estimate ground vegetation biomass of Q. mongolica or Q. variabilis forest ecosystems (Li et al. 2010a).
The mean BCEF, BEF, R, and EBEF for Q. mongolica was 1.0383 Mg/m3 (N = 27; standard deviation [SD], 0.5515), 1.3572 (N = 27; SD, 0.1355), 0.2017 (N = 32; SD, 0.0447), and 1.0216 (N = 7; SD, 0.0232), respectively. The mean BCEF, BEF, R, and EBEF for Q. variabilis was 0.7164 Mg/m3 (N = 17; SD, 0.3232), 1.2464 (N = 17; SD, 0.0823), 0.1660 (N = 8; SD, 0.0632), and 1.0496 (N = 8; SD, 0.0725), respectively. According to Son et al. (2005), the mean values of BEF and R over age classes were 1.4200 and 0.2643 for Q. mongolica, and 1.2007 and 0.2453 for Q. variabilis, which were different from our results. The discrepancy appeared to be due to the young age classes (<20 years old) that were excluded from our study (Tables 1 and 2). Compared to our findings, different BCEF values of 0.87
and 1.02 Mg/m3 for oak species (Quercus robur) in Europe indicated that species-specific BCEF value for oak was more variable than other broadleaf trees (Van Camp et al. 2004).
The IPCC (2003) provides tables of the default values for the BEF of temperate broadleaf forests and the R of temperate oak forests. The IPCC (2006) also provides default values for the BCEF of temperate hardwoods forest. For a better understanding of applicability of the biomass expansion factors in the Korean case, the mean BCEF, BEF and R values for Q. mongolica and Q. variabilis were calculated based on the default values given by the IPCC to compare the mean values in this study. The mean default BCEF value with forest ages of 21-100 years old was 1.5500 Mg/m3, which was much higher than the mean value of BCEF for the two species in our study, whereas the mean default values (1.4000 and 0.3500) of BEF and R seemed to be slightly higher than the mean default value of BEF and R in this study. The results of a comparison suggested that the biomass expansion factor values in this case may be more representative estimates of forest biomass in Q. mongolica or Q. variabilis forests of Korea than those
using the default values.
Recent studies indicate that the relationships between BEF and stand age are heteroscedastic and non-linear. For example, Li et al. (2010c) reported that BEF is expressed as a logarithmic equation of stand age for natural Japanese red pine (Pinus densiflora), whereas a exponential equation was used between BCEF and stand age for Finnish birch (Betula pubescens) (Lehtonen et al. 2004). However, no significant relationship between BEF and stand age was observed in the present study (Fig. 1). Different results may be due to different tree species and the relatively small scale data in this study. Because the biomass expansion factors of different tree species are sensitive to stem biomass and volume, any change in stem biomass or stem volume may influence the values of the biomass expansion factors. For example, studies on the heartwood of living hardwood trees rotted by microorganisms growing on wood biodegradation with increasing age were more than that of coniferous species (Chi 2001, Chi et al. 2004), suggesting that further evidence for the relationship between BCEF and stand age will take different species into account and require more data from future studies.
Ground vegetation is an important biomass component in oak forest ecosystems. The EBEF was easily defined, and it is believed that the EBEF is a complementary method for estimating the ground vegetation biomass in Q. mongolica or Q. variabilis forests in Korea.
