Large rate of uptake of atmospheric carbon dioxide by planted forest biomass in Korea

Transcription

1 GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 16, NO. 4, 1089, doi: /2002gb001914, 2002 Large rate of uptake of atmospheric carbon dioxide by planted forest biomass in Korea Sung-Deuk Choi, Kitack Lee, and Yoon-Seok Chang School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, Republic of Korea Received 11 April 2002; revised 19 August 2002; accepted 30 August 2002; published 16 November [1] The Republic of Korea, henceforth referred to as Korea, has successfully implemented intensive programs of reforestation and forest management over the last 30 years to restore its once-rich forests. This nationwide effort has resulted in a massive accumulation of less than 30-year-old tree biomass, which now accounts for about 72% of the total forest biomass in Korea. Here we use a forest tree inventory data set for Korea to calculate the effectiveness of these planted trees in absorbing excess carbon dioxide from the atmosphere during the period The forest carbon density in Korea has increased from 5 7 megagrams of carbon per hectare (Mg C ha 1,Mg=10 6 grams) in the period to more than 30 Mg C ha 1 in the late 1990s. The calculated carbon uptake has increased from a mean rate of petagrams of carbon per year (Pg C yr 1, Pg = grams) in the period to as high as Pg C yr 1 in recent years, largely due to the 30-year implementation of reforestation and forest management projects. The contemporary rate of carbon uptake by the total Korean tree biomass is approximately one-half of the mean rate of carbon uptake by the total Chinese forest biomass of Pg C yr 1 [Fang et al., 2001]; the Chinese forest biomass has recently been found to be a significant carbon sink in northern temperate regions. The observed uptake rate for Korea is remarkably high, considering the fact that the total area of Korean forests is approximately 16 times smaller than that of Chinese forests. Our results show that long-term rates of carbon sequestration by nationwide forests can be increased substantially through reforestation and forest management. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1610 Global Change: Atmosphere (0315, 0325); 1699 Global Change: General or miscellaneous; KEYWORDS: forest carbon uptake, atmospheric carbon dioxide, forest biomass, reforestation, forest management, biomass expansion factor Citation: Choi, S.-D., K. Lee, and Y.-S. Chang, Large rate of uptake of atmospheric carbon dioxide by planted forest biomass in Korea, Global Biogeochem. Cycles, 16(4), 1089, doi: /2002gb001914, Introduction [2] Improved information about the partitioning of carbon between the atmosphere, the terrestrial biosphere, and the oceans enables more accurate predictions of future atmospheric carbon dioxide (CO 2 ) concentrations under various fossil-fuel CO 2 emission scenarios. One of the more poorly quantified processes is the temporal variability in the net carbon uptake by the terrestrial biosphere that can offset fossil-fuel CO 2 emissions [Francey et al., 1995; Keeling et al., 1995; Lee et al., 1998; Battle et al., 2000; Bousquet et al., 2000]. Several independent lines of evidence support the existence of a terrestrial carbon sink in the temperate Northern Hemisphere [Tans et al., 1990; Kauppi et al., 1992; Dixon et al., 1994; Fan et al., 1998; Fang et al., 2001; Pacala et al., 2001; Schimel et al., 2001]. Nonetheless, the location, size, and temporal variability of this sink Copyright 2002 by the American Geophysical Union /02/2002GB remain unresolved, partly due to the sharp contrast between the results of atmospheric inverse methods and those of ground measurements of forest inventories [Pacala et al., 2001]. Indirect methods such as remote sensing and atmospheric inverse modeling are potentially powerful tools for the improvement of our knowledge of the spatial and temporal coverage of the terrestrial biomass, but have met with little success for heterogeneous forests [Wu and Strahler, 1994; Hall et al., 1995]. At present, the best method for obtaining objective information on the long-term trend of the net carbon uptake by forests is to make ground measurements for a sufficient number of sample plots located in systematic grids [Kauppi et al., 1992; Dixon et al., 1994]. [3] Forest inventory surveys have been carried out on a regular basis in industrialized countries over a long period, providing a basis for policy-making decisions concerning the past and future development of forest resources [Kauppi et al., 1992; Dixon et al., 1994; Brown and Schroeder, 1999; Brown et al., 1999]. Since the Kyoto Protocol of 1997 established greenhouse gas emission targets for indus- 36-1

2 36-2 CHOI ET AL.: CO 2 UPTAKE BY PLANTED FOREST BIOMASS IN KOREA trialized countries, considerable research has focused on the role of forests in the global carbon cycle. Unlike other mid- and high-latitude countries, the total forest biomass in Korea has significantly increased between 1973 and 2000, through reforestation and forest management programs. The objective of this paper is to quantify the net aboveground accumulation of carbon in Korean forests, and show that continuous forest inventory surveys in Korea can be used to validate such an offset. We compare our results with the rates of carbon uptake by Chinese forests, because the Chinese forest biomass has become a significant sink for atmospheric CO 2 through China s programs of reforestation, afforestation, and forest management [Fang et al., 2001]. 2. Calculation Methods and Error Analysis [4] We used a national timber volume data set for Korea collected for the years This data set was obtained from the Statistical Yearbook of Forestry that is published annually by the Korea Forest Service [Korea Forest Service, 2000a]. The total timber volume present in each year was estimated from analysis of 3500 permanent plots that are approximately evenly distributed across the total forest area of hectares. Each plot has an area of 0.05 hectares (500 m 2 )[Korea Forest Service, 2000b]. Timber volumes by age class and tree type (such as coniferous or deciduous) were measured for each plot but did not include estimates for the volumes of branches, leaves, and roots. The following assumptions are made here in estimating the total timber volume: (1) that the distributions of tree type and age class of each plot are similar to those of the adjacent forest that contains the plot; and (2) that the overall tree growth rate measured for the plot also applies to the adjacent forest area. [5] To convert this timber volume to the total tree biomass M TREE, including branches, leaves, and roots, we used a volume-derived method that requires the specification of the dried wood specific density, and the ratios of aboveground tree biomass (timber + branches + leaves) to timber biomass and of total tree biomass (timber + branches + leaves + roots) to aboveground tree biomass: M TREE ¼ V D R a R t where V (m 3 ) is a 3-year averaged tree volume; D (Mg m 3 ) is the measured dried specific density, which is 0.47 for Korean Pine and 0.80 for Oriental Chestnut Oak [Korea Forest Research Institute, 1994]; R a is the measured ratio of aboveground tree biomass to timber biomass, which is 1.29 for Korean Pine and 1.22 for Oriental Chestnut Oak [Kim and Kim, 1988]; and R t is the measured ratio of total biomass to aboveground tree biomass, which is 1.28 for Korean Pine and 1.41 for Oriental Chestnut Oak [Kim and Kim, 1988]. The biometric parameters (D, R a, R t ) were only required for Korean Pine and Oriental Chestnut Oak [Kim and Kim, 1988; Korea Forest Research Institute, 1994; Lee et al., 2001], because these two types of trees (Pinus and Quercus species) account for more than two-thirds of all coniferous and deciduous trees in Korea, respectively ð1þ [Korea Forest Research Institute, 1996; Lee et al., 1997; Lee et al., 2002]. The assumption was made in our analysis that Korean Pine and Oriental Chestnut Oak can be used to represent all coniferous and deciduous trees in Korea, respectively. [6] The resulting total tree biomass was then converted to carbon mass C t (Mg) using the ratio of carbon-to-total tree mass C c : C t ¼ M TREE C c A value of 0.5 for C c [Martin et al., 2001], as recommended by the Intergovernmental Panel on Climate Change (IPCC) [1996], was used in this calculation. [7] Systematic ground measurements of forest tree volume provide direct evidence of temporal changes in forest tree volume. However, estimated rates of forest carbon uptake are subject to errors due to uncertainties in the following procedures: (1) In the estimation of tree volume, the total number of permanent plots and the area of each plot were selected to provide an estimate of the total timber volume with an error of less than 5% (2) In accounting for land use change, the total timber volume data set for each year takes into account changes in forest area and tree volume caused by historical land use change. (3) In the selection of representative plots, the Korea Forest Service selected the permanent plots that best represented the adjacent forest areas. The type and age class of forests, and the tree growth and mortality rates observed in the selected plots are used to represent those of the adjacent forest areas, although the accuracy of this procedure varies from place to place. Quantifying the uncertainties in estimated forest carbon uptake rates produced by this approximation is presently not possible because the relevant survey data are not available [Korea Forest Service, 2000a]. (4) In the estimation of the biometric coefficients, since measured biometric coefficients for other types of trees in Korea were not available [Lee et al., 2001], we used biomass expansion factors (BEF), defined as the ratio of all stand biomass to living stock volume, for various types of coniferous and deciduous trees of China, to explore the uncertainties in estimated carbon uptake rates produced by this approximation. The forest survey data set of Korea shows that one third of the coniferous and deciduous forest types are not Pinus and Quercus species, respectively [Lee et al., 1997]. Therefore, an upper estimate of the carbon uptake rate for each year can be calculated by assuming that the remaining coniferous and deciduous forests have BEF values equivalent to the highest values for such forests found in China. For a lower estimate of the uptake rate, the lowest BEF values for such forests in China are used. The contrast between these two estimates yields an uncertainty of approximately 40% in annual uptake rates. This analysis suggests that the use for practical reasons here of only two major tree groups to represent all Korean forests is a reasonable approach. [8] The uncertainty associated with estimating forest carbon uptake rate is dominated by that inherent in the third and fourth approximations. However, the current lack of adequate data as to the accuracy of these approximations ð2þ

3 CHOI ET AL.: CO 2 UPTAKE BY PLANTED FOREST BIOMASS IN KOREA 36-3 Figure 1. Temporal variations in (a) total forest area, (b) total forest C biomass, (c) forest carbon density, and (d) forest carbon uptake rate for the period The shaded area in Figure 1d depicts the difference between maximum and minimum carbon uptake rates produced by uncertainties in the relative proportions of coniferous or deciduous trees in Korean forests. The 5-year mean values for the period are shown as open circles. makes it impossible to evaluate the true magnitude of the errors in estimated carbon uptake rates [Korea Forest Service, 2000a; Lee et al., 2001]. To estimate the maximum magnitude of the errors produced by the third and fourth approximations, probable annual maximum and minimum carbon uptake rates are estimated. The maximum carbon uptake rate for an individual year is estimated by assuming that all the trees are exclusively deciduous, and the minimum carbon uptake rate by assuming that all the trees are coniferous. The maximum uncertainty in the estimated carbon uptake is the difference between the annual maximum and minimum rate. This yields an error in the annual uptake rates of about 60% (the shaded area in Figure 1d). This approach seems a reasonable way to explore the upper and lower limits of the rates of carbon uptake by forests in Korea. 3. Results and Discussion [9] The total carbon content in the Korean forest biomass remained as low as 0.03 Pg C for the period , and then rapidly increased to 0.2 Pg C for the period (Figure 1b), whereas the total forest area decreased by 3.7% over the last 45 years (Figure 1a). In the 1950s, Korea had hectares of forest area with an area-weighted mean carbon density of 4.8 Mg C ha 1 (Figure 1c), which is an order of magnitude lower than those of forests in other northern extratropical countries [Kauppi et al., 1992; Fang et al., 2001]. This low carbon density in the forest resource during the 1950s derived mainly from forest exploitation in the period , and massive destruction during the Korean War in the years The Korean forest biomass did not accumulate carbon in the period , due largely to the less efficient carbon uptake of mature baseline forests and to a lesser extent to continued forest exploitation. Since 1973, the Korean government has initiated three 10-year forest restoration programs to protect water, soil, and biological resources. Consequently, the total forest carbon inventory has significantly increased by a 30- year total of 0.17 Pg C, with an annual uptake rate ranging from as low as Pg C yr 1 during the early 1980s to as high as Pg C yr 1 in the late 1990s (Figure 1d). [10] The large increase in the total carbon content over this 30-year period is a consequence of the increase of forest carbon density produced by reforestation and forest management practices (Figure 1c). Forest carbon density has increased from less than 7 Mg C ha 1 for the period to more than 30 Mg C ha 1 in the late 1990s, with a carbon accumulation rate of 1.5 Mg C ha 1 yr 1 in the late 1990s. This rate is much higher than the rates reported for other countries in which reforestation or forest management have been practiced successfully [Dixon et al., 1994; Fang et al., 2001] and close to the rate required in northern midlatitude forests of 1.5 Mg C ha yr 1 to fully offset the unbalanced 1.8 Pg C yr 1 in the global carbon budget [Dixon et al., 1994] (Figure 2). [11] One of the key processes affecting rates of carbon sequestration by forests is temporal variability in the proportion of planted to baseline forests [Wofsy et al., 1993; Barford et al., 2001; Fang et al., 2001; Schimel et al., 2001]. However, the forest inventory data set for Korea does not include temporal changes in planted and baseline forest volumes. A less direct method was used here to quantify the relative contribution of planted and baseline

4 36-4 CHOI ET AL.: CO 2 UPTAKE BY PLANTED FOREST BIOMASS IN KOREA Figure 2. Contemporary rates of carbon uptake by forest tree biomass in Korea for , in China for [Fang et al., 2000], in Europe for [Dixon et al., 1994], and in the USA (continental USA and Alaska) for [Dixon et al., 1994]. All annual carbon uptake rates were derived from ground measurements of forest biomass. Error bars for estimates for Korea s, Europe s, and the USA s forests indicate upper and lower estimates of carbon uptake rates (Mg C ha 1 yr 1 ). forests to the rates of carbon sequestration by the total forest biomass. Planted forests in this study refers to both planted and re-grown trees that are younger than 30 years old, whereas baseline forests refers to trees that are older than 30 years. Rates of carbon uptake by baseline forests were close to zero during the period (Figure 1d), which suggests that the same may have been true for the last 30 years. Forest biomass has significantly accumulated since 1973 and we attribute this forest accumulation to the active growth of planted forests. This planted forest pool currently accounts for more than 72% of the total forest biomass in Korea. This is a large fraction of young forest trees (<30 years old), which are effective carbon sinks and will continue to sequester excess CO 2 until they mature. [12] The ratio of planted to total forest tree biomass in Korea has increased from less than 0.1 in the early 1970s to 0.8 in the late 1990s. This ratio is much higher than that in China, known as one of the few countries that have successfully implemented reforestation and forest management programs over a long period of time (Figure 3). Such a high ratio of planted-to-total tree biomass is attributable to low baseline forest biomass and high forest accumulation through the successful implementation of vigorous reforestation and forest management programs. The rate of carbon uptake by the total forest tree biomass in Korea has notably increased over the last 30 years (Figure 1d). The estimated rate for annual carbon uptake has not increased linearly with time, as assumed in global carbon cycles [Houghton et al., 1983, 1987; Melillo et al., 1988]. Interannual variations of the carbon uptake rate are probably induced by weather and climatic variations such as temperature, growing-season length, and the availability of light [Braswell et al., 1997; Myneni et al., 1997; Rayner et al., 1999; Yang and Wang, 2000]. However, much of the long-term trend is a consequence of the active growth of planted forest trees. [13] The uptake rate of Pg C yr 1 found for the late 1990s is comparable to the 5-year mean rate of Pg Cyr 1 during the period [Fang et al., 2001] of uptake by the entire Chinese forest biomass, which is known to be a significant sink for excess CO 2 in the atmosphere. Note that the area of Korean forests is approximately 16 times smaller than that of Chinese forests. The estimated rate of carbon uptake by planted trees in Korea is certainly an underestimate of the total carbon uptake rate, as it does not include other carbon sinks such as nontree vegetation, nonliving organic matter, wood products in use and in landfills, and fine litter [Lee et al., 2002]. Furthermore, Korean forests will maintain this high rate of carbon sequestration over at least the next few decades, because the young (<30 years old) forest trees that account for more than 72% of the total forest biomass will maintain relatively high growth rates. It is well known that midsuccession forest biomass ( years) is a major contributor to present-day terrestrial carbon uptake in northern temperate regions, due to its high growth rate [Wofsy et al., 1993]. 4. Conclusion [14] The 30-year practice of reforestation and forest management in Korea has resulted in a remarkably high annual rate of carbon sequestration by planted forest trees. This restoration project has made Korean forests, accounting for 1% of the total midlatitude forests in the northern hemisphere, a significant carbon sink, although they were not specifically designed for that purpose. Our results show that national forests can be effective sinks for excess CO 2 in the atmosphere through successful implementation of intensive reforestation and forest management programs. However, it Figure 3. Temporal variations in the measured ratio of planted to total forest tree biomass in Korea (solid circles) and China (open circles) for the period Planted forests in this study include both planted and regrown trees that are younger than 30 years old. Open circles represent 5-year mean values and are centered at median years.

5 CHOI ET AL.: CO 2 UPTAKE BY PLANTED FOREST BIOMASS IN KOREA 36-5 is not clear how long today s high uptake rates will be sustained in the future, because the high growth rate that is currently causing this uptake is likely to diminish with time. [15] Acknowledgments. This paper benefited a great deal from many critical suggestions by S. Kang and an anonymous reviewer. This study would not have been possible without the painstaking work of many personnel in the Korea Forest Service over the last 45 years. We thank K.H. Lee of the Korea Forestry Research Institute for providing valuable advice about data interpretation. Special thanks are further extended to K.H. Kim of the Korea Forest Service for providing government documents concerning the history of land use change, reforestation, and deforestation. 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Lee (corresponding author), School of Environmental Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Nam-gu, Pohang, , Republic of Korea. (ktl@postech.ac.kr)