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https://www.tandfonline.com/doi/full/10.1080/21580103.2013.846876
Abstract
With the recent threat of climate change, the issue of carbon emission mitigation has become one of the fastest growing international agendas. The major cause of pan-international climate change is the increase of CO2 intensity in the atmosphere. It is possible, nonetheless, to sustain the carbon cycle when green plants, the only organic sink, exist in proper quantity. The main purpose of this current study is to quantitatively analyze the urban forest volume required to absorb carbon from a densely populated area. The study calculates CO2 emissions and uptake by urban forest volume in a given area, as well as the total urban forest volume required to absorb all of the emitted CO2. In a survey in the city of Chuncheon, the volume of CO2 emissions and that of the CO2 that can be absorbed was 2,211,586.74 and 30,455.48 tons, respectively, which means that approximately 2,181,129.26 additional tons need to be absorbed by urban forest volume. By the second-grade vegetation standard, it is necessary to secure 101,921.93 ha of coniferous forest, 103,371.06 ha of broad-leaved forest, or 103,863.30 ha of mixed forest. The urban area within the city of Chuncheon covers only 5303.04 ha, which clearly implies that urban forest alone cannot absorb the CO2 emissions in the area. Therefore, it follows that additional mitigation efforts need to be taken along with securing as much urban forest as possible.
Keywords: carbon dioxide, global warming, carbon account, urban forest, carbon offset
Introduction
Climate change mitigation has become one of the top international agendas. Excessive CO2 emissions increase CO2 intensity in the atmosphere, disturbing its dynamic equilibrium and causing global warming, which increases the frequency of disasters occurring through global climate change. Green plants are the only organic bodies on Earth that absorb or store CO2 emissions (American Forests 2000; Nowak & Crane 2011; Scott et al. 1998; McPherson 1998). A healthy ecosystem helps sustain the cycle in which the carbon emissions from, for example, animal activities are absorbed by plants. In a city full of human activities, however, it is difficult to secure a sufficient plant volume and the many species required to absorb and store excessive CO2 emissions. It is often the case that many of these species have disappeared from urban areas. Efforts to reduce emissions and increase carbon sinks are needed to face this global issue, and the latter must be dealt with in urban areas in particular.
Urban management that incorporates the reduction of carbon emissions along with securing urban forest in cities, thereby absorbing carbon emissions, would contribute to carbon offset policies for local and national government. In addition, if the total urban forest needed to absorb carbon emissions in cities could be quantified, a quantitative target for urban forest management could be suggested, thereby assisting related decision-making. Studies in Korea on such urban forests include, inter alia, Kim and Lee (1997), Choi et al. (2008), and Oh (2005). However, these studies only examine the total volume needed in terms of the area and quantity of urban forest.
One of the prominent examples of the management of urban forests as carbon sinks and the related policy decision-making is the US Forest Service (American Forests 2000). By calculating the carbon storage and sink capacity in urban areas by tree crown area per unit area according to tree age distribution, the resulting data can be converted into economic values. These can then be used in decision-making related to urban forest management. Whitford et al. (2001) proposed quantitative indicators of CO2 absorption by urban forest for urban ecosystem assessment. Studies by Jo (1999), Cho & Cho (1998), Choi (2007), Hwang et al. (2007) and Jo and Ahn (2001) proved to be useful in quantitatively assessing the CO2 absorption and storage capacity of urban forests in the central region of South Korea. These studies were the basis for calculating CO2 absorption and storage coefficients in the study conducted by the Ministry of Environment in South Korea (2001), but a cross-analysis with carbon emissions from the cities was not conducted. The existing analyses do not provide for cross-examination between the carbon emissions in the cities and the volume of urban forest needed to absorb them. The concept of total urban forest volume, as suggested in this paper, focuses on calculating the urban forest volume capable of absorbing carbon emissions from the urban areas, and hence it could complement existing studies by simultaneously analyzing and presenting the volume of urban forest and its function along with an index for carbon emissions in urban areas.
Similar studies were conducted that calculated urban CO2 emissions and the urban forest volume needed to absorb them (Jo & McPerson 1995; Lee 2008; Lee et al. 2009a, 2009b, 2009c; Changfu & Xiaoma 2012). These studies, however, conducted analyses in terms of the volume of urban forest carbon storage and not in terms of absorbing urban CO2 emissions. Lee (2003) Lee (2007) and Lee and Lee (2007) estimated the total urban forest volume required to absorb and store CO2 emissions within development project sites. These studies were mostly in line with the concept used in this paper, but urban or metropolitan areas as a whole were not set as the object of study.
This current study was conducted based on the concept of the urban carbon cycle with the aim of quantifying the environmental capacity of urban areas into an urban forest variable. The study focuses on estimating the total urban forest volume required to absorb urban CO2 emissions and presents the current urban forest and target urban forest volumes. The results will provide indicators that will assist decision-making in meeting quantitative targets for urban forest management in this era of “low carbon, green growth.”
Materials and methods
Concept of total urban forest volume considering the carbon cycle in an urban area
Because plants are the only organic bodies that can absorb CO2 emissions in urban areas, the total volume of urban forest needed to absorb all the urban CO2 emissions is considered the total urban forest volume required for such cities. Of all the greenhouse gases that induce global warming, CO2 accounts for more than 50% of such effects (Ciborowski 1989; Rodhe 1990). Urban forests are one of the main components of ecosystems that can naturally absorb and store CO2; hence, they will be used as criteria for estimating total urban forest. The concept of total urban forest volume considering the carbon cycle in an urban area would mean the total volume of urban forest needed to absorb the CO2 emissions in the urban area.
Calculating CO2 emissions at local government level
The IPCC (Intergovernmental Panel on Climate Change) has recommended ways to calculate CO2emissions at the local government level by a greenhouse gas estimation methodology (IPCC 1996). Under the assumption that carbon flow in or out of the city is disregarded, CO2 emissions were calculated using fossil fuel consumption statistics for the past 5 years (Equations (1) and (2)). The fossil fuel consumption data were taken from the annual statistics of the city of Chuncheon (Chuncheon 2008). For the carbon emission coefficients by fuels, the product absorption ratio and the average combustion efficiency by fossil fuels, coefficients recommended by the IPCC (1996), were used. For carbon emission coefficients by fossil fuels, the coefficients recommended by the Korea Energy Management Corporation (KEMCO) were used. The molecular ratio of carbon to CO2 is 44/12.
(1)
(2)where ECO2 indicates the amount of CO2 emissions EC indicates the amount of carbon emissions, F is the consumption by fossil fuels (ton/yr), Cf is the carbon emission coefficients by fossil fuels (ton/TOE = Ton of Oil Equivalent), S is the product absorption ratio and Ca is the average combustion efficiency by fossil fuels (KEMCO 2004; 2008).
(1)(2)where ECO2 indicates the amount of CO2 emissions EC indicates the amount of carbon emissions, F is the consumption by fossil fuels (ton/yr), Cf is the carbon emission coefficients by fossil fuels (ton/TOE = Ton of Oil Equivalent), S is the product absorption ratio and Ca is the average combustion efficiency by fossil fuels (KEMCO 2004; 2008).Calculating CO2 emissions from urban areas within local governments
Generally, the CO2 emissions of an area at the local government scale can be calculated using carbon emission coefficients by energy sources as suggested by the IPCC because domestic statistics are provided at the level of local administrative governments. CO2 emissions could be calculated using energy consumption statistics from KEMCO, but these data also have the local government level as the minimum area.
Because statistical data do not exist that will allow for calculating CO2 emissions in urban areas within local government administrative regions, per capita carbon emissions (ton/person) data were used instead, which have proved to be the only data available at this time for calculating urban CO2 emissions.
Therefore, the CO2 emissions in urban areas were computed by multiplying per capita CO2 emissions, as suggested by KEMCO, by the population of that urban area. KEMCO has developed carbon emission statistics based on energy consumption data from the industry, household and commerce, and transportation sectors (KEMCO 2008) (Table 1).
Table 1. Statistics used in calculating carbon emissions.
Calculating CO2 absorbable by urban forests within an urban area
The amount of CO2 that can be absorbed by vegetation can be estimated using the vegetation increase, obtained by subtracting last year's vegetation from the current year's and converting the increase into the CO2 amount. Another approach is to use an infrared gas analyzer to measure the real CO2 amount that is absorbed and emitted by vegetation. Jo and Ahn (2001) suggested CO2 absorption coefficients according to urban forest grade in urban areas by applying the results of the existing vegetation amount equation (Lim et al. 1981, 1982; Park 1985; Lee & Park 1987; Park & Lee 1987, 1990; Park & Moon 1994; Song & Lee 1996; Park et al. 1996; Jo 1999). Their CO2 absorption analysis was conducted over a period of 2 years (Jo & Ahn 2000, 2001) in urban areas as case studies (Ministry of Environment in South Korea 2001). The current study further improved CO2 absorption coefficients by urban forest types and grades as suggested in the Ministry of Environment Report (2001) from the studies of Jo (2001) and Jo and Ahn (2000, 2001). The current study also calculated the CO2 amount absorbable by urban forests in urban areas by multiplying the former by the vegetation area classified according to urban forest types and grades. Forest type map data, as updated by the Korea Forest Service in 2008, were used for area by urban forest types and grades. The CO2 absorption coefficients by urban forest types and grades as suggested by the Ministry of Environment (2001) were the result of applying the vegetation amount equation and yearly CO2 absorption indices in terms of a single tree, which were obtained by measuring the CO2 exchange rate to the target areas in the Central region. Regression was used to complement the coefficient values for Grades 1, 5, and 6 (Table 2).
Target sites for case studies
Taking into account the readability of the result of CO2 emissions and urban forest absorption analysis in local governments or urban areas within local governments, the city of Chuncheon in Gangwon Province was chosen for its mixture of urban area and urban forest (Figure 1). The city of Chuncheon has a surface area of 115.09 ha. The urban area was drawn in accordance with the Act on the Planning and Use of National Land. The urban area within the city of Chuncheon comprises 5303 ha, accounting for 4.61% of the total area.
Analysis
First, a comparative analysis was conducted between the CO2 emissions of the entire city of Chuncheon and the amount absorbable by the urban forests. The ability of Chuncheon's urban forests to absorb all the emissions was analyzed, and the total urban forest required for full absorption and the additional urban forest required to absorb the remainder were calculated as a result. Second, a comparative analysis was conducted between the CO2 emissions of the urban area within Chuncheon and the amount absorbable by the urban forest in that region. The ability of the urban forest to absorb all the emissions was analyzed, and the total urban forest required for full absorption and the additional urban forest required were calculated as a result. Third, from the above results, it was considered whether the total urban forest volume, taking into account the carbon cycle in an urban area, could serve as an urban forest management strategy for low carbon in the region.
Results
Total CO2 emissions in the city of Chuncheon as a whole
Using the IPCC's greenhouse gas estimation methodology, the total carbon emissions in the city of Chuncheon were calculated and were then translated into CO2 emissions, which resulted in an average of 2,549,638 tons of CO2 for the period between 2001 and 2005 (Table 3).
Table 2. CO2 absorption coefficients by urban forest types and grades (ton/(ha·yr)).
Table 3. CO2 emissions of the city of Chuncheon (ton/yr).
CO2 emissions in the urban area within Chuncheon
Korea's per capita CO2 emissions in 2007 were approximately 10.09 tons (KEMCO 2008), and the population of Chuncheon's urban area reached 219,186 (the City of Chuncheon 2008). Hence, the yearly CO2 emissions in the urban city of Chuncheon amounted to approximately 2,211,587 tons.
However, the total CO2 emissions of the whole city of Chuncheon from the above Equations (1) and (2) multiplied by the urban area population ratio gives 2,151,735 tons of CO2 emissions in the urban area. Note that the error is relatively small compared with the calculation result using per capita emissions. Some errors exist between the two emissions because one is based on 2007 while the other uses an average figure over the 5-year period from 2001 to 2005.
CO2 absorption by total urban forest in the city of Chuncheon as a whole
The urban forest in the city of Chuncheon has an area of 89,278 ha, accounting for approximately 78% of the total 115,098 ha. Grade 1 covers 1867 ha, Grade 2 covers 24,600 ha, Grade 3 covers 35,469 ha, Grade 4 covers 22,521 ha, Grade 5 covers 3613 ha, and Grade 6 covers 1208 ha. Of the whole urban forest, coniferous forest covers 33,486 ha, broad-leaved forest 35,336 ha, and mixed forest 20,456 ha, accounting for 38%, 40%, and 22% of the total urban forest, respectively (Table 4).
Table 4. CO2 absorption by urban forest in the city of Chuncheon by urban forest types and grades.
The CO2 absorption by the total urban forest in the city of Chuncheon is estimated as 2,246,188 tons of which 767,309 tons is absorbed by coniferous forest, 940,132 tons by broad-leaved forest, and 538,755 tons by mixed forest, accounting for 34%, 42%, and 24%, respectively, of the total absorption. In terms of grades, Grade 1 absorbs 36,841 tons, Grade 2 522,606 tons, Grade 3 912,525 tons, Grade 4 619,157 tons, Grade 5 112,133 tons, and Grade 6 42,925 tons; the three grades account for 41% of the total absorption (Table 4, Figure 2).
Figure 1. Target site: the city of Chuncheon and the urban area within the city of Chuncheon.(dark shaded: urban area; source: the city of Chuncheon census (Chuncheon 2008 census).
Figure 2. Distribution map of urban forest CO2 absorption in the city of Chuncheon ( 
<10,000 tons, 
∼20,000 tons, 
∼30,000 tons, 
∼40,000 tons, 
>40,000 tons).
<10,000 tons, ∼20,000 tons, ∼30,000 tons, ∼40,000 tons, >40,000 tons).
CO2 absorption by urban forest in the urban area within the city of Chuncheon
The urban forest in the urban area within the city of Chuncheon has an area of 1292 ha, accounting for approximately 1% of the total urban forest area of 89,278 ha. Of this total, coniferous forest covers 940 ha, broad-leaved forest 135 ha, and mixed forest 217 ha, accounting for 73%, 10%, and 17% of the total urban forest in the urban area, respectively. Note that coniferous forest has the highest proportion in the urban area (Table 4, Figure 3).
Figure 3. Distribution map of urban forest CO2 absorption in the urban area within the city of Chuncheon ( <250 tons, ∼500 tons, ∼750 tons, ∼1000 tons, > 1000 tons).
The CO2 absorption by the urban forest is estimated to be 30,455 tons, of which 21,368 tons (71%) are absorbed by coniferous forest. Broad-leaved forest absorbs 5362 tons and mixed forest absorbs 3726 tons of CO2. Grades 2 and 3 cover 986 ha (76%) of the total urban forest, and they absorb 22,508 tons (74%) of the total CO2 absorption (Table 4, Figure 3).
Total urban forest volume considering the carbon cycle
The total CO2 emissions in the city of Chuncheon as a whole were calculated as 2,549,638 tons and the total urban forest volume to absorb them by coniferous, broad-leaved, and mixed forests was 35,684 ha, 32,623 ha, and 33,698 ha, respectively, when an equal proportion of the three types was assumed. Considered in terms of each grade absorbing CO2 emissions equally by urban forest types, Grades 1 and 2 in coniferous forest require respectively 7118 ha and 6619 ha, whereas Grade 3 in broad-leaved forest requires 5151 ha. The required areas for the other urban forest types and grades are indicated in Table 5.
Table 5. Total and required urban forest volume considering the carbon cycle in an urban area within the city of Chuncheon.
Urban area within the city of Chuncheon
The total CO2 emissions in the urban area within the city of Chuncheon were calculated as 2,211,587 tons and total urban forest volume to absorb them by coniferous, broad-leaved, and mixed forests was 30,952 ha, 28,298 ha, and 29,231 ha, respectively, when an equal proportion of the three types was assumed. Considered in terms of each grade absorbing CO2 emissions equally by urban forest types, Grades 1 and 2 in coniferous forest require 6174 ha and 5741 ha, respectively, whereas Grade 3 in broad-leaved forest requires 4468 ha. The required areas for the other urban forest types and grades are indicated in Table 5.
Discussion
The city of Chuncheon as a whole can absorb 2,246,188 tons of CO2 as indicated in Table 4; hence, an additional 303,450 tons need to be absorbed. It follows, therefore, that −6267 ha of Grade 3 coniferous forest, −2455 ha and −10,110 ha of Grades 2 and 3 broad-leaved forest, respectively, and −3398 ha of Grade 4 mixed forest are required in terms of current urban forest types and grades. This means that, if secured, the total urban forest would be able to absorb all the CO2 emissions in the area.
The urban area within the city of Chuncheon can absorb 30,455 tons of CO2 as indicated in Table 4; hence, an additional 2,181,131 tons need to be absorbed. It follows, therefore, that 4991 ha of Grade 3 coniferous forest, 5808 ha and 4408 ha of Grades 2 and 3 broad-leaved forest, respectively, and 4469 ha of Grade 4 mixed forest are required in terms of current urban forest types and grades. This means that, if secured, the total urban forest would be able to absorb all the CO2 emissions in the area.
The city of Chuncheon as a whole needs an additional 12,724 ha of urban forest, of which 2197 ha are coniferous, −2714 ha are broad-leaved, and 13,241 ha are mixed when equal CO2 absorption among urban forest types and grades is assumed. This means that an additional 11.05% of urban forest in terms of the total Chuncheon area (115,098 ha) needs to be secured. Considering that it would be difficult to acquire new urban forests with a grade greater than Grade 2, as Grade 2 urban forest ages are between 11 and 20 years, more urban forest would be required than the above calculation shows. Realistically, then, it would be extremely difficult to secure enough urban forest volume that all the current CO2emissions in the city of Chuncheon could be absorbed.
The urban area within the city of Chuncheon requires an additional 87,189 ha of urban forest, of which 30,012 ha are coniferous, 28,164 ha are broad-leaved, and 29,013 ha are mixed. Considering that the total urban area is 5303 ha, it is also practically difficult to secure the required urban forest. The result implies that not only securing CO2 sinks but also CO2 mitigation efforts should be given more weight.
Gwangwon Province has set as its greenhouse gas mitigation target a reduction of 6% from 2003 levels by 2012 (Green Korea 2008). The city of Chuncheon emitted 2,499,215 tons of CO2 in 2003 using the IPCC methodology, meaning that it should present a reduction of 6%, or 149,953 tons, by 2012. If the 6% reduction is to be attained by the CO2 absorption of urban forest alone, approximately 7007 ha of coniferous forest, 7107 ha of broad-leaved forest, or 7141 ha of mixed Grade 2 forest (ages between 11 and 20 years) would be required to absorb 149,953 tons. If the burden is shared by 5% carbon source mitigation and 1% urban forest absorption, an urban forest sufficient for the absorption of 24,992 tons of CO2 would be required. When equal absorption of 24,992 tons is assumed among the urban forest types and grades, the urban forest target could be set as 350 ha, 330 ha, and 320 ha for coniferous, broad-leaved, and mixed forest, respectively (Table 6).
Table 6. Required urban forest for the city of Chuncheon under the greenhouse gas mitigation target.
Conclusions
This current study was conducted with a particular focus on calculating the urban forest required to absorb CO2 emissions in an urban area, to determine the total urban forest volume considering the carbon cycle in the urban area. By establishing a methodology for estimating the total urban forest volume required to absorb CO2 emissions in a local government area as a whole and its urban area and the additional urban forest that needs to be secured, the results of the study can be summarized as follows.
First, the CO2 emissions of the city of Chuncheon amount to 2,549,638 tons of which 2,246,188 tons can currently be absorbed. The additional urban forest would need to absorb 303,450 tons of CO2, and hence 12,724 ha of urban forest would have to be secured.
Second, for the urban area within the city of Chuncheon, it was estimated that 87,189 ha of urban forest is required, 30,455 tons is currently absorbable, and 5301 ha of additional Grade 2 urban forest would absorb 2,181,131 tons of CO2.
Third, when the 6% greenhouse gas mitigation target of Gangwon Province is applied, a quantitative urban forest target can be established that requires 7077 ha of Grade 2 coniferous forest, 7107 ha of Grade 2 broad-leaved forest, or 7141 ha of Grade 2 mixed forest to be secured.
This current study suggests that it is feasible to present the total required urban forest volume by quantifying the cyclic relationship between CO2 emissions in an urban area and the urban forest that can absorb them. These results can be fully applied as an indicator to establish a target for the total required urban forest volume for urban management while taking into account the economy, society, and the environment. Usually, however, the urban forest absorption would account for a small proportion against CO2 emissions; hence, it proved to be extremely difficult to secure the required urban forest volume suggested in theory. This situation implies that where CO2 mitigation in the atmosphere is concerned, securing urban forest alone will not suffice and that mitigation efforts regarding carbon source and CO2emissions must precede or accompany urban forest procurement to achieve the target. Nevertheless, if the target of mitigation by urban forest were to be set in proportion (e.g., mitigation by) and not in CO2emissions, it could be a useful indicator.
This current study has some limitations and further research is needed, as suggested below. Improvements in the following would contribute to converting the methodology set out in this paper into an indicator to be applied in practical urban management.
First, scientific data are needed for the methodology to calculate CO2 emissions in urban areas. The data for calculating CO2 emissions only exist at the national and regional levels; thus, it was impossible to obtain data for the local urban level. If data for calculating CO2 emissions in a local urban area are prepared, a more precise estimation of CO2 emissions would be possible.
Second, the CO2 absorption coefficients used in this current study were only for the central region; hence, national application would not be possible. Therefore, subsequent basic studies would need to be conducted to calculate absorption coefficients for areas such as the southern region.
Third, part of the CO2 in the atmosphere enters the soil through the plants’ photosynthesis. When this occurs, CO2 that is not degraded by microbes is stored in the soil in the form of organic carbon, which means that the soil's CO2 absorption capacity would need to be considered to calculate the CO2absorption by urban forest. Therefore, further studies are needed because the total urban forest volume could have been undervalued.
Fourth, this current study cannot represent the whole country because only the city of Chuncheon was examined. Further case studies on other cities could be conducted for general application.
For further details log on website :
https://www.tandfonline.com/doi/full/10.1080/21580103.2013.846876
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