Assessment on forest carbon sequestration in the Three-North Shelterbelt Program region, China

Journal of Cleaner Production 215 (2019) 382e389

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Assessment on forest carbon sequestration in the Three-North Shelterbelt Program region, China Xi Chu a, Jinyan Zhan a, *, Zhihui Li b, c, d, Fan Zhang b, c, d, Wei Qi e a

State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China c Center for Chinese Agricultural Policy, Chinese Academy of Sciences, Beijing 100101, China d University of Chinese Academy of Sciences, Beijing 100049, China e Institute of Polar Meteorology, Chinese Academy of Meteorological Sciences, Beijing 100081, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 March 2018 Received in revised form 17 December 2018 Accepted 29 December 2018 Available online 4 January 2019

Forest ecosystems are a major component of the terrestrial ecosystems as they provide a variety of important ecosystem services, especially climate regulation via carbon sequestration. The Three-North Shelterbelt Program (TNSP), a pioneer for China's large-scale ecological construction, has promoted massive afforestation and forest dynamics since 1978. Quantitative analysis of carbon sequestration and its economic value of sub-type forest landscapes in the TNSP region is crucial for better understanding the capacity of forest carbon sequestration and providing reasonable forest management. Therefore, we assess the forest carbon sequestration in the TNSP region using the InVEST model based on spatial datasets during 1990e2015. Our results showed that forests in the TNSP region had strong carbon sequestration capacity. Total carbon sequestration fluctuated and overall showed a decreasing trend, with a reduction rate of 1.92% during the period of 1990e2015. The carbon sequestration of each carbon pool (namely aboveground biomass, belowground biomass, soil, and dead organic matter) also decreased slightly and the changes mainly happened in the north of northeastern China and along the southeast of central north China. Additionally, in monetary terms, the economic value of carbon sequestration reflected that the TNSP was worth implementing, with a small amount of investment in exchange for large carbon sequestration benefits. This work provides an up-to-date attempt to calculate carbon sequestration of forest more accurately, quantify economic values of carbon sequestration for forest ecosystems, which will give a baseline reference for related studies in the TNSP region, as well as other similar reforestation area. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Forest carbon sequestration Carbon pools Economic value InVEST model Three-North Shelterbelt Program region

1. Introduction Rising carbon emissions caused by human activities have caused great climate changes, characterized by global warming (Rawlins et al., 2011). It has result in severe challenges to sustainable development of human society and natural ecosystems (Liu and Deng, 2011; Deng and Bai, 2014). China and other emerging counties are playing expanding roles to solve global and local environmental challenges and China itself is responsible for 38% fewer emissions in 2007 (Wang et al., 2013; Liu et al., 2013). Studies

* Corresponding author.. E-mail addresses: [email protected] (X. Chu), [email protected] (J. Zhan), [email protected] (Z. Li), [email protected] (F. Zhang), [email protected] (W. Qi). https://doi.org/10.1016/j.jclepro.2018.12.296 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

have shown that approximately 12%e20% of global human-caused greenhouse gases were caused by deforestation and forest degradation (Schrope, 2009; Van der Werf et al., 2009). As the major part of the terrestrial biosphere, forest ecosystems not only play a key role in energy balance and water circulations, but also play a crucial role in regulating climate-carbon cycle and climate warming mitigation (Pan et al., 2011; Wang et al., 2014). It is measured that approximately 50% of the organic carbon in terrestrial biosphere can be stored by forest. Meanwhile, they can exchange over 90% of terrestrial carbon with the atmosphere (IPCC, 2013; Wei et al., 2014). Forest ecosystems are an important carbon sink and carbon storage is a crucial service of forest ecosystems. In recent years, numerous studies about the capacity of forest ecosystems for carbon storage and to act as a carbon sink have been carried out on different scales such as landscape (Kline et al., 2016),

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regional (Ma et al., 2015; Sullivan et al., 2017), country (Seidl et al., 2014) and global scale (Bradshaw and Warkentin, 2015). These studies have shown that forest ecosystems made substantial contributions to the regional and global carbon balance. Saatchi et al. (2011) presented a “benchmark” map of above- and belowground biomass carbon stocks of tropical forests across Latin America, subSaharan Africa, and Southeast Asia. There was no obvious latitudinal pattern of total forest carbon sequestration (vegetation þ soil), while the distribution ratio of forest carbon sequestration between vegetation and soil had a negative logarithmic relationship with latitude. In addition, carbon stored by vegetation decreased with the increase of latitude, while carbon stored by soil was opposite (Wen and He, 2016). In the Qilian Mountains, researches showed that the remaining mountain forests of spruce and other conifers at the northern fringe of the Tibetan Plateau played an important role in the regional carbon budget and needed urgent conservation (Wagner et al., 2015). With the implementation of a series of forestry ecological construction projects in China, the forest area and forest accumulation volume continue to increase. It is necessary to re-calculate the carbon reserves in each region to reflect the status of carbon storage in the forest ecosystem. However, while historically most studies have focused on a single carbon pool such as biomass carbon stocks or soil carbon stocks at many different scales (Qi et al., 2016; Fang et al., 2018), few studies have thoroughly assessed the various carbon pool sizes and the economic value of the forests covered by major ecological construction programs (Wei et al., 2014). The task of monitoring carbon storage is time-consuming and costly. Accurate estimation of forest carbon sequestration is still a challenge as forests grows with complex structure and plants in unique geographical situation that give much difference to the capacity of carbon sequestration in different areas (Omasa et al., 2003). A wide variety of methods is available for assessing forest carbon storage. Each one has its own strengths and weaknesses in estimating carbon sequestration at different scales. One type of method is based on estimation of the forest biomass to calculate carbon storage (Fang et al., 2001; Houghton, 2005). This method is suitable for measurements at large scale, but it requires comprehensive forest inventories that take time to organize and summarize the data (Zhang et al., 2009). Other methods using micrometeorological approaches (Miller et al., 2011; Wolf et al., € rner et al., 2005; Verbeeck 2013) and some other techniques (Ko et al., 2006) to measure the forest carbon flux have advantages in acquiring accurate carbon storage data, but the high cost means they are usually only suitable for monitoring carbon stocks at a small scale. Based on integrated use of satellite imaging, RS (Remote Sensing) and GIS (Geographic Information System), many problems can be solved (Asner et al., 2010). The use of satellite imaging is conducive to data visualization of forest carbon storage (Deng et al., 2011; Ren et al., 2012). The InVEST model (Integrated Valuation of Ecosystem Services and Tradeoffs model) is a spatially explicit integrated modeling tool that can be linked to GIS and has many advantages for assessing carbon stocks (Pareta and Pareta, 2011; Delphin et al., 2013). It is widely used because of its data accessibility, ease of visualization of results and flexibility of study scale. Previous carbon sequestration studies based on the InVEST model focused on terrestrial ecosystems in response to land use/cover change (LUCC), which were mainly divided into six types, namely cultivated land, forest area, grassland, water body, built-up area and unused land. Studies about carbon sequestration based on sub-type LUCC remain poorly understood (Polasky et al., 2011; Zhang et al., 2017). In general, the capacity of carbon sequestration for each ecosystem depends largely on the size of four carbon pools, namely aboveground biomass, belowground biomass, soil, and dead organic matter (Sharp et al., 2016). Many researches concentrated

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on just one or two carbon pools (biomass carbon and soil carbon). The research on carbon stored by dead organic matter were largely ignored to some extent (Rawlins et al., 2011; Ma et al., 2015). The InVEST model aggregates the amount of the four carbon pools and gives comprehensive calculation for carbon sequestration. The Three-North Shelterbelt Program (TNSP), a pioneer for China's large-scale ecological construction, is the World's Best Ecological Project and is highly named as the “Green Great Wall” underway in China since 1978. Designed to protect forest resources and realize sustainable forests development, the TNSP altered much of the distribution and amount of forests. Meanwhile, ecosystem services, especially carbon sequestration, which are of great significance to human wellbeing, are severely affected by LUCC (Deng et al., 2016a; Wang et al., 2018). Because of the important role of forests in providing various ecosystem services, there is a need to assess changes in forest ecosystem services, especially carbon sequestration, in the TNSP region. The forest in the TNSP region plays a vital role in sustaining the carbon balance of terrestrial biosphere in China. Many scholars have carried out research in the TNSP region on dynamic changes in afforestation patterns (Wang et al., 2014; Zhang et al., 2016), forestry production efficiency and its driving factors (Xiong et al., 2018), ecosystem services (Jiang et al., 2016; Zhan et al., 2018), carbon storage with one or two carbon pools (Liu et al., 2014; Wei et al., 2014; Fang et al., 2018), and so on. However, to date few researches focused on estimation of various carbon pool sizes and the economic value of different forest types in the TNSP region (Czerepowicz et al., 2012). Addressing this gap in knowledge, the current study aims to quantify carbon sequestration based on four carbon pools and the economic values of sub-type forest during the period of 1990e2015 in the TNSP region. In addition, the InVEST model was chosen to complete this process because it is freely available and suitable for a wide range of environmental assets. China has a cumulative total investment in the TNSP exceeding 7.857 billion CNY. The average subsidy for planting forests was increased by 35.33 times from 53.1 yuan/ha during 1978e1985 to 1875.8 yuan/ha during 2000e2007 for the TNSP region (China Forestry Administration, 2008). Huge fiscal expenditure has been invested in the TNSP region and it has given substantial economic value to the environment. Therefore, another objective of this study was to evaluate the costs and economic value of forest carbon sequestration by comparing the economic values with the fiscal expenditure of the TNSP. This study will give a baseline reference for calculating carbon stored by subtype forest landscape and their economic values on a main afforestation area, and conduce to formulation and implementation of reasonable forest management policies. 2. Study area An ambitious policy to maintain the pace of environmental protection is considered to be the top priority in China (Wang et al., 2011). The TNSP, designed to protect forest resources and realize sustainable forests development, would transform natural and terrestrial ecosystems at the largest scale and over the longest time span in mankind's history. It has been implemented through the northwestern, central north and northeastern parts of China, accounting for approximately 40.2% of China's landmass; about half of these zones are barren or sparsely vegetated (Zhang et al., 2016). TNSP began in 1978 and was planned to be finished in 2050, lasting 73 years and being completed in three stages (1978e2000, 2001e2020, and 2021e2050) following eight engineering schedules. The program was initially designed to be constituted of 551 counties in 13 provinces (Beijing, Tianjin, Hebei, Shanxi, Inner Mongolia, Liaoning, Jilin, Heilongjiang, Shaanxi, Gansu, Qinghai, Ningxia and Xinjiang), ranging from 73 260 E to 127 500 E, 33 300 N

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to 50120 N. However, as it has now developed to the end of the second stage (2001e2020), the program has already covered 725 counties, nearly covering the whole 13 provinces. Therefore, the TNSP region in this study is the whole of the 13 provinces mentioned above (Fig. 1). Approximately 85% of the land surface is classified as deserttype land in the TNSP region, consisting of arid and semiarid lands of more than 1.6 million km2 (Wang et al., 2010). The growth of vegetation here are greatly influenced by weather condition including sunlight intensity, temperature, evapotranspiration and precipitation, etc. In TNSP region, where covers famous fragile ecosystems (the loess plateau and Qinghai-Tibetan Plateau) is extremely sensitive to these effects (Deng et al., 2107). The average annual temperature ranges between 2  C and 8  C and varies significantly across the region. Additionally, southwestern TNSP region (mainly Qinghai Province) is the center of frigid highlands with the lowest temperature in China (Deng et al., 2016b). Annual precipitation increases from the northwest to the southeast, with an average annual precipitation less than 400 mm (Zhang et al., 2016). The altitude ranges between 100 m and 5000 m, showing an increasing trend from east to west. According to multiple characteristics including regional differentiation, climate conditions, fragile ecological environment, soil properties, and regional economic levels, the area can be divided into four sub-regions, namely, western Northeast China, Inner MongoliaeXinjiang sub-regions, the Loess Plateau, and northern North China (China Forestry Administration, Bureau of Three North Shelterbelt Development Program, 1987). Nearly 67% of the area in the TNSP region faces

with severe ecological disasters, such as water and soil erosion, catastrophic flooding, and loss of biodiversity caused by the desertification process (Deng et al., 2010; Liu et al., 2014). A unique forest shelterbelt system was to be implemented in each subregion, with the aim of improving local forest coverage from 5% to 15% to regulate climate as well as to combat desertification and to control dust storms. Additionally, the four kinds of shelterbelt systems of afforestation are mainly farmland shelterbelt, comprehensive shelterbelt, eco-economic shelterbelt coordinated agriculture, forestry and animal husbandry, and shelterbelt for wind prevention and sand fixation and water conservation. In addition, because of poor soil and climate conditions, most of the plants in the TNSP region are shrub herbaceous plants in dry regions, which results in a high mortality rate. According to some reports, there was only 15% of the planted trees survived (Cao, 2008). However, increasing investments and applicable policies may help improve the efficacy of the TNSP (Wang et al., 2010). 3. Materials and methods 3.1. Carbon sequestration modeling: use of the InVEST model The InVEST (Integrated Valuation of Environmental Services and Tradeoffs) model is a suite of spatially explicit integrated models, which can be used to value and map multitudinous ecosystem services. The model was developed and is continuously updated by the Natural Capital Project team (Tao et al., 2015). The toolkit is a large-scale scenario model, which is currently used on a wide range of terrestrial, freshwater, and marine ecosystems, including over twenty sub-models (Bottalico et al., 2016). For modeling carbon stocks of forest ecosystems, the InVEST Carbon Storage and Sequestration model (version 3.4.2) was used in this paper, which aggregates the amount of carbon sequestration in four carbon pools based on forests landscape maps over time (Garrastazú et al., 2015). The size of these four carbon pools, aboveground biomass (Cabove), belowground biomass (Cbelow), soil (Csoil), and dead organic matter (Cdead), largely determines carbon sequestration on a given land parcel. Using classification maps about different forest landscapes and the carbon density of each carbon pool (biophysical table), the amount of carbon sequestration in different forest landscapes over time can be estimated. Additionally, with a social value of carbon sequestration, the model can also evaluate the economic values of the carbon sequestration (Sharp et al., 2016). The specific data used in this study and its sources are as follows. 3.1.1. Forest land types The forest data with 1 km  1 km grids used in this study were based on Landsat TM/ETM remote sensing images taken in 1990, 1995, 2000, 2005, 2010 and 2015, provided by the Data Center for Resources and Environment Sciences, Chinese Academy of Sciences (http://www.resdc.cn). The forest land was classified into four categories, including mixed woodland, closed shrublands, open shrublands and woody areas (such as nursery gardens and various kinds of orchard land) (Zhan et al., 2013).

Fig. 1. Location of study area (a and b) and distribution of forest cover in Three-North Shelterbelt Program (TNSP) region in 1990 (c) and 2015 (d).

3.1.2. Carbon density Values of carbon density for the four carbon pools (Cabove, Cbelow, Csoil and Cdead) are needed for every forest land type. Because of the lack of data, the carbon density of closed shrublands and open shrublands were acquired from default values in the InVEST model. Aboveground biomass carbon density data of mixed woodland and soil carbon density data of woody areas were derived from Liu et al. (2014) and Wang et al. (2016), respectively. The other three carbon density data of mixed woodland and woody areas came from the

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3.1.3. Market value The market value of carbon is optional according to the Guide of InVEST model (Sharp et al., 2016). The value of forest carbon sequestration is determined by the net present value (NPV) of the perpetuity annuity or a constant cash flow over time until infinity (Mohammadi et al., 2017). The net present value of annual carbon sequestration was calculated as in Eq. (1).

the first stage of the TNSP was under construction, forestry investment completed in CNC was much higher than that in NEC and NWC. However, with the implementation of the second stage (2001e2020) of the TNSP, CNC's forestry investment completed dropped rapidly and stayed at a low value until 2007. In NWC, the forestry investment completed increased slowly until 2007. The forestry investment completed in these three sub-regions all increased rapidly during the period of 2008e2015. The year of 2008 was an important year in China, with the 29th summer Olympics held in Beijing. This might explain the high value of forestry investment completed at this time to protect the environment and regulate climate.

. NPVc ¼ Ca  Pc ð1 þ iÞt

4. Results and analysis

InVEST model. Additionally, the values of carbon density of carbon pools, which can be used to stand for carbon sequestration of the forest landscape in every grid, were regarded as fixed values during the study period, ignoring it's time variation.

(1)

where NPVc reflects the net present value of forest carbon sequestration (CNY), Ca is the annual stored carbon (Mg), Pc is the carbon price (CNY/Mg), i is the real discount rate and t is the time period. In our study, the carbon price was 5.2 USD/Mg (Goldstein et al., 2014), which was equal to 32.39 CNY/Mg in 2015 according  of to the exchange rate published on the Statistical Communique the PR of China on the 2015 National Economic and Social Development. Meanwhile, the economic values of forest carbon sequestration from 1990 to 2015 were modified by the consumer price index (CPI) and all the values were represented by the price of year 1990. The real discount rate in the capital market was assumed to be equal to 8% (Manley and Maclaren, 2012). 3.2. Socio-economic data To calculate the cost and economic value of the carbon sequestration service provided by forest ecosystems in the TNSP region, we collected forestry investment completed data from the China Forestry Yearbook (1990e2015). The forestry investment completed during the period of 1990e2015 were modified by CPI and represented at the price of year 1990 (Fig. 2). The total forestry investment completed was approximately 16.96 billion CNY. The average annual growth rate of forestry investment completed in central north China (CNC), northwestern China (NWC) and northeastern China (NEC) were nearly the same, with an annual growth rate of 9.46%, 11.70% and 10.95%, respectively. Before 2000, when

4.1. Estimated carbon sequestration among different forest covers Carbon sequestration was estimated in the study forest landscape for the years 1990, 1995, 2000, 2005, 2010, and 2015 (Table 2 and Fig. 3). The amount of carbon sequestration differed among forest cover of the entire study landscape with different reference years. The forest in the TNSP region have strong carbon sequestration capacity. At the beginning of the study period (1990), total carbon sequestration was 697.40 Pg C, with over two thirds (495.88 Pg C, 71.10%) of this value comprising mixed woodland. The contribution of closed shrublands, open shrublands and woody areas were on the decline, accounting for approximately 112.30 Pg C (16.10%), 66.56 Pg C (9.53%), and 22.76 Pg C (3.26%), respectively, of total carbon sequestration in the study forest landscape. In 2015, mixed woodland also accounted for the largest part, contributing 483.38 Pg C (70.67%), with closed shrublands, open shrublands and woody areas accounting for approximately 116.34 Pg C (17.01%), 66.44 Pg C (9.71%), and 17.86 Pg C (2.61%), respectively, of total carbon sequestration in the study forest landscape. During the period of 1990e2015, carbon sequestration notably differed among different forest landscapes. The total carbon sequestration fluctuated and ultimately showed a decreasing trend, with a reduction rate of 1.92%. Total carbon sequestration of closed shrublands increased by 3.60%, while woody areas decreased by 21.52%, which contributed substantially to the decrease in total carbon sequestration in the TNSP region. This was mainly because

Million CNY 800 Central North China (CNC) 600

Northeastern China (NEC) Northwestern China (NWC)

400

200

0

1991 1993 1995 1997 1999 2001 2003 2005 2007 2009 2011 2013 2015 Fig. 2. Forestry investment completed in CNC, NEC and NWC during the period of 1990e2015.

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Fig. 3. Spatial distribution of carbon sequestration in TNSP region from 1990 to 2015.

Fig. 4. Estimated carbon sequestration of different carbon pools in TNSP region during the period of 1990e2015.

Table 1 Biomass and soil carbon density (Mg/ha) in different forest landscape. LULC_Name

Cabove

Cbelow

Csoil

Cdead

Mixed Woodland Closed Shrublands Open Shrublands Woody Area

9.61 10.51 4.51 20.12

14.1 6.7 7.3 23.6

60 60.1 65.5 95.68

12.1 1.3 1.1 2.6

of the conversion from forest landscape to other landscape such as built-up land or cultivated land by human activities. Another reason many be that the newly planted trees died due to pest and disease problems caused by simplification of afforestation tree species or drought problem in the TNSP region, resulting in a decreasing trend of forest landscape. The area of mixed woodland, open shrublands and woody area all decreased except for closed shrublands during the study period. According to Table 1, the carbon sequestration ability of mixed woodland and woody area are much stronger than that of closed shrublands. Thus, total carbon sequestration decreased during the study period was mainly due to the reduction of mixed woodland and woody areas. 4.2. Changes in carbon sequestration of different carbon pools In summary, two thirds of carbon was stored in soil (approximately 67.00%), followed by belowground biomass (approximately 13.27%), aboveground biomass (approximately 10.30%), and dead organic matter (9.44%) in 1990. All carbon sequestration of the four carbon pools evaluated decreased slightly during the period of 1990e2015 (Fig. 4). Carbon sequestration of belowground biomass decreased by 2.31 from 92.52 Pg C in 1990 to 90.21 Pg C in 2015, with the highest rate of decrease of 2.50%. The lowest rate of decrease (1.72%) in carbon sequestration was in soil, which

Fig. 5. Distribution changes in carbon sequestration of different carbon pools in TNSP region during the period of 2000e2015.

decreased by 8.05 Pg C from 467.21 Pg C in 1990 to 459.20 Pg C in 2015. The carbon sequestration of aboveground biomass and dead organic matter decreased by 1.98% and 2.43%, respectively, during the period of 1990e2015. Changes in carbon sequestration of different carbon pools during the period of 1990e2015 mainly occurred in the north NEC and along the southeast CNC (Fig. 5). In the northernmost part of NEC, carbon sequestration of aboveground biomass, belowground biomass and soil declined, while that of dead organic matter increased. Meanwhile, carbon sequestration of the four carbon pools changed rapidly with no clear trend in CNC.

4.3. Economic value of carbon sequestration Due to the availability of land use data, we calculated carbon sequestrations and their economic values just for year of 1990,

Table 2 Total carbon sequestration calculated for each forest landscape in TNSP region during the period of 1990e2015. Forest landscape

1990

1995

2000

2005

2010

2015

1990e2015 Change

Mixed woodland Closed shrublands Open shrublands Woody area Total (Pg C)

495.88 112.30 66.46 22.76 697.40

473.09 119.68 77.55 17.12 687.44

485.39 116.86 66.34 13.82 682.41

484.00 117.14 66.93 17.23 685.30

483.55 117.12 66.68 17.91 685.26

483.38 116.34 66.44 17.86 684.02

12.50 (2.52%) 4.04 (3.60%) 0.01 (0.02%) 4.90 (21.52%) 13.37 (1.92%)

X. Chu et al. / Journal of Cleaner Production 215 (2019) 382e389

1995, 2000, 2005, 2010 and 2015. In order to make the economic value of carbon sequestration and the total investment (costs) comparable, we considered the costs (forest investment completed) in the corresponding year. In monetary terms, we estimated economic value of carbon stored by each forest landscape in 1990, 1995, 2000, 2005, 2010 and 2015 in the TNSP region (Table 3). The economic value of carbon sequestration continuously increased during the period of 1990e2015. The economic value of carbon stored by mixed woodland contributed over two thirds of the total value. As for the cost of carbon sequestration, it refers to the forestry investment completed financed by the Chinese government in the TNSP region. During the period of 1990e2015, economic value of carbon sequestration increased with an average annual growth rate of 3.5%. Meanwhile, the investment increased with an average annual growth rate of 10.30% (Table 4). During the period of 1990e1995, economic value of carbon sequestration decreased a lot with an average annual growth rate of 4.61%. While it showed an average annual growth rate of 5.93% during the period of 1995e2000. The rapid increase of economic value may largely due to the huge amount of investment which has an average annual growth rate of 25.35 during the period of 1995e2000. Although the investment decreased during the period of 2000e2005, the government investment in the last five years was still work. They both contributed to the increase of economic value which has an average annual growth rate of 6.66%. During the period 2005e2015, with the continual stable growth of forestry investment, the economic value of carbon sequestration has also risen steadily. In summary, the forestry investment was helpful for the growth of economic value of carbon sequestration. Additionally, the past four decades of TNSP can be considered to have realized tremendous recovery of ecoenvironment in northern China, which was confirmed by the economic value of forest carbon sequestration. From a certain point of view, the forests in the TNSP play a crucial role in carbon sequestration and provide substantial benefits in terms of climate regulation. 5. Conclusions and discussion In this study, a detailed research of carbon sequestration changes in the forest landscape and the economic values of the corresponding landscapes were undertaken in the TNSP region. The research was carried out with the InVEST model. We found that total carbon sequestration in the TNSP region fluctuated and ultimately showed a decreasing trend, with a reduction rate of 1.92% during the period of 1990e2015. Despite the large-scale afforestation campaign carried out in the TNSP region, some mixed

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woodland and woody area reduced due to the dry weather and the pests and diseases problems caused by single tree species planted together. This kind of reduction caused the decrease of carbon sequestration in the mixed woodland and woody areas, which were an important component of regional carbon dynamics. Carbon sequestration of the four carbon pools evaluated decreased slightly and changes mainly happened in the north NEC and along the southeast CNC. Additionally, in monetary terms, the forestry investment was helpful for the growth of economic value of carbon sequestration. To some extent, it reflects that the TNSP was worth implementing, with a small amount of investment in exchange for substantial carbon sequestration benefits. Our empirical results provide with potential policies to improve carbon sequestration of afforestation campaign in the TNSP region and other arid and semi-arid regions. Geographical and climatic conditions differ across the TNSP region, and the water scarcity is the main limiting factor for plant growth in the region. When the plants grow, woodlands use much more water than shrublands, and they are easier affected by the pests and diseases problem. Thus, in the sandy areas, arid areas and areas where water resources are scarce, it is an effective measure to increase the survival rate of afforestation by moderately reducing the proportion of tree species and increasing the proportion of shrublands. The shrublands can also play an important role in carbon sequestration and improving local environment. This work is not inclined to offer accurate absolute values, but it is significant for providing an up-to-date attempt to study carbon sequestration in various kinds of forest landscape specifically on a major afforestation region. It offers an analysis framework of carbon sequestration that can reflect the capacity and potential of different forest in global warming mitigation. In addition, the process of assessing economic value of carbon sequestration in TNSP region can be a reference for other related researches. Determining the values of uncommercial forest products can give better understanding of forest management decisions and help set future policies. However, the current study involved several limitations that should be considered. The accuracy of forest landscape data used in this paper was a 1 km  1 km grid, and the results might be more accurate with higher resolution forest data. And the land use data before the year 1978 when the TNSP implemented of would also help the results analyses. Additionally, there are some uncertainties, which are resulted from the fact that the majority of carbon density data used in the study are default values available in the InVEST model because of missing data. Besides, some values of carbon density derived from literature review used in this paper were fixed values, ignoring their temporal variation. Meanwhile, the value of carbon density provided by the model is different from

Table 3 Economic value and cost (forest investment completed) of carbon sequestration represented by the price of year 1990. Economic value and cost

1990

1995

2000

2005

2010

2015

Total value (trillion CNY) Mixed woodland Closed shrublands Open shrublands Woody area Forestry investment completed (billion CNY)

3.30 2.35 0.53 0.31 0.11 0.17

2.60 1.79 0.45 0.29 0.06 0.23

3.47 2.47 0.59 0.34 0.07 0.72

4.79 3.39 0.82 0.47 0.12 0.40

6.10 4.30 1.04 0.59 0.16 1.15

7.79 5.51 1.33 0.76 0.20 1.94

Table 4 The average annual change rate of economic value and cost of carbon sequestration represented by the price of year 1990. Average annual growth rate (%)

1990e1995

1995e2000

2000e2005

2005e2010

2010e2015

1990e2015

Economic value Forestry investment completed

4.61 6.71

5.93 25.35

6.66 11.11

4.92 23.65

5.03 11.06

3.50 10.30

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