Biomass carbon storages and carbon sequestration potentials of the Grain for Green Program‐Covered Forests in China

Abstract The Grain for Green Program (GGP) was the most all‐embracing program of ecological reconstruction implemented in China. To estimate carbon storages and carbon sequestration potentials of the GGP forests, the study presented in the paper collected data spanning from 1999 to 2010, such as tree species, tree planting area relevant to the GGP, empirical growth curves suitable for different planted tree species in China, as well as wood density (WD), biomass expansion factor (BEF), carbon fraction (CF) of different trees species, and estimated the carbon storages of the biomasses of GGP forests from 1999 to 2050. It showed that the total carbon storage of the biomass of GGP forests was 320.29 Tg upon the GGP completion in 2010; the total carbon sequestration is higher during the early GGP‐implementation stage than at the late GGP‐implementation stage, and the annual mean carbon sequestration of GGP forests was 26.69 Tg/year. The potential of GGP forests as carbon sink presented an increasing increment. In China, the potential increments of GGP forests as carbon sinks were estimated to be 397.34, 604.00, 725.53, and 808.90 Tg in 2020, 2030, 2040, and 2050, respectively, and the carbon sequestration rates were 1.72, 0.89, 0.52, and 0.36 Mg ha−1 year−1, respectively, corresponding to 2010s, 2020s, 2030s, and 2040s. Therefore, the GGP forests had bigger carbon sequestration capacities and potentials in China.

evant to the GGP, empirical growth curves suitable for different planted tree species in China, as well as wood density (WD), biomass expansion factor (BEF), carbon fraction (CF) of different trees species, and estimated the carbon storages of the biomasses of GGP forests from 1999 to 2050. It showed that the total carbon storage of the biomass of GGP forests was 320.29 Tg upon the GGP completion in 2010; the total carbon sequestration is higher during the early GGP-implementation stage than at the late GGP-implementation stage, and the annual mean carbon sequestration of GGP forests was 26.69 Tg/year. The potential of GGP forests as carbon sink presented an increasing increment. In China, the potential increments of GGP forests as carbon sinks were estimated to be 397. 34, 604.00, 725.53, and 808.90 Tg in 202034, 604.00, 725.53, and 808.90 Tg in , 203034, 604.00, 725.53, and 808.90 Tg in , 204034, 604.00, 725.53, and 808.90 Tg in , and 2050, respectively, and the carbon sequestration rates were 1.72, 0.89, 0.52, and 0.36 Mg ha −1 year −1 , respectively, corresponding to 2010s, 2020s, 2030s, and 2040s. Therefore, the GGP forests had bigger carbon sequestration capacities and potentials in China.

K E Y W O R D S
carbon sequestration potential, carbon storage, China, Grain for Green focusing on carbon storages (Yu et al., 2007), carbon sequestration (Deng, Liu, & Shangguan, 2014), and carbon sequestration potential of forestland soils (Chang, Fu, Liu, & Liu, 2011;Zhang, Dang, Tan, Cheng, & Zhang, 2010) in China. However, there are a few researches on carbon sequestration capacities of plantations in China . As the biggest developing country of the world, China had the largest plantation area in the world (Peng et al., 2014). Since the 1980s, China government had carried out six major forestry programs, making its new plantation area gradually expand, so that these programs had played not only an important role in ecological environment improvement but also a positive role in atmospheric CO 2 fixation (Deng, Liu, et al., 2017).
However, currently, there are few studies conducted on effects of major forestry policies and forestry programs of China on it forest carbon sequestration capacities (Yin, Yin, & Li, 2010).
The Grain for Green Program (GGP) of China was one most allembracing ecological reconstruction program implemented by China (Deng & Shangguan, 2011), which afforested the largest area of forests (He et al., 2011). By its large-scale forestations, the Program had established large areas of new forest vegetation, hence enhancing sequestration capacities of terrestrial ecosystems in China (Chen et al., 2009;Deng et al., 2014), and consequently, its effects on carbon sequestration potential of forests should not be ignored (Chen et al., 2009). At present, effects on the GGP forests as carbon sinks are mainly investigated in specific regions or in terms of forestland soil in China. Chen et al. (2009) studied effects of the GGP on forest carbon sequestration potentials in Yunnan of China. Chang et al. (2011) studied soil organic carbon sequestration potentials of the GGP-covered Loess Plateau. And Zhang et al. (2010) researched on effects of the GGP on soil organic carbon sequestration rate in China. Although Liu et al. (2014) have estimated the changes in carbon fluxes and stocks caused by GGP using a process-based ecosystem model, there are still limited researches carried out on effects of the GGP on carbon storages and carbon sequestration potentials of forest biomasses in China as a whole using inventory-based method.
Therefore, in order to correctly assess effects of relevant forestry policies and forestry programs on forest carbon sequestration and in the hope of providing theoretical basis and data supports for plantation ecological assessment, the study investigated effects of the GGP on forest carbon sequestration capacities by estimating biomass carbon storages of GGP forest biomasses after the GGP implementation. The study was mainly intended to investigate effects of the GGP on (a) forest carbon storages, (b) carbon sequestration capacities, and (c) carbon sequestration potentials in the future 40 years.

| Program profile
In China, the Grain for Green Program was implemented for more than 10 years as a measure to control soil erosion (Deng &

| Geographical zones
According to Fang's classification (Fang et al., 2001) Figure   The GGP mostly afforested on barren hills and farmlands with slope gradient >15°, of which the soil was relatively poor and the soil moisture was lower, so that the forestation survival rates were unlikely to reach 100%. Thus, a correction factor, namely actual afforestation survival rate, was introduced. China's State Forestry Administration (SFA) survey (SFA, 2005) showed that the afforestation survival rate was only 90.2%, and thus, the actual planting areas were the results of the cited afforestation areas times the correction factor of 0.902.

| Carbon storage estimation
Because the objects of the study were short-time plantations, the carbon storages of their dead woods and litters were too low to take into account although they would increase after cropland or barren land afforestation or reforestation. Therefore, the study only took carbon storages of forest biomasses into account in its carbon storage estimation.

| Carbon storage estimation of forest biomasses
In general, carbon storages of forest vegetations are estimated by measuring and then multiplying their biomasses with their corresponding carbon fractions (CF). The carbon storage estimations of forest biomasses dynamics are related to the estimation of forest biomass changes. This study employed the empirical growth curve of plantations to estimate carbon storages of forest biomasses. The formulae for carbon storage estimation are as follows: In which, C Bi stands for carbon stocks (Mg) in living tree biomass in year i; S jk stands for the area (ha) of species j planted or to be planted in year k; V ijk stands for stand volume per hectare(m 3 /ha) of species j planted in year k in year i; B ijk stands for stand biomass per hectare(Mg/ha) of species j planted in year k in year i for; D j stands for the basic wood density (Mg/m 3 ); BEF j stand for the biomass expansion factor to convert stem biomass of species j into its stand biomass (whole trees, including stems, branches, foliages, and roots); and CF j stands for the carbon fraction of species j. Equation (1) is suitable for arbor forests, and Equation (2) is suitable for shrub forests.

Stand volume or biomass estimation
In the model of carbon storage change in forest biomass estimation, stand volume (V) is a function change over time (forest age).
The study adopted allometric growth equations of stand volume and forest age depended on them (Table 1). In addition, the study assumed that the age of a forest was one when it was afforested.
Biomass expansion factor, wood density, and carbon content The study cited its basic wood densities (WD) and biomass expansion factors (BEF) from China initial national communication (

| Carbon sequestration rate estimation
The method for carbon accumulation rate was adopted as follows (Zhou et al., 2008): where Cr represents carbon accumulation rate (Mg ha −1 year −1 ); C tk, and C tj are carbon storage in year k and j (Mg); t k-j is the interval of time between years k and j. In our study, the Cr in the periods of 2010-2020, 2020-2030, 2030-2040, and 2040-2050 was the mean values of the 10 years of each period.

| Data analysis
One-way ANOVA was used to analyze the means of the each period among the different periods and different regions.
Differences were evaluated at the 0.05 significance level. When significance was observed at the p < 0.05 level, LSD (least significant difference) post hoc test was used to carry out the multiple comparisons. The data used for one-way ANOVA have passed the homogeneity test. All statistical analyses were performed using the software program SPSS, ver. 17.0 (SPSS Inc., Chicago, IL, USA).
Because the GGP-covered regions had different planting areas of GGP forests and tree species, the GGP-covered regions had different carbon storage properties in annual carbon storage of forest biomass. Like the carbon storage of forest biomass of the different GGP-covered regions, the annual mean forest carbon sequestration of Central south China was the highest, reaching 7.30 Tg/ year (Table 3), and the annual mean forest carbon sequestration of Northeast China was the lowest, reaching 2.12 Tg/year (Table 3) and accounting for about 7% of the total forest carbon sequestration of China. The forest carbon sequestrations of the six regions varied consistently with those of China, that is, the forest carbon sequestrations were higher at the late GGP-implementation stage (2005)(2006)(2007)(2008)(2009)(2010) than at the early GGP-implementation stage (1999)(2000)(2001)(2002)(2003)(2004) (Table 3).

| Carbon sequestration potential of forest biomass
From the GGP completion in 2010-2050, the forest carbon sequestration potential of China will increase with time ( Figure 4)

| The contribution of the GGP to carbon sinks in China
Forestry policies and forestry projects of China not only play an important role in improving its ecological environment but also modify its carbon sequestration capacity (Deng et al., 2014;Deng, Liu, et al., 2017;Wang et al., 2016). The carbon sequestration capacity due to implementation of forestry policies and forestry projects attract more and more attentions, and particularly after the Kyoto protocol took effect in 2005, which has prompted more countries and regions of the world carried out relevant researches (Deng et al., 2014;Jandl, Lindner, et al., 2007;Niu & Duiker, 2006

| Tree growth equation accuracy
There are several methods in estimating forest biomass C stock at national or regional scales (Deng, Liu, et al., 2017), but no universally accepted approach for predicting future changes in forest biomass C stock (Xu, Guo, Piao, & Fang, 2010). The growth curves of forest volume (biomass) play a crucial role in estimating carbon storages in living tree biomasses (Chen et al., 2009;Deng, Liu, et al., 2017). Xu et al. (2010) have proved that the growth curves of forest volume method performed well in predicting the forest biomass C stocks dynamic at national scale. However, the estimation of future forest biomass C sequestration potential of GGP still has large difference (Deng, Liu, et al., 2017). In the current study, the potential increments of GGP forests as carbon sinks were estimated to be 397.34 and 808.90 Tg in 2020 and 2050, respectively. The results are higher than the estimation of Liu et al. (2014) but lower than the estimation of Deng, Liu, et al. (2017). The different results could mainly attribute to different methods used in estimating forest biomass C stock.
Moreover, the study adopted allometric growth equations of stand volumes (forest biomass) suitable for local Chinese planta-

| Carbon stock estimation of trees coarse wood residues
The study only estimated the biomass carbon storages of GGP forests. Due to lacks of relevant data, the study did not taken into account the effects of GGP on carbon storages of litters and dead woods. In general, farmland to forestland conversions will increase carbon storages of litters and dead woods to a certain extent. For a long period of time, the carbon storage of forest litters will increase obviously without forest logging, so that forest litters should not be ignored; and there will gradually appear dead standing trees, fallen logs, big diameter drying branches, and other coarse wood residues (Delaney, Brown, Lugo, Torres-Lezama, & Quintero, 1998). Litter and dead woods will become important forest carbon pools. Thus, the two parts of GGP forests as carbon pools will significantly affect carbon sequestration potentials of the forests. Woods as raw materials are more and more fully exploited and recyclable economy is expanded, so that felled trees will be transformed as carbon pools in the forms of forest products rather than being immediately converted as carbon emissions. The forest products include building materials, decoration materials, furniture, and paper. Niu and Duiker (2006) reported that the carbon storage of wooden forest products could make up as high as 32% of the biomass carbon storage of felled trees from which the products were processed. So to cut GGP forests appropriately is helpful to extend the carbon sequestration F I G U R E 3 Annual carbon sequestrations of the biomasses of GGP forests in China and its six GGP regions capacities of the forests. In order to predict carbon potentials of GGP forests for a long time, further investigations and researches on litters and coarse wood residues as carbon pools as well as carbon storages of wooden forest products will be needed so as to estimate benefits of the GGP to increase forest products as carbon pools.

| Forest managements
Because the study was a preliminary attempt to quantitatively assess ecological benefits of forestry policies and forestry projects of China and its main regions and in particular more and more attention was paid to the issue of carbon sequestration by man-made management; therefore, the study still had many shortcomings.
Because of lacks of relevant data and proper estimation methods, the study did not taken into account carbon sequestration benefits evolving from strengthening forest tending managements, pest control, fire prevention, etc. However, influence of management measures on carbon sequestration potentials should not be underestimated (Nabuurs et al., 2000). Considering hydrothermal con- F I G U R E 4 Carbon sequestration potentials of the biomasses of GGP forests in China and its six GGP regions Neumann, et al. (2007) reported that forest managements could control carbon inputs and outputs by cutting, thinning, and intervening and optimized forest managements could not only maintain higher productivity but also increase forest soil carbon storage at the same time. Forest fires can increase soil carbon storage, but cause huge losses of biological carbon pools (Serrano-Ortiz et al., 2011;van der Werf, Randerson, Collatz, & Giglio, 2003), and postdisaster managements play an important role in determining the function of forest ecological systems as carbon sinks (Serrano-Ortiz et al., 2011). For example, postdisaster afforestation can quickly accelerate the transformation of ecosystems from carbon sources to carbon sinks (Merino, Real, Álvarez-González, & Rodríguez-Guitián, 2007). Incidences of forest diseases and insect pests as common biological natural disasters can severely harm forests. In 2010, China had 11.52 million ha of forests attacked by various hazards including forest diseases and insect pests, of which 87, 300 ha was most severely attacked (SFA, 2011). Incidences of forest diseases and insect pests indirectly affect forest soil carbon accumulation by directly influencing forest productivity. Incidences of forest diseases and insect pests can significantly reduce forest productivity, hence significantly reducing forest soil carbon accumulation. Therefore, strengthen managements of GGP forests, and promote implementations of their management measures, will greatly improve carbon sequestration capacities of the forests.

| CON CLUS ION
During the GGP implementation from 1999 to 2010, the total carbon sequestration was higher at the early stage than at the late stage. Upon the GGP completion, the forest carbon sequestration was 320.29 Tg, and the annual mean carbon sequestration was 26.69 Tg/year from 1999 to 2010. By 2050, the potential increment of GGP forests as carbon sinks will be 808.90 Tg. The study showed that GGP forests had higher carbon sequestration capacities and potentials. The annual mean carbon sequestration of GGP forest biomasses peaked in 2010. For afforestation, choosing tree species with higher carbon sequestration capacities and improving forest operations and managements can first produce bigger carbon benefits.

CO N FLI C T O F I NTE R E S T
None declared.

AUTH O R S' CO NTR I B UTI O N S
Kaibo Wang, Zhouping Shanguan, and Lei Deng designed research and analyzed data; Dongfeng Hu and Juan Deng collected data and contributed to discussion; Kaibo Wang, Zhouping Shangguan, and Lei Deng wrote the paper.