4.1. Forest Biomass C Sink Potential in the YREB
China’s key forestry ecological projects, dominated by afforestation, have increased land greening [
17]. Therefore, afforestation is one of the most feasible and effective options for offsetting the greenhouse gas emissions and mitigating climate warming [
9,
12,
14,
16]. Since the implementation of the Grain for Green Program, the Yangtze River and Zhujiang River Shelter Forest Projects, and the Natural Forest Protection Project at the end of the 20th century [
15,
35], the forest area in the YREB increased continuously and was predicted to reach a value of 92.7 million hectares in 2060 (
Table A1 and
Table A3). In this study, the predicted forest biomass C storage in the YREB (including arbor, shrub, and bamboo in the existing and new afforestation forests) will increase by 3.67 Pg C from 2015 to 2060 (
Figure 3 and
Table A8), which is equivalent to 120% of the C storage of the existing forests in 2015. Meanwhile, the forest biomass C density will increase from 33.75 Mg C hm
−2 in 2015 to 66.12 Mg C hm
−2 in 2060 (
Figure 5a and
Table A8). These results indicated that the forests in the YREB have a large C sink potential in the future.
A previous study predicted that the total CO
2 emission in China will increase first from 2086 Tg C in 2016 to 2980 Tg C in 2027, and then decrease to 1370 Tg C in 2050 [
39]. As a result, China is expected to emit 88,700 Tg C of CO
2 emission between 2016 and 2050. According to the predictions of this study, it is conservatively estimated that the forest in the YREB will absorb 4.1% of the CO
2 emission in China during the same period, which is equivalent to 68.3% of China’s forest biomass C sink potential [
9,
12]. Moreover, the proportion of forest biomass C sink in the YREB to CO
2 emission in China decreased first from 3.2% in 2016–2020 to 2.9% in 2021–2030, and then increased to 4.3% in 2041–2050 (
Figure 7a), suggesting higher forest biomass C sequestration efficiency in the future. However, forest biomass C sink gradually decreased over time (
Figure 6a), similar to previous studies on China’s forest vegetation [
9,
12,
20]. A reason is related to the high percentage of mature and over-mature forests in the future [
8]. Although this study predicted that forest biomass C sink in the new afforestation forests increased from 5.67 Tg C yr
−1 in 2016–2020 to 16.95 Tg C yr
−1 in 2041–2050 (
Figure A1), there is a low total area of new afforestation forests (
Table A3). To achieve a high C sequestration rate in the future, therefore, it is important to perform more efficient forest management (such as forest structure adjustment and forest quality improvement) in the YREB.
Interestingly, the predicted average forest biomass C storage between 2020 to 2050 in the YREB was closer to that in the same region reported by Qiu et al. [
12] rather than by He et al. [
9] and Gu et al. [
22] (
Figure 7b). One possible reason was the differences in the vegetation types. This study and Qiu et al. [
12] collected data from the NFI in China. They predicted forest biomass C storage in similar vegetation types (arbor, shrub, and bamboo forests for this study versus arbor, shrub, bamboo forests, and nursery land for Qiu et al. [
12]. While He et al. [
9] collected data from the field survey of China’s forests that only included arbor forests such as deciduous broadleaf forest, deciduous needleleaf forest, evergreen broadleaf forest, evergreen needleleaf and needleleaf, and broadleaf mixed forest. And Gu et al. [
22] predicted C storage in vegetation types, including forest, shrubland, grassland, and cropland. In these studies, the differences in vegetation types will result in different forest areas and affect the prediction of forest C storage. Moreover, compared to this study and Qiu et al. [
12], the underestimated forest area reported by He et al. [
9] and the overestimated vegetation area reported by Gu et al. [
22] would lead to a high C density and a comparable C density in the YREB, respectively (
Figure 7b). On the other hand, a previous study showed that global forest C balance was mainly regulated by soil nutrient availability [
12], indicating the limitation of low soil nutrients across subtropical China to forest C sequestration [
35]. Therefore, forest C storage based on climate-induced prediction [
8,
26] may differ from that based on multivariate (e.g., climatic factors, soil properties, stand attributes, and topographic features) prediction (e.g., this study and Qiu et al. [
12]). Additionally, identifying the effects of biotic and abiotic factors on forest C storage across scales and ecosystems is beneficial to further understanding these differences [
40,
41].
4.2. Differences in the Forest Biomass C Sink Potential Amongst Regions
Between 2015 and 2060, Yunnan and Sichuan provinces would have the greatest forest biomass C sink with a total value of 0.99 and 0.66 Pg C, respectively (
Figure 3), and the highest forest biomass C density with the average value of 64.81 and 66.83 Mg C hm
−2, respectively (
Figure 5b). Benefited from the complex landform (
Figure 1), the subalpine and alpine area of the western region in the YREB (such as Yunnan and Sichuan in this study) represents the second largest natural forest in China [
8,
22], which has diverse vegetation types and high forest productivity [
42]. As a result, these forests are the most important C sink in YREB (
Figure 6). However, the proportion of forest biomass C storage in these provinces to the total forest biomass C storage in the YREB during the same period decreased over time (
Figure 4). These results indicated that the forest biomass C sequestration rate in these regions might weaken in the future, as illustrated by the decreasing forest biomass C sink (
Figure 6a). This may be related to the weakening of annual C sink capacity caused by the gradual maturity of existing forests and new forests in the future and the decline in growth rate [
43]. Therefore, it is necessary to focus on strengthening forest protection and management in the upper reaches of the YREB to maintain and enhance its forest biomass C storage function.
Consistent with the results of previous studies [
9,
12,
22], the lower reaches, such as Shanghai municipality and Jiangsu province in the Yangtze River, had the lowest forest biomass C storage, C density, and C sink in the historical period and in the future (
Figure 3,
Figure 4,
Figure 5 and
Figure 6). Two mechanisms, a large proportion of young- and middle-age arbor forests and the low forest area, were the main reason for the lowest forest biomass C sink potential in these regions [
8]. In addition, there is dense population, lack of forest resources, huge man-made destruction, and serious utilization interference, which will result in poor forest quality and low C sequestration. Surprisingly, forest biomass C density in these two regions has shown a steady increase from 2015 to 2060 (
Figure 5a), indicating that the C sink function of forest biomass is continuously increasing after improved forest management.
In this study, the predicted forest biomass C sink potential of various regions (
Figure 3,
Figure 4,
Figure 5 and
Figure 6) differed from previous studies [
9,
12] due to differences in the estimation methods, the estimated forest types, and the setting of new afforestation scenarios. Moreover, the forest biomass C sink potential of various regions was unbalanced (
Figure 3,
Figure 4,
Figure 5 and
Figure 6). Overall, the highest forest biomass C density was in the upper reaches of the Yangtze River (e.g., Sichuan and Yunnan), followed by the middle reaches (e.g., Chongqing, Hubei, Hunan, Guizhou, and Jiangxi), and the lowest was in the lower reaches of the Yangtze (e.g., Anhui, Jiangsu, and Shanghai). These results were closely related to the forest coverage and the economic development level in these regions and showed an important function in increasing forest biomass C sinks and reducing emissions in the upper and middle reaches of the Yangtze River in the future.
4.3. Uncertainty and Limitation of This Study
In this study, although we predicted the increased forest biomass C sink potential in the YREB (
Figure 7a), there was still some uncertainty. On the one hand, since future deforestation and logging will be greatly affected by the policy, it is difficult to predict and has high uncertainty. Therefore, the scenario set in this study is to assume that the forest will not be subjected to deforestation and logged in the future, and the forest will grow according to its natural growth process. Indeed, if the forests encounter the effects of anthropogenic and natural disturbances that lead to deforestation or death during the growth process, low-density young forests will replace high-density mature forests, which will cause deviations in the prediction results. On the other hand, one of the scenarios assumed that the soil fertility in all permanent plots remains constant in the future. Unfortunately, soil nutrients changed over time in subtropical China, resulting in uncertain predictions [
13,
35]. Moreover, the total area of new afforestation forests in each province and municipality and the allocation ratios of forest area in different periods, different origins, and different tree species may change with the adjustment of policies and management methods in the future, suggesting the changed C sink potential of new afforestation forests. Therefore, how to accurately estimate and predict forest biomass C sink potential in the YREB needs to be further studied. More importantly, taking into account the C sink potential of dead organic matter C pools (litter and deadwood), soil organic C pools, and harvested wood forest products C pools is necessary to assess regional carbon neutrality [
22]. Furthermore, due to the lack of reliable C emission data in the YREB, we cannot make an accurate estimate of the future contribution of forest biomass C sink in offsetting CO
2 emissions in this region and only estimate the national contribution. In future research, it is necessary to strengthen the research on C emissions in different regions of the country.