Land Use Simulation and Identification of Core Carbon Sink Areas in the Beijing–Tianjin–Hebei Region

陆地生态系统碳循环能够综合反映全球气候变化及区域响应，是全球及区域气候变化及人类活动影响研究的重要内容。青海高原作为青藏高原的重要组成部分，在全球及区域气候与环境变化中具有极其重要的作用。因此，研究青海高原植被碳源/汇时空变化及气候因子的影响具有重要意义。采用土壤呼吸模型和改进的CASA模型，结合MODIS、气象数据估算了青海高原2000-2015年植被净生态系统生产力（NEP），分析了植被NEP、碳汇的年际时空分布、年际动态变化、多年累积空间分布与植被NEP变异系数，定量分析了降水量、气温对植被NEP的影响。结果表明：1）2000-2015年，青海高原植被年NEP空间分布特点呈东高西低、南高北低，由西北向东南逐步增加趋势，年NEP多年平均值为128.40 gC m^-2a^-1；2）青海高原不同生态区植被NEP与碳汇量空间分异显著，碳汇区约占植被分布区面积的73.11%，其中，祁连山生态区和三江源生态区为主要的碳汇区；3）2000-2015年，青海高原植被碳汇功能逐步增强，年固碳量为-3.2-64.42 TgC，年际变化呈平稳上升；4）受自然与人为因素的协同影响，青海高原年NEP呈现逐步好转的趋势，平均趋势系数为1.52，NEP增加的区域占植被总面积的25.72%；5）青海高原植被NEP变异系数空间分布以较低、中等波动为主，稳定性颇高；6）降水量对植被NEP以促进作用为主，气温以抑制作用为主，气温对青海高原植被NEP的影响占主导地位。;The carbon cycle of terrestrial ecosystem relates to global climate change and its regional response comprehensively. Therefore, it is an important part of climate changes research and the impact of human activities. As an important part of the Qinghai-Tibet Plateau, the Qinghai Plateau plays an important role in the global and regional climate and environmental changes. Therefore, it is of great significance to study the temporal and spatial variation of carbon source/sink of vegetation and the influence of climate factors in Qinghai Plateau. By using the soil respiration model and the improved Carnegie Ames Stanford Approach (CASA) model, combined with MODIS and meteorological data, we estimated the net ecosystem productivity (NEP) of the Qinghai Plateau from 2000 to 2015, analyzed the inter-annual spatial and temporal distribution, the inter-annual dynamics and the multi-year accumulation spatial distribution of vegetation NEP and carbon sink, calculated the coefficient variation of vegetation NEP. We quantitatively discussed the impact of precipitation and temperature on vegetation NEP. The results indicate that 1) during 2000 and 2015, the spatial distribution characteristics of the annual vegetation NEP are high in the east and the south, low in the west and the north, and gradually increase from the northwest to the southeast. The annual average of NEP is 128.40 gC m^-2a^-1. 2) There is a significant spatial distribution of vegetation NEP and the amount of carbon sinks between different ecological regions, the total carbon sink area accounts for about 73.11% of the total vegetation area in Qinghai Plateau, of which the Qilian Mountain Ecological Area and the Sanjiangyuan Ecological Area are the main carbon sink areas.3) From 2000 to 2015, the carbon sink function of vegetation in Qinghai Plateau is gradually enhanced, and the annual carbon sequestration is -3.2—64.42 TgC. And the inter-annual variation of the carbon sequestration increases stably. 4) Influenced by the combination of natural and human factors, the annual NEP of Qinghai Plateau shows a gradually improvement trend, with an average trend coefficient of 1.52, and the area where NEP increases accounted for 25.72% of the total vegetation area. 5) The spatial distribution of NEP variation coefficient in Qinghai Plateau is mainly low and medium fluctuation, and the stability is quite high. 6) The precipitation mainly promotes vegetation NEP accumulation, the temperature inhibits the accumulation of NEP. And the influence of temperature on NEP accumulation is dominant.

Net ecosystem productivity (NEP) plays a vital role in quantifying the carbon exchange between the atmosphere and terrestrial ecosystems. Understanding the effects of dominant driving forces and their respective contribution rates on NEP can aid in the effective management of terrestrial carbon sinks, especially in rapidly urbanizing coastal areas where climate change (CC) and human activities (HA) occur frequently. Combining MODIS NPP products and meteorological data from 2000 to 2020, this paper established a Modis NPP-Soil heterotrophic respiration (Rh) model to estimate the magnitude of NEP in China’s coastal zone (CCZ). Hotspot analysis, variation trend, partial correlation, and residual analysis were applied to explore the spatiotemporal patterns of NEP and the contributions of CC and HA to the dynamics of NEP. We also explored the changes in NEP in different land use types. It was found that there is a clear north–south difference in the spatial pattern of NEP in CCZ, with Zhejiang Province serving as the main watershed for this difference. In addition, NEP in most regions showed an improvement trend, especially in the Beijing–Tianjin–Hebei region and Shandong Province, but the pixel values of NEP here were generally not as high as that in most southern provinces. According to the types of driving forces, the improvement of NEP in these regions primarily results from the synergistic effects of CC and HA. NEP changes in provinces south of Zhejiang are mainly dominated by single-factor-driven degradation. The area where HA contributes to the increase in NEP is much larger than that of CC. From the perspective of land use types, forests and farmland are the dominant contributors to the magnitude of NEP in CCZ.

Carbon sink enhancement is of great significance to achieving carbon peak and carbon neutrality. This study firstly estimated the carbon sink in the Beijing–Tianjin–Hebei Region using the carbon absorption coefficient method. Then, this study explored the differentiation of carbon sink enhancement potential with a carbon sink–economic carrying capacity index matrix based on carbon sink carrying capacity and economic carrying capacity under the baseline scenario and target scenario of land use. The results suggested there was a remarkable differentiation in total carbon sink in the study area, reaching 2,056,400 and 1,528,300 tons in Chengde and Zhangjiakou and being below 500,000 tons in Langfang and Hengshui, while carbon sink per unit land area reached 0.66 ton/ha in Qinhuangdao and only 0.28 t/ha in Tianjin under the baseline scenario. Increasing area and optimizing spatial distribution of arable land, garden land, and forest, which made the greatest contribution to total carbon sinks, is an important way of enhancing regional carbon sinks. A hypothetical benchmark city can be constructed according to Qinhuangdao and Beijing, in comparison with which there is potential for carbon sink enhancement by improving carbon sink capacity in Beijing, promoting economic carrying capacity in Qinhuangdao, and improving both in the other cities in the study area.

Accurate regional carbon sequestration estimates are essential for China’s emission reduction and carbon sink enhancement efforts to address climate change. Enhancing the spatial precision of vegetation carbon sink estimates is crucial for a deeper understanding of the underlying response mechanisms, yet this remains a significant challenge. In this study, the Beijing–Tianjin–Hebei (BTH) region was selected as the study area. We employed the GF-SG (Gap filling and Savitzky–Golay filtering) model to fuse Landsat and MODIS data, generating high-resolution imagery to enhance the accuracy of NPP (Net Primary Productivity) and NEP (Net Ecosystem Productivity) estimates for this region. Subsequently, the Sen+MK model was used to analyze the spatiotemporal variations in carbon sinks. Finally, the land use intensity index, which reflects human activity disturbances, was applied, and the bivariate Moran’s spatial autocorrelation method was used to analyze the response mechanisms of carbon sinks. The results indicate that the fused GF-SG NDVI (Normalized Difference Vegetation Index) data provided highly accurate 30 m resolution imagery for estimating NPP and NEP. The spatial distribution of carbon sinks in the study area showed higher values in the northeastern forest regions, relatively high values in the southeastern plains, and lower values in the northwestern plateau and central urban areas. Additionally, 58.71% of the area exhibited an increasing trend, with 11.73% showing significant or strongly significant growth. A generally negative spatial correlation was observed between land use intensity and carbon sinks, with the impact of land use intensity on carbon sinks exceeding 0.3 in 2010. This study provides methodological insights for obtaining vegetation monitoring data and estimating carbon sinks in large urban agglomerations and offers scientific support for developing ecological and carbon reduction strategies in the BTH region.

Net ecosystem productivity (NEP) is a key indicator of ecosystem carbon balance and vegetation carbon source–sink dynamics. The Heihe River Basin (HRB), a representative arid and semi-arid inland basin in northwestern China, provides an ideal natural laboratory for long-term carbon dynamics research. Despite its ecological significance, few studies have systematically quantified long-term, large-scale patterns of net primary productivity (NPP) and NEP in this basin. Using an improved Carnegie–Ames–Stanford Approach (CASA) model and a soil respiration model, we quantified NPP and NEP in 2000, 2005, 2010, 2015 and 2022. Results show that the HRB functioned overall as a carbon source, while NEP increased from −113.10 to −96.33 g C·m⁻²·a⁻¹. NEP exhibited pronounced seasonality, peaking in July and reaching a minimum in January. Spatially, higher NEP occurred in the south and east, with Qilian and Minle counties acting as carbon sinks. Temperature, precipitation and altitude are dominant drivers, with strong temperature‒precipitation interactions.

The carbon balance of terrestrial ecosystems is intertwined with climate and changes in land use. Over the past 30 years, the Loess Plateau (LP) has experienced temperature increases and an expansion of forest and grassland. The net ecosystem productivity (NEP) underlying these changes is worth investigating. Using three periods (i.e., 1990–2000, 2000–2010, and 2010–2019) of annual average NEP and climatic, topographic, and land use data, we analyzed changes in the spatial distribution of carbon sources and sinks of the LP. Using an optimal parameter-based geographical detector model to discuss the driving factors of carbon sources and sinks, we found that: (1) The area of carbon sinks has been increasing continuously, and that the distributions of both of these elements are zonal. The carbon sinks show a downward trend from south to north, which is mainly driven by climate and land use type. (2) Carbon sources are mainly concentrated in the middle temperate zone, and they are mainly linked to impervious land, unused land, and grassland. The carbon sinks are mainly concentrated in the south temperate zone and plateau climatic zone, and they are mainly linked to forest, grassland, and cultivated land. Additionally, the southern temperate zone has been the most green, due to its superior hydrothermal conditions that sustain carbon sinks. (3) It is not uncommon for some forests, grasslands, and cultivated land to transition between being carbon sources and carbon sinks, especially when affected by human intervention and inadequate management measures. (4) NEP was primarily influenced by CO2 concentration, temperature, and precipitation, and the interaction of these factors greatly influenced the dynamics of carbon sources and sinks, while terrain exerted insignificant impacts on the NEP. This study highlights the importance of the carbon balance in terrestrial ecosystems and can be used to guide the creation of vegetation-based carbon sequestration policies.

The transformation of ecosystem types caused by land use change plays an extremely important role in the regional carbon cycle. Studying the response of vegetation carbon source/sink systems to land use change is helpful to improve the understanding of the vegetation carbon sink effect in the process of land use change. However, few studies have focused on the response of vegetation carbon sources/sinks to land use change. The CASA model and soil microbial respiration model were combined to estimate the net ecosystem productivity （NEP） of vegetation in the Yanhe River Basin in the Chinese Loess Plateau from 2000 to 2020 based on remote sensing, meteorological, and land use data. The spatiotemporal pattern evolution characteristics of the carbon source/sink and land use were identified using a significance test, univariate linear regression analysis, and land use status transition matrix methods, and the response of the carbon source/sink to land use change was further analyzed. The results showed that from 2000 to 2020, the multi-year average NEP in the Yanhe River Basin showed a spatial distribution pattern of lower in the upstream and higher in the midstream and downstream. The Yanhe River Basin belonged to a weak carbon sink area as a whole, with this type of area accounting for 88.81% of the basin area. The annual NEP of the basin showed a significant increase trend in fluctuations, and the carbon sequestration capacity was gradually improving. The areas with significant and extremely significant increases in annual NEP accounted for 65.78% of the basin area, and the types of annual NEP restoration, basic stability, and degradation accounted for 79.70%, 10.15%, and 10.15% of the basin area, respectively. Over the past 20 years, the land use transformation of Yanhe River Basin mainly included five types, that is, cropland was converted into grassland, woodland, and construction land, and grassland was converted into cropland and woodland. The land use in the Yanhe River Basin was mainly shifting towards promoting the improvement of the carbon sink capacity, and the transformation of land type to woodland had a more significant effect on improving carbon sink capacity. During the five main land use transformation processes in the Yanhe River Basin, the area ratio of NEP recovery-recovery type for cropland shifting to woodland was the highest at 80.78%. The area ratios of NEP recovery-recovery type for grassland shifting to cropland and cropland shifting to grassland were relatively low, at 48.05% and 51.97%, respectively. The stability of NEP restoration when shifting cropland to woodland was the strongest, and the fluctuation of NEP restoration when shifting between cropland and grassland mutually was strong. When adjusting cropland and grassland mutually, attention should be paid to select suitable vegetation types and increase vegetation coverage reasonably to improve carbon sequestration and sink enhancement capabilities, so as to avoid carbon losses during land transformations. The research methods and results in this study can provide reference for land management departments to formulate scientific and reasonable land use decisions to promote vegetation carbon sequestration and sink enhancement.

Understanding the spatiotemporal dynamics of terrestrial ecosystem carbon sinks, as well as the underlying driving mechanisms, is crucial for guiding regional carbon neutrality policies. Using Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data, field measurements data, and multi-source environmental data, we estimated net primary productivity in subtropical zone from 2000 to 2020 with the Carnegie-Ames-Stanford approach (CASA) model, and assessed net ecosystem production (NEP) by subtracting heterotrophic respiration. Regression analysis, coefficient of variation, Hurst exponent, and geodetector were applied to examine the spatiotemporal patterns and driving forces of NEP. The results identified distinct spatial heterogeneity in NEP across the study area, characterized by a west-south high and east-low gradient, with moderate levels in the north. The NEP exhibited positive persistence (H > 0.5) in 73.2% of the study area. Notably, natural forest areas showed strong persistent improvement (H > 0.65), whereas the Chang-Jiu urban agglomeration was characterized by strong persistent degradation (H < 0.35). The elevation range of 550-750m exhibited the peak carbon sink capacity (345.6g C m⁻² year⁻¹); Normalized difference vegetation index and elevation, with the q value of 0.37 and 0.34 respectively, were identified as the key individual factors influencing NEP variation. The strongest interactive effect on NEP variation was detected between soil type and land use type (q = 0.586). This evidence, combined with the impact of the climate-land use interaction on NEP, implies that synergistic management of these factors could enhance carbon sink potential. Our research reveals that the carbon sink dynamics in subtropical zone are governed by the interaction of topographic, climate, and human activity. Future efforts must implement zonal management strategies (e.g., conserving mountainous areas and promoting forest-grain intercropping on plains) to bolster forest carbon sinks.

Spatial and temporal analysis of carbon sources and sinks through land use/cover changes in the Beijing-Tianjin-Hebei urban agglomeration region

In the context of the challenge posed by climate change and the pursuit of the "dual carbon" goals， accurately assessing the efficiency of forest carbon sinks in various regions， identifying the reasons behind efficiency differences， and accordingly proposing effective resource allocation pathways are of great significance for promoting coordinated regional development， improving resource utilization efficiency， and achieving carbon sequestration and emission reduction. Based on provincial data from China spanning from 2004 to 2021， a three-stage DEA model was constructed to measure the forest carbon sink efficiency of 30 provinces and municipalities， and a regional difference analysis was conducted. Simultaneously， a prediction model based on GAN and KOA-CNN-BiLSTM-Attention was established to forecast the total carbon sink targets that each region intends to achieve by 2030， and scenarios were set up to enhance the accuracy of the model. Based on the three-stage theory， further improvements were made to the inverse DEA model to discuss the resource allocation path planning schemes for various regions to achieve their predicted carbon sink targets. In particular， feedback was provided on the current input redundancies and deficiencies in each region. The results showed that： ① Although China's forest carbon sink efficiency showed a trend of growth， it was still at a moderately low level overall. Moreover， there were significant regional differences. Regions with higher efficiency levels were mainly distributed in the southwest and northeast， while those in North China had lower levels and require attention. The remaining regions fell in between and still had room for development. ② The core reasons for the differences in forest carbon sink efficiency among regions were natural resource endowments （including forest resources and natural conditions） and the degree of concern for forestry （including policy support and resource allocation preferences）. Scientific management and operation played a reinforcing role in enhancing the forest carbon sink capacity of the regions. ③ The northeast and southwest regions have made significant contributions in terms of total forest carbon sink. Nationally， the total forest carbon sink is expected to increase by 7% to 30% by 2030， and the forest carbon sink will continue to play a crucial role in carbon emission reduction and sequestration. ④ Each region needs to make incremental improvements in at least one input area. Land input will be the main challenge in achieving future carbon sink targets and is also a key limiting factor for the current level of forest carbon sink efficiency. In the long run， Heilongjiang and Inner Mongolia have the best prospects for forest carbon sink development. The research can provide decision-making references for governments and related industries in pursuing the "dual carbon" goals and help enhance carbon sequestration efficiency and resource allocation efficiency.

Net ecosystem productivity (NEP), which plays a key role in the carbon cycle, is an important indicator of the ecosystem's carbon budget. In this paper, the spatial and temporal variations of NEP over Xinjiang Autonomous Region, China from 2001 to 2020 were studied based on remote sensing and climate re-analysis data. The modified Carnegie Ames Stanford Approach (CASA) model was employed to estimate net primary productivity (NPP), and the soil heterotrophic respiration model was used to calculate soil heterotrophic respiration. Then NEP was obtained by calculating the difference between NPP and heterotrophic respiration. The annual mean NEP of the study area was high in the east and low in the west, high in the north and low in the south. The 20-year mean vegetation NEP of the study area is 128.54 gC·m-2, indicating that the study area is a carbon sink on the whole. From 2001 to 2020, the annual mean vegetation NEP ranged between 93.12 and 158.05 gC·m-2, and exhibited an increasing trend in general. 71.46% of the vegetation area showed increasing trends of NEP. NEP exhibited a positive relationship with precipitation and a negative relationship with air temperature, and the correlation with air temperature was more significant. The work reveals the spatio-temporal dynamics of NEP in Xinjiang Autonomous Region and can provide a valuable reference for assessing regional carbon sequestration capacity.

Grassland Carbon Sequestration Ability in China: A New Perspective from Terrestrial Aridity Zones

The interest in national terrestrial ecosystem carbon budgets has been increasing because the Kyoto Protocol has included some terrestrial carbon sinks in a legally binding framework for controlling greenhouse gases emissions. Accurate quantification of the terrestrial carbon sink must account the interannual variations associated with climate variability and change. This study used a process‐based biogeochemical model and a remote sensing‐based production efficiency model to estimate the variations in net primary production (NPP), soil heterotrophic respiration (HR), and net ecosystem production (NEP) caused by climate variability and atmospheric CO2 increases in China during the period 1981–2000. The results show that China's terrestrial NPP varied between 2.86 and 3.37 Gt C yr−1 with a growth rate of 0.32% year−1 and HR varied between 2.89 and 3.21 Gt C yr−1 with a growth rate of 0.40% year−1 in the period 1981–1998. Whereas the increases in HR were related mainly to warming, the increases in NPP were attributed to increases in precipitation and atmospheric CO2. Net ecosystem production (NEP) varied between −0.32 and 0.25 Gt C yr−1 with a mean value of 0.07 Gt C yr−1, leading to carbon accumulation of 0.79 Gt in vegetation and 0.43 Gt in soils during the period. To the interannual variations in NEP changes in NPP contributed more than HR in arid northern China but less in moist southern China. NEP had no a statistically significant trend, but the mean annual NEP for the 1990s was lower than for the 1980s as the increases in NEP in southern China were offset by the decreases in northern China. These estimates indicate that China's terrestrial ecosystems were taking up carbon but the capacity was undermined by the ongoing climate change. The estimated NEP related to climate variation and atmospheric CO2 increases may account for from 40 to 80% to the total terrestrial carbon sink in China.

The Hindu Kush Himalayan (HKH) region is one of the most ecologically vulnerable regions in the world. Several studies have been conducted on the dynamic changes of grassland in the HKH region, but few have considered grassland net ecosystem productivity (NEP). In this study, we quantitatively analyzed the temporal and spatial changes of NEP magnitude and the influence of climate factors on the HKH region from 2001 to 2018. The NEP magnitude was obtained by calculating the difference between the net primary production (NPP) estimated by the Carnegie–Ames Stanford Approach (CASA) model and the heterotrophic respiration (Rh) estimated by the geostatistical model. The results showed that the grassland ecosystem in the HKH region exhibited weak net carbon uptake with NEP values of 42.03 gC∙m−2∙yr−1, and the total net carbon sequestration was 0.077 Pg C. The distribution of NEP gradually increased from west to east, and in the Qinghai–Tibet Plateau, it gradually increased from northwest to southeast. The grassland carbon sources and sinks differed at different altitudes. The grassland was a carbon sink at 3000–5000 m, while grasslands below 3000 m and above 5000 m were carbon sources. Grassland NEP exhibited the strongest correlation with precipitation, and it had a lagging effect on precipitation. The correlation between NEP and the precipitation of the previous year was stronger than that of the current year. NEP was negatively correlated with temperature but not with solar radiation. The study of the temporal and spatial dynamics of NEP in the HKH region can provide a theoretical basis to help herders balance grazing and forage.

While the simple model of the total atmospheric carbon sink effect as a linear function of concentration has provided excellent prediction results, several problems remained to be investigated and solved. The most obvious open issue is the correct treatment of land use change emissions. It turns out that the model improves by mostly neglecting these emissions after 1950. This effectively implies that land use change emissions have been constant and small since then. The key investigation starts with the observation that the total carbon sink has a short-term component that can be explained by temperature changes. The apparent paradox, why contrary to the short-term changes no temperature-caused trend can be detected, despite the fact that several contributing processes exhibit clear temperature dependency, is analyzed and explained. The result of this analysis leads to the model extension, where the total effect of absorptions and natural emissions are a linear function of concentration and temperature. This extended model not only explains current measurements but also paleo-climate data from ice core time series.