Exploring the Transmission Process of Carbon Sequestration Services and Its Applications: A Case Study of Hainan

Abstract

The pressing need to address climate change and advance global sustainable development has heightened the emphasis on ecosystem services, especially carbon sequestration. This research assesses the supply and demand dynamics of carbon sequestration services on Hainan Island, China, highlighting its significant contributions to global biodiversity conservation and carbon balance. The analysis considers the spatial distribution and interrelation of these services in light of recent land use and ecological policy changes. The methodology incorporates land use and land cover data, the Normalized Difference Vegetation Index (NDVI), meteorological data, and soil data. A gravity model is employed to elucidate the supply–demand relationship for carbon sequestration services, examining the flow across different regions and identifying spatial connections and their intensities. The results indicate a notable increase in carbon sequestration supply in Hainan from 2000 to 2020, particularly in the central mountainous areas. Conversely, the demand for these services has risen, especially in the northern plains’ urban areas and southern coastal towns. The gravity model reveals a strong spatial interdependence between the central mountainous supply zones and the high-demand urban locales. This study underscores the disparities in carbon sequestration supply and demand on Hainan, emphasizing the need for the strategic management of these elements. It provides critical data for ecological compensation policies and offers insights into the roles of regional ecosystems in climate change mitigation. The research highlights the necessity of incorporating ecosystem services into land-use planning and decision-making to foster sustainable development and strengthen climate resilience.

1. Introduction

Climate change stands out as the foremost non-traditional security challenge confronting human development today, characterized by substantial greenhouse gas emissions, rising global temperatures, increasing sea levels, and profound impacts on biodiversity [1,2]. The 2015 Global Climate Change Conference urged nations to improve their carbon sequestration capacity and slow global warming [3]. Addressing global climate change and fostering sustainable development are areas that have progressively gained global consensus. Currently, more than 30 countries and regions globally have pledged to achieve carbon neutrality and formulated corresponding low-carbon development strategies [4]. Initiatives such as “peak carbon, carbon-neutral” have emerged as international efforts to combat climate change [5].

Ecosystem services include both direct and indirect contributions of ecosystems to human well-being [6]. Ecosystem services (ES) are the goods and benefits that human obtain directly or indirectly from ecosystems, and the stable supply of ES maintains the smooth operation of socio-economic systems. Due to regional development disparities, ES are spatially heterogeneous, and the acceleration of population growth and urbanization is encroaching on ecosystems. This has led to the widespread existence of a mismatch between the supply and demand for ES. With growing recognition and mounting evidence of the value of ecosystem services in informing land-use planning and decision-making, governments and non-governmental organizations worldwide are adopting the ecosystem approach to address sustainability challenges [7]. An ecosystem service flow is the temporal and spatial transfers of ecosystem services driven by both natural and human factors between supply and use regions. Ecosystem service flows emphasize the flow of materials and energy within ecosystems that are utilized or consumed by humans, and such flows can be transmitted across different regions. The study of ecosystem service flows is of significance for understanding the complex relationships between nature and humans, as well as for adjusting patterns to achieve sustainable development goals [8]. Ecosystem carbon sequestration, a crucial component of ecosystem services, involves the process by which ecosystems capture carbon through interactions among vegetation, soil, animals, and microorganisms, contingent on carbon inputs and outputs [9]. Carbon sequestration services sustain robust ecosystems by establishing carbon sinks that facilitate the growth of above-ground vegetation and soil formation, and serve as a key defense against global warming [10,11]. The regional capacity for carbon sequestration influences the region’s participation in the global carbon cycle, thereby impacting the overall ecosystem carbon sequestration service benefits of the region. Therefore, effective identification and monitoring of the carbon sequestration capacity are crucial for a region’s ecosystem services.

The academic community has extensively researched the spatial distribution patterns and influence mechanisms of carbon sequestration services, yielding significant results [12,13]. These studies have quantified carbon sequestration in diverse ecosystems at various scales from the supply perspective of carbon sequestration services [14,15]. The research on the carbon balance of terrestrial ecosystems primarily focuses on individual ecosystems, such as forests, agriculture, and soil ecosystems, with comparatively limited exploration of the entire terrestrial ecosystem [15]. Some research methods involve using changes in net primary productivity (NPP), while others consider net carbon emissions (the balance between carbon emissions and sequestration) to assess regional carbon balance [16]. Terrestrial carbon sequestration and storage services are among the most recognized ecosystem services. However, there is a notable shortage of studies analyzing the carbon balance of terrestrial ecosystems from the perspective of carbon sequestration supply and demand.

Hainan Island, China’s largest tropical island and a global biodiversity conservation hotspot, boasts extensive tropical forests and abundant species resources, playing a pivotal role in the global carbon cycle and carbon balance due to its substantial carbon sequestration capacity [17]. Ren et al. previously estimated the carbon stock distribution in Hainan’s forest ecosystems by integrating four periods of national forest inventory data from 1993 to 2008 with measured data [17]. However, over the past decade, particularly with the development of the Hainan Tropical Rainforest National Park, the quantity and quality of Hainan’s forest resources have undergone considerable changes. The forest cover rate increased from 58.5% in 2008 to 62.1% in 2021, and the forest stock volume rose from 0.79 billion m³ in 2008 to 1.61 billion m³ [18]. To meet the practical management needs of balancing the ecosystem service supply and demand on Hainan Island, the aims of this study were the following: (1) utilize the CASA and NEP model to calculate the supply of carbon sequestration and employ the emission factor method to determine the demand for carbon sequestration; (2) elucidate the characteristics of carbon sequestration supply and demand; (3) apply the gravity model to understand the strength and spatial variation of the linkages between carbon sequestration service supply and demand, and visually present the transmission process of carbon sequestration services on Hainan Island; (4) propose targeted suggestions for ecological environment management and high-quality development on Hainan Island.

2. Materials and Methods

2.1. Study Area

Hainan Island, situated between 18°10′–20°10′ N and 108°37′–111°3′ E, is China’s second largest island, with an area of 32,900 km². It experiences a tropical monsoon climate characterized by year-round warmth, considerable heat, abundant rainfall, and distinct wet and dry seasons, earning it the moniker of a “natural greenhouse”. Hainan boasts an average annual temperature range of 23–25 °C and an annual cumulative temperature of 8200–9200 °C (for temperatures ≥10 °C). The annual precipitation averages 1720 mm, although it is unevenly distributed across the island. The windward eastern zone receives annual precipitation ranging from 2000 to 2500 mm, while the leeward western zone registers below 1000 mm. The island’s topography is complex, with mountains, hills, plateaus, and plains forming a circular, layered land-scape. The central region has elevated areas such as Wuzhi Mountain and Parrot Mountain, which serve as the core of the elevation, gradually diminishing in all directions. Functioning as a critical tropical agricultural base and ecotourism destination in China, Hainan’s landscape is predominantly characterized by forests and farmland, exhibiting high vegetation coverage and productivity levels, as well as a stable ecosystem structure (Figure 1). Positioned as an integral component of the national ecological security pattern, Hainan’s terrestrial ecosystem belt serves as a regional ecological security barrier, significantly impacting both the Chinese and global ecological environment.

The Hainan Tropical Rainforest National Park, with the main purpose of protecting nationally representative natural ecosystems and biodiversity, was established and selected as one of the first national parks in 2021. The Hainan Tropical Rainforest National Park was established in 2021 with the primary purpose of protecting nationally representative natural ecosystems and biodiversity. Located in the central mountainous area of Hainan Island, it spans 4269 km², which is approximately one-seventh of the island’s total area (https://parks.theways.cn/ (accessed on 9 January 2025)).

Hainan Island is also famous for its unique tropical natural landscape and rich human characteristics. In terms of natural landscape, Hainan Island is blessed with diversified natural scenery such as blue sea and sky, sandy beaches and coconut groves, and tropical rainforests. In terms of human characteristics, Hainan Island has a long history and culture, such as Li culture and Qiong opera. At the same time, Hainan Island is rich in folk activities, such as the Dragon Boat Festival and Water Splashing Festival.

2.2. Data Source

The study employed diverse data sources, including land use and land cover (LULC), NDVI, total solar radiation, precipitation, temperature, and soil data, among others (

Table 1. Data sources in this study.

Type	Time	Source
LULC	2000, 2005, 2010, 2015, 2020	Resource and Environment Science and Data Center (http://www.resdc.cn (accessed on 9 January 2025))
NDVI	2000–2020 (half-monthly)	Land Processes Distributed Active Archive Center (https://lpdaac.usgs.gov (accessed on 9 January 2025))
Precipitation	2000–2020 (monthly)	Chinese National Metrological Information Center (http://data.cma.cn (accessed on 9 January 2025))
Temperature	2000–2020 (monthly)	Chinese National Metrological Information Center (http://data.cma.cn (accessed on 9 January 2025))
Total solar radiation	2000–2020 (monthly)	Chinese National Metrological Information Center (http://data.cma.cn (accessed on 9 January 2025))
Soil texture	/	Land–Atmosphere Interaction Research Group (http://globalchange.bnu.edu.cn/research/data (accessed on 9 January 2025))
Bulk density	/	Land–Atmosphere Interaction Research Group (http://globalchange.bnu.edu.cn/research/data (accessed on 9 January 2025))
Soil organic matter	/	Land–Atmosphere Interaction Research Group (http://globalchange.bnu.edu.cn/research/data (accessed on 9 January 2025))
Population	100 m (yearly)	WorldPop (https://hub.worldpop.org/ (accessed on 9 January 2025))
CO2 emissions	County	figshare (https://doi.org/10.6084/m9.figshare.c.5136302.v2 (accessed on 9 January 2025))

). The LULC data, acquired from the Resource and Environment Science and Data Center (RESDC) (http://www.resdc.cn (accessed on 9 January 2025)), have a resolution of 30 m and span the years 2000, 2005, 2010, 2015, and 2020. These data were categorized into seven groups based on the land’s natural attributes and utilization patterns, aligning with Pouderoux’s classification [19]. The NDVI data originated from the Moderate Resolution Imaging Spectroradiometer (MODIS) database (https://lpdaac.usgs.gov/ (accessed on 9 January 2025)), covering the period from 2000 to 2020. Meteorological data, covering monthly precipitation, temperature, and total solar radiation data from 2000 to 2020, were obtained from the Chinese National Meteorological Information Center (CNMIC) (http://data.cma.cn (accessed on 9 January 2025)). Soil data, inclusive of soil texture (clay, silt, sand, rock fragment), bulk density, and soil organic matter, were provided by the Land–Atmosphere Interaction Research Group (http://globalchange.bnu.edu.cn/research/data (accessed on 9 January 2025)).

2.3. Defining Carbon Sequestration

The flowchart of the study exploring the transmission process of the carbon sequestration service and its application is shown in Figure 2. First, we utilized the CASA and NEP model to calculate the supply of carbon sequestration and employed the emission factor method to determine the demand for carbon sequestration. Next, the characteristics of carbon sequestration supply and demand were analyzed. Then, we applied the gravity model to understand the strength and spatial variation of the linkages between carbon sequestration service supply and demand, and visually present here the transmission process of carbon sequestration services on Hainan Island. Finally, suggestions are made for ecological environment management and high-quality development on Hainan Island (Figure 2).

2.4. Carbon Sequestration Quantification

2.4.1. Supply of Carbon Sequestration

Vegetation, as a crucial element of regional ecological stability and ecological security patterns, serves a pivotal role as a carbon sink in the global carbon balance. The assessment of its carbon sequestration services utilizes net ecosystem productivity (NEP) [20]. The NEP (net ecosystem productivity) is quantified as the difference between the vegetation’s net primary productivity (NPP) and soil heterotrophic respiration (Rh) [16]. The model equation is expressed as follows:

$$\mathrm{NEP}=\mathrm{NPP}-\mathrm{Rh}$$

(1)

where NEP denotes the value of carbon sequestration over time (gC/(m²·yr)), NPP represents the net fixation of CO₂ by an ecosystem (gC/(m²·yr)), and Rh represents heterotrophic respiration (gC/(m²·yr)). Rh can be calculated using the following method:

$$\mathrm{Rh}=0.592\times {\mathrm{Rs}}^{0.714}$$

(2)

where Rs represents soil respiration, quantifying the release of carbon dioxide (CO₂) from the soil (gC/(m²·yr)). Rs can be estimated according to Chen’s methods [21]. The calculation of NPP can be accomplished using the CASA model, applying the light use efficiency (LUE) principle established by Potter et al. [22].

$$\mathrm{NPP}=\mathrm{APAR}\times \mathsf{\epsilon }$$

(3)

where APAR (absorbed photosynthetically active radiation) denotes the photosynthetic effective radiation (MJ/m²), depending on the total solar radiation and the proportion absorbed by the vegetation. The ε denotes the actual light utilization rate (gC/MJ), which is mainly affected by temperature and moisture.

$$\mathrm{APAR}=\mathrm{Sr}\times \mathrm{FPAR}\times 0.5$$

(4)

The APAR is calculated by using the data on solar surface irradiance Sr (MJ/m²) and the fractions of photosynthetic active radiation absorbed by green vegetation (FPAR). In addition, the constant 0.5 accounts for the fact that approximately half of the incoming solar radiation is in the photosynthetic active radiation waveband [22].

$$\mathsf{\epsilon }={\mathsf{\epsilon }}_{\max }\times \mathrm{Ts}\times \mathrm{Ws}$$

(5)

The factor ε can be determined as the product of ε_max (monthly maximum utilization rate of light energy, g C/MJ), obtained by a calibration with field data, as well as stress scalars representing the availability of soil moisture Ws and the suitability of the temperature Ts [22].

2.4.2. Demand of Carbon Sequestration

Evaluating the demand for carbon sequestration services primarily relies on the emission factor and measured methods [23]. Given the expansive study area, employing the real measurement method is impractical. Therefore, this paper uses the emission factor method to estimate the regional carbon sequestration demand. Using county administrative units, the characteristics of carbon sequestration demand within the region are depicted using energy consumption data and population density data. The calculation formula is as follows:

$$\mathrm{CE}=\mathsf{\delta }\times \mathrm{P}\left(\mathrm{i}\right)\times \mathsf{\varphi }\left(\mathrm{t}\right)$$

(6)

where CE is the carbon sequestration demand (kg/yr), P(i) is the population at pixel i (capita/ha), and ϕ(t) is the CO₂ emission per capita in year t (kg/capita), which is obtained from the county-level CO₂ emissions divided by population. We estimated the carbon content in CO₂ (δ) as 27% of the total, based on the relative atomic weight [24]. The spatial distribution of the carbon sequestration demand was calculated using ArcGIS 10.3.

2.5. Application of Gravity Modeling

To understand the degree of correlation and the extent of spatial correlation between the region’s carbon sequestration supply and demand, we introduce the concept of regional supply and demand gravity. This concept posits that the influence of the carbon sequestration services’ supply area on the external demand area is related to the distance between them [25]. Leveraging the gravitational formula from Newtonian mechanics, we incorporate the gravitational model to effectively characterize the correlation relationship resulting from the flow between the region’s carbon sequestration supply and demand. This model quantitatively expresses the significance of the transmission corridor facilitating this correlation. The gravity model captures the spatial flows of geographic elements and the strength of interconnections between different elements, operating at both individual and macro levels [26]. A higher gravitational force between two locations indicates that the transport corridor connecting them facilitates a greater flow of sequestered carbon streams, signifying its heightened importance [27,28]. The gravity model is calculated as follows:

$$\mathrm{F}=\mathrm{G}{\displaystyle \frac{{\mathrm{M}}_{\mathrm{i}}-{\mathrm{M}}_{\mathrm{j}}}{{\mathrm{d}}^{\mathrm{r}}}}$$

(7)

where F denotes the magnitude of the gravitational force (degree of connection) between supply and demand places; M_i and M_j denote the supply and demand of carbon sequestration services in supply place i and demand place j after equal weight superposition; d is the spatial transmission distance between the two places, i.e., the length of the transmission corridor; r is the resistance coefficient (r = d/l), l is the straight-line distance between the two places, and r is 1 because d = l in this paper; G is the gravitational force coefficient, which is the mutual attraction relationship between different individuals, and G is set to 1 in this paper.

3. Results

3.1. Supply Characteristics of Carbon Sequestration

To validate the simulation outcomes of the carbon sequestration model, we conducted a comparative analysis of the NEP results simulated by the model with findings from existing studies. The mean NEP value in the Hainan tropical rainforest was determined to be 277.78 gC/(m²·yr), slightly surpassing the NEP estimate in the region reported by Chen et al. [29], yet falling below the estimates provided by Ye et al. and Zhang et al. [30,31]. Furthermore, the spatial pattern of NEP exhibited fundamental consistency with the outcomes of the aforementioned studies, affirming that the simulated NEP in this study effectively captured the carbon sequestration services of terrestrial ecosystems in Hainan.

According to our model calculations, the total carbon sequestration supply (NEP) in Hainan was estimated to be 4.74 × 10⁶ tC/yr in 2020, with an average capacity of 140.04 gC/(m²·yr). Notably, areas where the NEP is negative (indicating carbon sources) constitute a relatively small and scattered portion, suggesting that Hainan, as a whole, falls under the category of a carbon sink. High carbon sequestration supplies are concentrated in the central and southern mountainous regions, while low supplies can be observed in the northern plains (Figure 3). Forest ecosystems account for more than 75% of the total carbon sequestration services.

From 2000 to 2020, the carbon sequestration supply in Hainan showed an upward trend, increasing by 4.42 gC/(m²·yr). However, it is crucial to note that not all regions followed a positive trend (Figure 4). In aggregate, the carbon sequestration supply increased in the central mountainous areas, particularly in the Hainan Tropical Rainforest National Park, which is obvious and statistically significant. Due to the higher altitude of the region, dense forest vegetation, and less anthropogenic interference, the natural recovery process of the vegetation is in a better condition and the carbon sequestration is significantly enhanced. Conversely, in the northern plains and the eastern artificial cultivation areas, there was a significant decrease in carbon sequestration supply due to the higher intensity of human activities [32]. This is consistent with the result of Hainan Island and other regions, such as the Qinling Mountain, and Yangtze River Delta [27,32,33].

3.2. Demand Characteristics of Carbon Sequestration

The total demand for carbon sequestration services (carbon emissions) in Hainan is estimated to be 13.35 × 10⁶ tC/yr in 2020, with an average capacity of 3.94 tC/(ha·yr). Unlike the spatial distribution of the supply, the demand is generally higher in the northern plains and lower in the central and southern mountainous regions (Figure 5). Areas with high demand values are predominantly concentrated around urban and cropland areas, where human activities are intense, and land development and utilization are frequent [34]. This leads to a considerably higher demand for carbon sink services in these areas compared to other regions.

Over nearly the past 20 years, the total carbon sequestration demand in the study area exhibited an increasing trend, likely influenced by various economic development policies and the intensity of urbanization expansion. The overall carbon sequestration demand in Hainan demonstrated an upward trajectory from 2000 to 2020, reflecting an increase of 0.72 tC/(ha·yr). With the migration of populations from townships to cities, especially large- and medium-sized cities (e.g., Haikou, Sanya), the population growth is very obvious. This population increase will lead to an increase in energy consumption and transportation demand, thereby increasing carbon emissions (i.e., carbon sink demand). In general, most urban areas show a significant growth trend (Figure 6), especially in the large- and medium-sized cities in Hainan, where the demand for carbon sinks has increased significantly. On the contrary, in the central mountainous areas, there is no significant change in carbon sequestration demand due to the rugged terrain and sparse human activities. In aggregate, the carbon sequestration demand increased in the northern plains, particularly in Haikou City (the capital of Hainan).

3.3. Spatially Linked Characteristics of Carbon Sequestration

To explore the impact of Hainan’s abundant carbon sequestration services on neighboring demand areas, we used a gravity model to analyze the spatial variability of the carbon sequestration services provided by different supply areas to each demand area. On the demand side, towns in the northern plains and coastal cities in the southern part of Hainan exhibit the highest demand for carbon sequestration, constituting over 80% of the total demand. These areas require the largest amount of carbon sequestration services from external suppliers. In contrast, towns and cities in the central mountainous areas, characterized by a sparse population distribution, have the smallest cumulative demand for carbon sequestration and the lowest need for external carbon sequestration services (

Table 2. Supply and demand matrix of carbon sequestration in Hainan.

Region	County	Supply–Demand Balance (105 tC)
		2000	2005	2010	2015	2020
Central mountains	Baisha	3.89 (+)	4.01 (+)	3.89 (+)	3.28 (+)	3.13 (+)
	Baoting	1.36 (+)	1.16 (+)	0.89 (+)	0.59 (+)	0.31 (+)
	Changjiang	1.18 (+)	1.11 (+)	0.84 (+)	0.14 (+)	0.04 (+)
	Qiongzhong	5.56 (+)	5.68 (+)	5.29 (+)	5.1 (+)	5.09 (+)
	Wuzhishan	2.82 (+)	2.88 (+)	2.69 (+)	2.53 (+)	2.42 (+)
Northern plains	Haikou	−7.02 (−)	−8.99 (−)	−13.98 (−)	−26.28 (−)	−35.28 (−)
	Chengmai	−0.91 (−)	−1.17 (−)	−1.78 (−)	−4.14 (−)	−5.08 (−)
	Ding an	−0.89 (−)	−0.99 (−)	−1.45 (−)	−2.66 (−)	−3.17 (−)
	Lin gao	−1.54 (−)	−1.73 (−)	−2.39 (−)	−4.35 (−)	−5.2 (−)
	Qionghai	−1.67 (−)	−1.93 (−)	−2.83 (−)	−4.91 (−)	−5.86 (−)
	Tunchang	0.19 (+)	0.11 (+)	−0.28 (−)	−1.45 (−)	−1.93 (−)
	Wenchang	−2.19 (−)	−2.43 (−)	−3.49 (−)	−5.83 (−)	−6.88 (−)
	Danzhou	−2.84 (−)	−3.32 (−)	−4.67 (−)	−9.02 (−)	−10.89 (−)
Southern coastal	Dongfang	1.21 (+)	0.98 (+)	0.47 (+)	−1.21 (−)	−1.97 (−)
	Ledong	2.53 (+)	1.89 (+)	1.31 (+)	−0.27 (−)	−1.21 (−)
	Lingshui	−0.85 (−)	−1.06 (−)	−1.71 (−)	−2.92 (−)	−3.6 (−)
	Wanning	−1.35 (−)	−1.74 (−)	−2.8 (−)	−4.89 (−)	−5.97 (−)
	Sanya	0.32 (+)	−0.71 (−)	−2.4 (−)	−6.29 (−)	−9.73 (−)

Notes: + indicates a surplus of carbon flows, − indicates a loss of carbon flows.

). From the supply side, the ecosystems in the central mountainous areas of Hainan are abundant in carbon sequestration resources, primarily due to the dense distribution of forests, enabling them to provide substantial carbon sequestration capacity. Conversely, the surrounding plains exhibit a lower total cumulative carbon sequestration supply due to the sparse distribution of forest resources (Table 2).

Using the gravity model, we can derive a ratio for the amount of sequestration services provided by supply zones with carbon flow surpluses to demand zones with carbon flow deficits. Multiplying this ratio by the total sequestration surplus in the supply zone gives the actual amount of sequestration services provided by the supply zone to the demand zone. Figure 7 illustrates the strength and spatial variation of the linkages between sequestration service flows of supply and demand. The degree of carbon sequestration contribution from the central mountainous region to neighboring regions is reflected by the thickness of the connecting lines between the regions in the figure. We found that the neighboring demand areas benefiting the most are mainly Haikou City, Danzhou City, and Sanya City, each receiving carbon flows greater than 60 × 10³ tC/yr. Conversely, other coastal cities and counties benefit less, with carbon flows of less than 30 × 10³ tC/yr. From 2000 to 2020, the intensity of the carbon sequestration from the central mountainous region to neighboring areas shows a decreasing trend year by year, indicating that the central mountainous region’s own carbon sequestration surplus is also decreasing due to Hainan’s urban expansion and population growth. This is consistent with the findings of Yang et al., who used the InVEST model to estimate the variation of the carbon storage capacity [35]. Additionally, with the rapid development of megacities such as Haikou in the north and Sanya in the south, the contribution of the central mountainous region to these cities also shows an increasing trend. The linkage of carbon sequestration supply and demand between them becomes closer, and the high-value areas of carbon sequestration service transmission exhibit a characteristic of concentrating in a longitudinal belt from Haikou to the central mountainous region and then to Sanya.

The chord diagram of carbon sequestration services provides a clear picture of the flow and direction of carbon sequestration services provided by supply counties, as well as the distribution and contribution of the services in demand counties (Figure 8). From 2000 to 2020, the number of supply counties decreased from 9 to 5, while the number of demand counties increased from 9 to 13. Among them, Sanya City experienced the most significant change, shifting from a supplying role in 2000 to a demanding role and becoming one of the top three areas in terms of carbon sequestration demand by 2020. The central mountainous regions of Qiongzhong, Baisha, and Wuzhishan are the most important carbon sequestration supply areas in Hainan, with their proportion of the total carbon sequestration rising from 65% in 2000 to more than 95% in 2020. In 2000, Haikou, Danzhou, and other major demand areas in Hainan benefited from more than 60% of the carbon sequestration from the central mountainous region, and by 2020 this proportion had risen to more than 95%. All indications are that the central mountainous area has become the most critical safety barrier for Hainan’s carbon sink services.

4. Discussion

Over the past two decades, Hainan’s built-up land area has rapidly expanded. This expansion is typically accompanied by changes in land cover types, often involving reductions in forested grassland and cropland [36]. Such land use changes can cause significant disturbances to natural ecosystems, resulting in a decline in ecosystem service carbon sequestration and loss of ecological diversity [30]. Notably, the spatial distribution of construction land growth primarily involves the conversion of cropland and some forested grassland around the city [37]. These changes are concentrated mainly in Haikou City in northern Hainan and Sanya City in the south. Simultaneously, amidst China’s robust ecological protection policies, the mountainous regions of central Hainan have implemented a series of ecological measures to address ecological degradation. These measures include reforestation, returning farmland to forests, and closing mountains to forests. The implementation of these policies has played a role in mitigating the negative impacts of peri-urban land use changes on carbon sequestration services. Initiatives such as returning farmland to forests and afforestation increase the forest cover, enhance the carbon fixation capacity in the central mountainous region, and improve the ecosystem service functions of forest ecosystems [16].

With Hainan’s economic development and urbanization over the past 20 years, the demand for carbon sequestration in the main urban areas of Hainan has seen a significant increase. In terms of spatial distribution, the areas with a higher degree of demand are primarily situated in the northern plains and the southern coastal towns and cities, characterized by intense human activities in urban areas, resulting in a heightened demand for carbon emissions [38]. The elevated demand for carbon sequestration in urbanized areas can be attributed to several factors. Firstly, the spatial expansion of towns and cities necessitates substantial land for construction, potentially leading to de-forestation and land development, thereby diminishing the local carbon sequestration capacity [38]. Secondly, the influx of large populations into urban spaces increases the population density in urbanized areas. The more productive and residential activities result in higher energy consumption, contributing to increased carbon emissions [39]. Thirdly, regional development often requires extensive transportation and infrastructure development, including roads and bridges. The construction and operation of these infrastructure projects typically demand significant energy, leading to elevated carbon emissions [39]. Moreover, the supply and demand for carbon sequestration services in Hainan exhibit noticeable geographical differences. Generally, the areas with high supply capacity are predominantly located in the central high mountainous region, characterized by lush vegetation, limited land development, and a high potential for carbon sequestration supply. Conversely, the regions with high demand levels are primarily situated in the coastal plain region, with a developed economy and intense human activities, resulting in elevated demand for carbon emissions [40]. This overall imbalance between supply and demand underscores the necessity for improved coordination in carbon management and climate change policies to achieve a more favorable carbon balance.

In this study, we estimated the supply and demand of carbon sequestration services in Hainan and identified the corresponding areas. Additionally, we introduced a gravity model to compare the spatial differences in the amount of carbon sequestration services across different supply–demand correlation areas. The model primarily considered the impacts of material quantities and spatial straight-line distances between the supply and demand areas on regional sequestration service replenishment. We acknowledge some limitations, such as the exclusive consideration of the material quantity and spatial straight-line distance while neglecting other influences such as climate change and socio-economic factors. Climate change is a significant factor affecting carbon sequestration services. It influences plant growth by affecting the net primary productivity and turnover rate of litter and root biomass through changes in precipitation and temperature. For every one percent increase in precipitation in arid regions, the aboveground biomass increases by 1.9%, while in humid regions, it increases by 0.5% [41]. The climate in different regions can also impact the abundance and persistence of soil organic carbon. Socio-economic factors are also crucial in influencing carbon sequestration services. Higher levels of economic development, larger populations, stronger energy demands, and greater consumption of material resources lead to increased carbon emissions [42]. The urbanization process, primarily characterized by the expansion of artificial land use, results in the conversion of large amounts of cultivated land, forestland, and grassland into artificial land. This not only accelerates threats to the structure and function of ecosystems leading to the loss of ecosystem services but also continuously weakens carbon sequestration functions. The research results from Haikou also confirm that urbanization and population growth increase the demand for carbon sequestration. From 2000 to 2020, Haikou experienced accelerated urbanization in Hainan Province, with the population increasing from 0.83 million to 3.0 million and the urbanization rate rising from 57.34% to 84.39%, further leading to an imbalance in the supply and demand of carbon sequestration. The methodology employed in this study effectively identifies the degree of correlation and spatial matching relationship between the supply and demand. The results emphasize the importance of the carbon sequestration services supplied by the mountain ecosystems in central Hainan to the neighboring coastal areas. Additionally, the study clarifies the spatial variability in the quality of these services from the central mountainous region, the primary supply area, to each demand area. These findings provide valuable data support for the development of subsequent ecological compensation policies.

5. Conclusions

This research paper presents the results of a comprehensive study to understand the dynamics of the carbon sequestration supply and demand on Hainan Island, China—a region vital for global biodiversity conservation and carbon cycling. Faced with the escalating crisis of climate change and its implications, this study has pioneered an innovative approach to address these issues. It was grounded in the growing global consensus to counteract climate change by enhancing carbon sequestration capacities and adopting sustainable development practices. Central to this study was the examination of Hainan Island’s carbon sequestration dynamics, expanding beyond the traditional focus on individual ecosystems to encompass the entire terrestrial ecosystem, thereby filling a significant gap in the existing literature. Leveraging a rich dataset that includes land use and land cover, NDVI, meteorological information, and soil data, we performed a detailed analysis of the spatial and temporal distribution of the carbon sequestration supply and demand across the island. The results revealed a nuanced picture; there has been an overall increase in the carbon sequestration supply over the past two decades, particularly in the central mountainous regions of Hainan. This finding is crucial for understanding regional carbon dynamics and underscores the effectiveness of recent conservation efforts. However, these positive developments in carbon sequestration supply are counteracted by a marked increase in demand, especially in urbanized areas. This imbalance highlights the challenges posed by rapid economic development and urbanization, which intensify carbon emissions.

The innovative application of the gravity model in our study distinguishes it from previous research. By integrating this model, we successfully quantified the flow of the carbon sequestration services between different regions, illuminating the interconnected nature of these services. This approach has revealed the central mountainous regions of Hainan as crucial suppliers of services to high-demand urban areas, given their richness in carbon sequestration resources. The significance of our study extends beyond its academic contribution to understanding carbon sequestration dynamics; it also holds practical implications. These insights are invaluable for the assessment of carbon sequestration effectiveness and the strategic planning and implementation of ecological compensation policies in the Hainan Tropical Rainforest National Park. The national park contains the most concentrated, strictly preserved, and extensive tropical rainforest in China. This study can be used to evaluate the level of carbon sequestration and assess the role of institutional effectiveness on carbon sequestration. Based on the results, there are four suggestions for improved carbon sequestration management in Hainan Island. Firstly, it is recommended to strengthen ecosystem monitoring and evaluation efforts and establish and improve a unified provincial system for natural resource surveying, monitoring, and evaluation, encompassing institutional frameworks, organizational structures, standard systems, technical systems, quality management systems, and application service systems. Secondly, the regulation of land use should be strengthened through hierarchical and categorized management. The dominant functions of various functional zones, such as farmland protection zones, rural development zones, ecological protection zones, ecological control zones, urban development zones, and marine development zones, should be clarified. Based on the functional orientation of each zone, industrial development plans, spatial plans, and other factors, a differentiated ecological environmental permit list should be compiled. Thirdly, it is necessary to establish a rigorous evaluation system for ecological environmental protection, clarifying key issues such as who will evaluate, whom will be evaluated, what will be evaluated, how to conduct the evaluation, how to use the results, and how responsibilities will be pursued. The final suggestions are leveraging Hainan’s ecological advantages, continuously optimizing the structure of the energy industry, and promoting green and low-carbon development. In terms of innovation, this study stands out by providing a comprehensive view of the carbon balance within the terrestrial ecosystem, a perspective that has been relatively underexplored in the prior research. The novel application of the gravity model in this context enhances our understanding of the spatial relationships involved in carbon sequestration services. However, this study had certain limitations, particularly the focus on physical quantities and distances in the gravity model, which may outweigh other crucial factors such as socio-economic changes and climate variations. Future research endeavors could build upon this foundation by incorporating these additional elements to offer a more comprehensive understanding of the dynamics of carbon sequestration.

In conclusion, our study represents a significant advancement in environmental science, offering fresh perspectives on the carbon sequestration capabilities of Hainan Island and introducing a novel methodological approach with potential for broader applications. The insights gleaned from this research contribute not only to the academic discourse but also hold practical significance for policymakers and environmental managers. This study underscores the critical importance of balancing ecological conservation with the imperatives of human development.