Spatial-Temporal Evolution, Trade-Offs and Synergies of Ecosystem Services in the Qinba Mountains

Abstract

A scientific understanding of the trade-offs and synergies of ecosystem services is the prerequisite for maintaining the structure, function and health of forest ecosystems, which is conducive to promoting the “win-win” goal of economic development and ecological protection. As an important ecological function area in China, the Qinba Mountain region is responsible for important ecological services such as biodiversity conservation and water conservation, and exploring the trade-offs and synergistic relationships of ecosystem services is important for ecological conservation and high-quality development in this region. This paper analyzes the spatial and temporal characteristics of water conservation, soil conservation, carbon sequestration, and habitat quality services and their synergistic/balancing relationships in the Qinba Mountain region from 1990 to 2020 using tools such as the InVEST model, ArcGIS, and Matlab. The results showed that (1) the overall trend of water connotation, soil conservation and carbon sequestration in the Qinba Mountains is increasing, while the trend of habitat quality is fluctuating and decreasing. The spatial distribution pattern of water conservation and soil conservation services was “high in the southwest and low in the northeast”, while the spatial pattern of habitat quality services was the opposite; the spatial distribution pattern of carbon sequestration services was “low in the middle and high around”. (2) Habitat quality in the Qinba Mountains has a trade-off relationship with water connotation and soil conservation, as water connotation–soil conservation is a synergistic relationship, while carbon sequestration is unrelated to water connotation–soil conservation and habitat quality. (3) The area of habitat quality–water conservation showed a decreasing trend; the area of habitat quality–soil conservation showed an increasing trend; the area of habitat quality-water conservation showed a decreasing trend; the area of habitat quality-soil conservation showed an increasing trend; the area of water conservation-soil conservation service synergistic relationship showed a decreasing and then increasing trend; while the area of carbon sequestration service and In most of the regions, carbon sequestration, soil conservation and habitat quality services are not related to each other.

1. Introduction

Ecosystem services are the direct and indirect contributions of ecosystems to human well-being and are important for global sustainable development [1]. However, there are significant differences in human needs and preferences for different ecosystem services, and if one ignores the fact that ecosystems are integrated and holistic systems, the blind pursuit of maximizing the production value of one service will lead to a significant decline in another ecosystem service and the trade-off/synergy between various ecosystem services is constantly shifting [2,3]. Significant loss of biodiversity and serious decline in ecosystem services causes secondary damage to the ecosystem [4]. In order to reduce the negative effects of interrelationships between services, ecosystem service trade-off/synergistic relationships have become an important topic in current ecosystem service research.

In recent years, the spatial and temporal dynamics of regional-scale ecosystem services and their trade-offs and synergistic relationships have increasingly become the focus of ecosystem services research [5], and proper cognition of the trade-off/synergistic relationship between ecosystem services can help carry out sustainable management decisions of multiple ecosystem services and improve ecological management efficiency. Current research mostly uses statistical analysis, geospatial mapping, scenario simulation, and other methods [6,7,8], spatial patterns and mechanisms of ecosystem service trade-offs/synergistic relationships with the help of InVEST model, CASA model and ARIES model [9]. For example, Morán-Ordóñez et al. [10] conducted a scenario simulation analysis of trade-offs/synergies in Mediterranean forest ecosystem services and found that the key factors affecting future ecosystem services are mainly policy management rather than climate. Ren et al. [11] evaluated the ecosystem services in the middle Yellow River region by using InVEST model, analyzed the trade-offs and synergies of ecosystem services by using correlation analysis method, and introduced pixel-by-pixel correlation analysis method to measure the spatial distribution and change the trend of the trade-off and synergy region among ecosystem services. Feng et al. [12] quantified ecosystem services and trade-offs in the Ansai watershed of the Loess Plateau and used redundancy analysis to explore the driving role of environmental factors on ecosystem service relationships. Ran et al. [13] concluded that the spatial variation of ecosystem service trade-offs and synergies in Poyang Lake is closely related to the spatial distribution of land use types. Most of the existing studies are based on the scales of watersheds and municipalities, and there are relatively few studies on the trade-offs and synergistic relationships of ecosystem supply and regulation services in key ecological function areas. Currently, it is urgent to identify the ecosystem service trade-offs/synergies among mountain ecosystems with important ecological functions, so as to provide a decision-making basis for promoting ecological protection and realizing sustainable management of resources and environment.

Key ecological function areas are the core components of the global ecosystem and are key areas for ecological service functions and national ecological security protection. In 2010, the Chinese government released the National Plan for Main Function Areas, which delineates 25 key ecological function areas whose main function is to provide ecosystem services. In order to realize their main functions, all kinds of development activities are strictly controlled within the national key ecological function zones to free up more space for maintaining the virtuous cycle of the ecosystem [14,15]. As an important ecological barrier area in China, the Qinba Mountains bear important ecosystem service functions. However, due to the interference of human activities and natural disasters, the ecological environment in the Qinba Mountains is fragile, and the conflict between conservation and development is very serious [16]. Clarifying the interactions among multiple ecosystem services in the Qinba Mountains will not only contribute to the sustainable management of resources and environment, but also to the promotion of socio-economic green transformation and sustainable development in the mountains. Facing the diversified characteristics of ecosystem services in the key ecological function areas, this paper takes the key ecological function areas as the target and selects the Qinba Mountain area as the reference unit to identify and analyze the spatial and temporal evolution characteristics of regional ecosystems from the perspectives of ecological space and ecosystem services and to explore the characteristics of changes in the trade-off/synergistic relationship of ecosystem services, so as to provide a decision reference for exploring the policy effects of key ecological function areas and serving ecological conservation management decisions.

2. Materials and Methods

2.1. Study Area

The Qinba Mountains span 80 counties (cities and districts) in 5 provinces and 1 city in Henan, Hubei, Chongqing, Sichuan, Shaanxi and Gansu [17] (Figure 1a), with a total land area of 2.25 × 10⁵ km², and are an important boundary between the north and south geographical divisions of China, as well as a north–south climate intersection and boundary line, straddling two climate zones, warm temperate and subtropical (Figure 1b). The Qinba Mountains are rich and diverse in natural vegetation types and biological resources, and are an important ecological security barrier in central China [18]. The Qinba key ecological function area is the first batch of key ecological function areas in China, occupying the main area of the Qinling Mountains and Daba Mountains, with plains or basins on the north and south sides [19]. At the same time, the fragile ecological environment, shortage of land resources, severe soil erosion and frequent natural disasters in the region have combined to constrain the sustainable development of the Qinba Mountains.

2.2. Data Sources

The data needed for this study include meteorological data, soil attribute data, land use data and digital elevation model (DEM) data. Among them, precipitation and other meteorological data from 1990 to 2020 were obtained from the China Meteorological Science Data Sharing Service Network (http://data.cma.cn/, accessed on 1 March 2022). The data on soil properties in Qinba Mountains were obtained from the Chinese soil dataset of the World Soil Database (v1.1) (https://data.tpdc.ac.cn/, accessed on 5 March 2022). The land use data and DEM data, respectively, are from the Resource and Environment Science and Data Center (http://www.resdc.cn/, accessed on 5 March 2022) and geospatial data cloud (http://www.gscloud.cn/, accessed on 1 March 2022). The raster data were uniformly resampled to 100 m, and the projection coordinate system was uniformly adopted from Albers_cgcs 2000.

2.3. Research Methodology

(1)

Water Yield model

In this study, the water yield module in the integrated valuation of ecosystem services and trade-offs (InVEST) model was used to calculate the water-saving capacity of the Qinba Mountain Area, which is an important water resource international functional area in our country [20]. The specific calculation method is:

$${\mathit{Y}}_{\mathit{xj}}=\left(1-\frac{{\mathit{AET}}_{\mathit{xj}}}{{\mathit{P}}_{\mathit{x}}}\right){\times \mathit{P}}_{\mathit{x}}$$

(1)

$$\frac{{\mathit{AET}}_{\mathit{xj}}}{{\mathit{P}}_{\mathit{x}}}=\frac{{1+\omega }_{\mathit{x}}{\mathit{R}}_{\mathit{xj}}}{{1+\omega }_{\mathit{x}}{\mathit{R}}_{\mathit{xj}}+\left(\frac{1}{{\mathit{R}}_{\mathit{xj}}}\right)}$$

(2)

$${\omega }_{\mathit{x}}=\mathit{Z}\frac{{\mathit{AWC}}_{\mathit{x}}}{{\mathit{P}}_{\mathit{x}}}$$

(3)

$${\mathit{R}}_{\mathit{xj}}=\frac{{\mathit{K}}_{\mathit{xj}}{\times \mathit{ET}}_{\mathit{x}}}{{\mathit{P}}_{\mathit{x}}}$$

(4)

where Y_xj is the annual water yield of land cover type j in raster cell x; AET_xj is the actual evapotranspiration of land cover type j in raster cell x; P_x is the precipitation of raster cell x; ω_x is the ratio of the annual water availability of the modified vegetation to the precipitation; R_xj is the drying index; Z is the Zhang coefficient, the reference to the relevant literature takes the value of 30 [21]; AWC_x is the effective soil water content of grid cell x; K_xj is the vegetation evapotranspiration coefficient for land cover type j in raster cell x; and ET_x is the reference crop evapotranspiration [22].

(2)

Sediment Delivery Ratio (SDR) model

Soil erosion is the main ecological and environmental problem that hinders the socio-economic development of the Qinba Mountains. Although soil erosion in the Qinba Mountains has improved in recent years, soil and water conservation is still a key concern in achieving sustainable development in the region [23]. Therefore, the sediment delivery ratio module in InVEST model was used to analyze the soil conservation capacity in the Qinba Mountain area. The calculation formula is as follows:

$$SEDRE{T}_x=RKL{S}_x-USL{E}_x$$

(5)

where SEDRET_x is the soil retention of raster cell x, RKLS_x and USLE_x denote the potential soil erosion and actual soil erosion, respectively. The specific calculation formula is as follows:

$$RKL{S}_x=R_x\times K_x\times L{S}_x$$

(6)

$$USL{E}_x=R_x\times K_x\times L{S}_x\times C_x\times P_x$$

(7)

where R_x is rainfall erosion force; K_x is soil erodability; LS_x is slope length-slope factor; C_x is vegetation cover factor; and P_x is management factor [22].

(3)

Carbon Storage and Sequestration model

The carbon sequestration service in the Qinba Mountains was calculated using carbon storage and sequestration in the InVEST model. The carbon density data is referenced from the relevant literature with the following equation [24]:

$$G=G_{above}+G_{dead}+G_{below}+G_{soil}$$

(8)

where G, G_above, G_below, G_dead and G_soil were total carbon sequestration of the ecosystem, aboveground carbon sequestration, underground carbon sequestration, dead organic carbon sequestration and soil carbon sequestration.

(4)

Habitat quality model

Habitat quality refers to the ability of an ecosystem to provide the necessary living conditions for the survival and reproduction of individuals and populations (the model assumes that areas with high habitat quality can better support all levels of biodiversity). The habitat quality map is generated by combining the information on landscape type sensitivity and biodiversity threat factors [25]. The calculation formula is as follows [26]:

$$Q_{ij}=H_j\times \left[1-D_{xj}^z÷\left(D_{xj}^z+k^z\right)\right]$$

(9)

where Q_ij is the habitat quality of raster cell x in land use type j in the study area, which ranges from 0 to 1, the closer to 1, the better its habitat quality, and vice versa; H_j is the habitat suitability of land use type j; D^z_xj is the degree of habitat degradation of raster cell x in land use type j in the study area; k is the half-saturation parameter, usually half of the degree of livelihood degradation; and z is the default parameter of the model [27].

(5)

Trade-off/synergistic relationships of ecosystem services

We use image-by-image correlation analysis to determine the trade-off/synergy relationship at the image scale by the positive and negative values of the correlation coefficients between two groups of ecosystem services and the bivariate correlation analysis of ecosystem services in the Qinba Mountains from 1990 to 2020 to construct the correlation matrix between ecosystem services [11]. The specific formula is as follows:

$$R=\frac{\sum \left(x-\overline{x}\right)\left(y-\overline{y}\right)}{\sqrt{\sum {\left(x-\overline{x}\right)}^2-{\left(y-\overline{y}\right)}^2}}$$

(10)

where R represents the trade-off/synergistic relationships of two ecosystem services. If it is positive, the relationship between the two services is synergy; otherwise, it is trade-off. If R is 0, there is no correlation.

3. Results

3.1. Spatial-Temporal Characteristics of Ecosystem Services

3.1.1. Spatio-Temporal Characteristics of Water Conservation Services

From 1990 to 2020, water conservation in the Qindaba Mountains generally showed a “W” type trend, from 8.58 × 10⁹ m³ in 1990 to 10.20 × 10⁹ m³ in 2020, with an increase of 18.88% (Figure 2a). Among them, from 1990 to 1995, water conservation decreased significantly from 8.58 × 10⁹ m³ to 6.69 × 10⁹ m³, with a decrease of 22.03%; from 1995 to 2005, the water conservation increased significantly, from 6.69 × 10⁹ m³ to 10.52 × 10⁹ m³, with an increase of 57.25%; from 2005 to 2015, water conservation decreased significantly from 10.52 × 10⁹ m³ to 6.98 × 10⁹ m³, with a decrease of 33.65%; there is another increase from 2015–2020, from 6.98 × 10⁹ m³ to 10.20 × 10⁹ m³, with an increase of 50.72%. In terms of the coefficient of variation, the coefficient of variation of water conservation services in the Qinba Mountains area showed a fluctuating downward trend from 1990 to 2020 (Figure 3), with a decrease of 8.36%, indicating that the regional differences in water conservation services tend to decrease.

From 1990 to 2020, the spatial distribution pattern of water conservation services in the Qinba Mountains was generally “high in the southwest and low in the northeast”, with the high-value areas expanding first and then contracting, and the low-value areas contracting in the opposite direction (Figure 4(a1–g1)). Specifically, from 1990 to 1995, the high-value area contracted while the low-value area expanded significantly, such as the southern part of the Longnan Mountains, the western part of the Qinling Mountains and the northern part of the Sichuan Basin. From 1995 to 2000, the high-value area expanded from the Daba Mountains and some counties in the northern Sichuan Basin. From 2000 to 2005, the spatial change in water conservation service was not obvious. From 2005 to 2015, the high-value area shrank gradually, while the low-value area expanded continuously in the Longnan Mountains, Qinling Mountains and Daba Mountains. From 2015 to 2020, the high-value area expanded from some counties at the southern foot of the Daba Mountains to the Three Gorges Reservoir area, and the Longnan Mountains and the western Qinling Mountains contracted significantly.

3.1.2. Spatio-Temporal Characteristics of Soil Conservation Services

From 1990 to 2020, soil conservation in the Qinba Mountains area showed an upward trend of fluctuation, from 1.40 × 10¹⁰ t in 1990 to 2.07 × 10¹⁰ t in 2020, with an increase of 47.86% (Figure 2b). Among them, from 1990 to 2010, soil conservation showed a year-on-year increasing trend, from 1.40 × 10¹⁰ t to 2.07 × 10¹⁰ t, with an increase of 47.86%; from 2010 to 2015, the soil conservation decreased significantly, from 2.95 × 10¹⁰ t to 1.46 × 10¹⁰ t, with a decrease of 50.51%; from 2015 to 2020, soil conservation increased significantly again, from 1.46 × 10¹⁰ t to 2.07 × 10¹⁰ t, with an increase of 41.78%. In terms of the coefficient of variation, the coefficient of variation of soil conservation services in the Qinba Mountains area showed a fluctuating upward trend from 1990 to 2020 (Figure 3), with an increase of 19.40%, indicating that the regional differences in soil conservation services tend to increase.

In terms of spatial distribution, the soil conservation services in the Qinba Mountains area showed a spatial distribution pattern of “high in the southwest and low in the northeast” from 1990 to 2020 (Figure 4(a2–g2)). The high-value areas for soil conservation services were mainly distributed in the southern Longnan Mountains, the northern Sichuan Basin, the central and western Qinling Mountains, the Daba Mountains and the Three Gorges Reservoir area. The low-value areas were mainly distributed in the eastern Qinling Mountains, Funiu Mountains and Han River valley basin. Specifically, from 1990 to 2010, the high-value area of soil conservation services was in an expansion state, gradually expanding from the Longnan Mountains and the northern part of the Sichuan Basin to the central and western Qinling Mountains, the central Funiu Mountains and the Daba Mountains; from 2010 to 2015, on the contrary, the high-value area was in a contraction state; from 2015 to 2020, the high-value area expanded again, with the high-value area extending northward from the southern Longnan Mountains and shifting from the Daba Mountains to the Three Gorges Reservoir area.

3.1.3. Spatio-Temporal Characteristics of Carbon Sequestration Services

From 1990 to 2020, the carbon sequestration in the Qinba Mountains area showed an upward trend of fluctuation, from 1.539 × 10⁹ t in 1990 to 1.543 × 10⁹ t in 2020, with an increase of 0.26% (Figure 2b). Among them, from 1990 to 1995, carbon sequestration increased significantly, from 1.539 × 10⁹ t to 1.556 × 10⁹ t, with an increase of 1.30%; from 1995 to 2000, carbon sequestration decreased significantly, from 1.556 × 10⁹ t to 1.534 × 10⁹ t, with a decrease of 1.41%; from 2000 to 2010, carbon sequestration showed an upward trend, from 1.534 × 10⁹ t to 1.544 × 10⁹ t, with an increase of 0.65%; from 2010 to 2015, carbon sequestration showed a downward trend, from 1.544 × 10⁹ t to 1.541 × 10⁹ t, with a decrease of 0.19%; and from 2015 to 2020, carbon sequestration increased again, from 1.541 × 10⁹ t to 1.543 × 10⁹ t, with an increase of 0.13%. In terms of the coefficient of variation, the coefficient of variation of carbon sequestration services in the Qinba Mountains area showed a weak increase from 1990 to 2020 (Figure 3), with an increase of 0.70%, indicating that the regional differences in carbon sequestration services tend to slightly increase.

In terms of spatial distribution, the carbon sequestration in the Qinba Mountains shows a spatial distribution pattern of “low in the middle and high in the surroundings” (Figure 4(a3–g3)). The high-value areas are mainly located in the western part of the Qinling Mountains, the Funiu Mountains, the Daba Mountains and the Three Gorges Reservoir area; the low-value areas are mainly located in the Hanjiang River valley basin and the Jialing River valley. Overall, the overall spatial distribution pattern of carbon sequestration services from 1990 to 2020 has changed relatively little. Among them, some counties in the northwestern part of the Sichuan basin and the Funiu mountain range area have increased carbon storage, while the carbon storage in the river valley basin and other counties has slightly decreased.

3.1.4. Spatio-Temporal Characteristics of Habitat Quality Services

From 1990 to 2020, the habitat quality in the Qinba Mountains area showed a downward trend of fluctuation, from 0.6969 in 1990 to 0.6843 in 2020, with a decrease of 1.81% (Figure 2b). Among them, from 1990 to 1995, habitat quality increased significantly, from 0.6969 to 0.7129, with an increase of 2.30%; from 1995 to 2000, habitat quality decreased significantly, from 0.7129 to 0.6977, with a decrease of 2.13%; from 2000 to 2005, habitat quality increased significantly, from 0.6977 to 0.7000, with an increase of 0.33%; from 2005 to 2010, habitat quality showed a downward trend again, from 0.7000 to 0.6803, with a decrease of 2.81%; from 2010 to 2020, habitat quality increased again, from 0.6803 to 0.6843, with an increase of 0.59%. In terms of the coefficient of variation, the coefficient of variation of habitat quality services in the Qinba Mountains area showed an increasing trend from 1990 to 2020 (Figure 3), with an increase of 5.70%, indicating that the regional differences of carbon sequestration services tend to increase.

In terms of spatial distribution, the habitat quality services in the Qinba Mountains area showed a spatial distribution pattern of “low in the southwest and high in the northeast” (Figure 4(a4–g4)). The overall spatial distribution pattern of habitat quality changed little from 1990 to 2020, and only a small part of counties in the western Hanjiang River Valley Basin had a slight decline in habitat quality. In general, the high-value areas are mainly distributed in the northwest of the Sichuan Basin, the west of the Qinling Mountains, the Daba Mountains, the Funiu Mountains and the north of the Three Gorges Reservoir area. The low-value areas are mainly distributed in the Hanjiang River valley basin and the northern part of the Sichuan Basin.

3.2. Trade-Offs and Synergies of Ecosystem Services

The correlation coefficients of carbon sequestration and habitat quality in the Qinba Mountains area were positive in the three time periods and passed the significance test at 0.01 level, the variation of the correlation coefficients showed a trend of decreasing first and then increasing, indicating that the degree of synergism between them was weakened first and then increased; the correlation coefficient of carbon sequestration and soil conservation only passed the significance test at 0.01 level during 2000 to 2010, indicating they were in a synergistic relationship during this period. The correlation coefficient of habitat quality and water conservation only passed the significance test at the 0.05 level from 2010 to 2020, and was negative, indicating they were in a trade-off relationship during this period; the correlation coefficients of water conservation and soil conservation passed the significance test at 0.01 level during 1990 to 2000 and during 2010 to 2020, and were both positive, but the correlation coefficients showed a downward trend, indicating that the synergistic relationship between them was weakened; the correlation coefficients of carbon sequestration–water conservation and habitat quality–soil conservation were not significant (

Table 1. Correlation coefficient of ecosystem services in Qinba Mountain area.

Ecosystem Service	1990–2000	2000–2010	2010–2020
carbon sequestration–habitat quality	0.534 **	0.294 **	0.451 **
carbon sequestration–water conservation	−0.142	0.101	−0.091
carbon sequestration–soil conservation	−0.051	0.375 **	−0.011
habitat quality–water conservation	0.104	−0.206	−0.254 *
habitat quality–soil conservation	0.131	−0.050	−0.101
water conservation–soil conservation	0.914 **	0.181	0.370 **

** and * were significant at 0.01 and 0.05 levels (bilateral), respectively.

).

The results of the meta-correlation analysis show that most of the areas in the Qinba Mountains exhibited no correlation between carbon sequestration and habitat quality from 1990 to 2020, and only a small number of areas exhibited a trade-off or synergistic relationship. The proportion of areas with synergistic relationships increased from 5.18% in 1990–2000 to 19.18% in 2010–2020, and the proportion of areas with strong synergistic relationships has been on an increasing trend (Figure 5). From a spatial perspective, the regions with a synergistic relationship between carbon sequestration and habitat quality from 1990 to 2020 are mainly located in the Longnan Mountains and the northwest Sichuan Basin, while the trade-off regions are concentrated in some areas of the Funiu Mountains, and the synergistic trade-off regions gradually spread to the Qinba Mountains over time, with no obvious boundary in the Qinba Mountains (Figure 6(a1–a3)).

From 1990 to 2020, most of the areas in the Qinba Mountains showed no correlation between carbon sequestration and water conservation, but a small number of areas showed trade-offs or synergistic relationships. The proportion of regional area represented by trade-off relationship showed an increasing trend, from 3.73% in 1990–2000 to 10.67% in 2010–2020. As shown in Figure 4, the proportion of regional areas of synergistic relationships increased first and then decreased. Among them, the proportion of weak synergistic relationships was always on the rise, from 0.57% during 1990–2000 to 1.92% during 2010–2020, while the proportion of strong synergistic relationships increased first and then decreased (Figure 5). From the perspective of space, carbon sequestration and water conservation services are mainly distributed in the Longnan Mountains and Funiu Mountains, while the synergistic relationship areas are scattered in the northwest of the Sichuan Basin. From 1990 to 2020, the cooperative trade-off region gradually spread to the whole study area, showing a point-like distribution without clear boundaries (Figure 6(b1–b3)).

From 1990 to 2020, the relationship between carbon sequestration and soil conservation in most areas of the Qinba Mountain area showed no correlation, but a few areas showed trade-off or a synergistic relationship, and the proportion of areas showing synergistic or trade-off relationship increased first and then decreased. Among them, the proportion of the regional area of cooperative relationships increased from 3.16% in 1990–2000 to 13.08% in 2000–2010 and then decreased to 11.84% in 2010–2020. The proportion of the regional area of trade-off relationship increased from 3.11% in 1990–2000 to 12.22% in 2000–2010 and then decreased to 11.34% in 2010–2020 (Figure 5). From the perspective of space (Figure 6(c1–c3)), the trade-off relationship of carbon sequestration and soil conservation was distributed in the Longnan Mountains and Funiu Mountains, while the regions with cooperative relationships were scattered in various counties and districts in the Qinba Mountains. From 1990 to 2020, the region of cooperative trade-off is expanding rapidly, showing an obvious point-like distribution.

From 1990 to 2020, the relationship between habitat quality and water conservation in most regions of the Qinba Mountains area was synergistic or trade-off, and the trade-off relationship was dominant. The proportion of areas showing trade-offs tends to decrease and then increase, from 70.56% in 1990–2000 to 45.15% in 2000–2010 and then to 54.16% in 2010–2020; the proportion of area showing synergies tends to increase and then decrease, from 24.5% in 1990–2000 to 42.40% in 2000–2010 and then to 36.7% in 2010–2020 (Figure 5). In terms of space (Figure 6(d1–d3)), In 1990–2000, it was mainly distributed in the Longnan Mountains, Qinling Mountains, Funiu Mountains, Daba Mountains and the northern part of Sichuan Basin. In 2000–2010, some regions with strong trade-off relations in the Longnan Mountains, the central Qinling Mountains, the central Funiu Mountains, the southern Daba Mountains and the northern Sichuan Basin changed into strong and weak synergistic relations, while most regions in the Three Gorges Reservoir area changed from weak synergistic relationships to strong trade-off relationships. In 2010–2020, most areas in the central and western Qinling Mountains and the northern Sichuan Basin changed from strong and weak synergistic relationships to strong trade-off relationships, while some areas in the Three Gorges Reservoir area and the northern Longnan Mountain area changed from strong trade-off relationships to strong synergistic relationships.

From 1990 to 2020, the relationship between habitat quality and soil conservation in most areas of the Qinba Mountains area was synergistic or a trade-off. The degree of trade-offs tends to increase and the degree of synergy tends to weaken. The proportion of areas showing trade-offs is decreasing and then increasing, with strong trade-offs increasing from 28.88% in 1990–2000 to 37.54% in 2010–2020; the proportion of areas showing synergies is decreasing from 48.04% in 1990–2000 to 34.17% in 2010–2020 (Figure 4). The proportion of areas showing synergistic relationships decreases from 48.04% in 1990–2000 to 34.17% in 2010–2020 (Figure 5). Spatially, the Qinba Mountain trade-off area expanded from the area west of the Jialing River to the east from 1990 to 2020 (Figure 6(e1–e3)). Among them, the synergistic relationship region spreads to the west of the Qinba Mountains from 2000–2010, and the trade-off relationship extends to the east, with the two spreading in a staggered manner within the Qinba Mountains; most of the Qinling Mountains, the Longnan Mountains, the northern Sichuan Basin, the eastern Daba Mountains and the Three Gorges Reservoir area from 2010–2020 shift from synergistic relationship to trade-off relationship.

From 1990 to 2020, water conservation and soil conservation in most areas of the Qinba Mountains area showed synergistic or trade-off relationships, and the synergistic relationship was dominant. The proportion of areas showing synergistic relationships tends to fall and then rise, from 88.04% in 1990–2000 to 56.04% in 2000–2010, and then to 84.24% in 2010–2020; the proportion of areas showing trade-offs tends to trend of rising and then falling, from 7.98% in 1990–2000 to 37.79% in 2000–2010 and then falling to 11.65% in 2010–2020 (Figure 5). Spatially, the Qinba Mountains were dominated by synergistic relationships in 1990–2000, except for the northeastern Qinling Mountains and the northern Funiu Mountains. From 2000 to 2010, the southwest of Qinba Mountains, the southern Funiu Mountains, the Daba Mountains, the northern Longnan Mountains, the Three Gorges reservoir area and the central and southern Qinling Mountains changed from synergistic relationships to trade-off relationships. From 2010 to 2020, the weighting area shrinks significantly, with the northern Longnan Mountains, the southern Fuyiu Mountains and the Daba Mountains evolving from a trade-off relationship to a synergistic relationship (Figure 6(f1–f3)).

4. Discussion

We analyze the characteristics of spatial and temporal changes of four ecosystem services and their trade-off synergistic relationships in the Qinba Mountains from 1990 to 2020. The study found that the soil conservation and water conservation in the Qinba Mountains show a fluctuating upward trend and a distribution characteristic of “high in the southwest and low in the northeast”. On the one hand, As a key factor affecting water conservation, the average rainfall in this region in the past 30 years presents a distribution pattern of "high in the south and low in the north", resulting in the spatial distribution of water conservation also presents a distribution pattern of "high in the south and low in the north" [28]. On the other hand, after the release of the National Main Function Zone Plan in 2010, the national policies of returning farmland to forest and grass, ecological compensation, and the construction of the main function zone of habitat quality limit human over-exploitation and reduce the burden on the environment. Further, vegetation conditions and ecological environment conditions have been continuously improved [18] and the continuous optimisation of natural ecological conditions promotes the further improvement of water connotation and soil conservation capacity.

The average annual growth rate of carbon storage in the Qinba Mountain area is relatively low, mainly because Qinling Mountain area, Funiu Mountain area and Daba Mountain area have good natural environments, moist climate, abundant rain, a large area of natural forest distribution, good forestry resource endowment, carbon sink function has basically reached saturation, and further investment in returning farmland to forest and grassland has a limited effect on improving the ecosystem carbon sink function. In addition, due to the obvious expansion of urban construction land in the Jialing River and Hanjiang River valley basins and the decrease in vegetation cover, the carbon sequestration services in the valley basins are reduced to some extent so that the carbon sequestration services in the valley basins show a slow growth trend in general, and the regional differences tend to expand. Sun et al. [29] also showed that the carbon sequestration capacity was lower in urban sprawl areas with serious human disturbance.

The habitat quality showed a fluctuating decreasing trend from 1990 to 2020, and the high-value areas were dominated by the Daba Mountains, Longnan Mountains and Qinling Mountains, while the low-value areas were mainly distributed in the Hanjiang River Valley Basin and Jialing River Valley areas, such as Hanzhong and Ankang City. Land use change is an important influence on habitat quality change, and the initial improvement in habitat quality was inextricably linked to the implementation of the reforestation and grass restoration project, but over time some of the disadvantages of the ecological restoration project have become apparent, with excessive vegetation restoration instead leading to adverse competition for the ecological environment. The study by Lin et al. [30] also confirms the potential for ecological and environmental problems to arise from fallow forestry (grassland) projects at long time scales. In addition, the rapid urbanisation of adjacent urban areas and urban expansion have eroded the original landscape types and their spatial distribution, resulting in increased habitat fragmentation and new sources of threats [31], with the result that the surrounding habitats have been squeezed and fragmented and the quality of regional habitats has decreased.

Water conservation and regulatory services are mutually enhanced and synergistic. Water conservation capacity is mainly derived from the water system, which evaporates into steam and then drops to surrounding areas in the form of precipitation to maintain local humidity and rainfall, thus playing a role in regulating climate. Natural rainfall, in turn, affects the supply of water resources and promotes the growth of vegetation, forest land and the increase in biological species in the forest, while tree roots are conducive to the stability of slope. Reduce the amount of soil erosion, and then play a role in soil and water conservation. This is consistent with the previous research results [11,32].

The trade-off area of habitat quality–soil conservation and habitat quality–water conservation was expanding. The reason may be that the project of returning farmland to the forest (grass) will increase the actual evapotranspiration of vegetation, continuously consume shallow groundwater resources, and decrease the soil water content [33]. In addition, the development of land for construction has gradually increased the demand for supply services, causing fluctuations in water containment and soil conservation capacity in the already fragile ecological environment of the Qinba Mountains. The rapid socio-economic development of the adjacent urban areas and the conversion of a large amount of ecological land to construction land has caused a reduction of vegetation cover and weakened habitat quality on the one hand; this has led to a stronger trade-off between the part of the area and the ecosystem services mentioned above, and these results are similar to those of existing research studies [34,35].

In this paper, InVEST model, ArcGIS visualizes the assessment results and realizes the spatialisation of quantitative assessment of ecosystem service function value. However, the simplification of some assumptions and algorithms leads to some limitations of the results. The image-by-image correlation analysis characterizes the trade-off/synergistic spatial relationship of ecosystem services, but still cannot fully reflect the internal mechanism and action mechanism of ecosystem services. In the future, it is necessary to further explore and analyze the InVEST model with other models and methods, combining qualitative or quantitative methods to assess ecosystem services, so as to improve the accuracy of the assessment results and explore the complex relationship between human activities, natural factors and ecosystem services.

5. Conclusions and Recommendations

5.1. Conclusions

Based on InVEST model, the temporal and spatial characteristics of water conservation, soil conservation, carbon sequestration and habitat quality services in the Qinba Mountain region were analyzed. The following conclusions have been obtained:

(1)

From 1990 to 2020, ecosystem service functions have improved, water conservation, soil conservation and carbon storage are on the rise, and only habitat quality has a small decline. In the past 30 years, the regional ecosystem structure has been relatively stable, its pattern has not changed significantly, and the spatial distribution of ecosystem services has obvious heterogeneity. The distribution pattern of water conservation and soil conservation services was “high in southwest and low in northeast”, while the spatial pattern of habitat quality services was just the opposite. The overall carbon sequestration service showed a spatial distribution pattern of “low in the middle and high around”.

(2)

We pay more attention to the correlation between ecosystem services. On the whole, the trade-off effect is stronger than the synergistic effect in the Qinba Mountain area. The trade-off relationship between habitat quality-soil and water conservation and habitat quality-soil and water conservation is only the synergistic relationship between soil and water conservation and habitat quality-soil and water conservation.

(3)

The regional area proportion of habitat quality–water conservation showed a decreasing trend, and it contracted in most areas of the Longnan Mountain area, eastern Daba Mountain area and the Three Gorges reservoir area with time. The area proportion of habitat quality–soil conservation trade-off relationship showed an increasing trend, and the distribution area gradually shrank to the west over time. The regional area proportion of water conservation and soil conservation service synergy decreased first and then increased, and the distribution area expanded from the Longnan Mountains, the western Qinling Mountains, and the Daba Mountains to the study area. The proportion of carbon sequestration service, water conservation, soil conservation, and habitat quality service trade-offs and synergies showed an increasing trend, but showed no correlation in most regions.

5.2. Recommendations

The results of the study show that with the increase in ecological protection, ecosystem services in the key ecological function areas have been optimised and enhanced, but the ‘coordination’ between ecosystem services in the key ecological function areas is clearly lacking, and there is even a change from ‘coordination’ to ‘trade-off’, which poses a serious challenge to ecosystem protection and management. There is a need for more targeted ecosystem conservation management decisions to improve overall ecosystem benefits and support the sustainable supply of regional ecosystem services. In particular, the trade-offs between habitat quality–water quality and habitat quality–soil conservation need to be addressed through scientific and rational ecological planning and the delineation of ‘arable land red lines’ and ‘ecological red lines “This will promote the development of a full range of ecosystem services in the Qinba Mountains and reduce the trade-offs between ecosystem services. In urban areas with more intensive human activity, such as the northern Sichuan Basin, the Han River Valley Basin region and the northern Longnan Mountains, priority conservation areas are incorporated into the delineation of urban growth boundaries to find a balance between protection and development and to mitigate conflicts between urban expansion and ecological red line reserves. At the same time, actively guide local residents’ awareness of environmental protection and the concept of paying for the consumption of ecological products, increase the ecological yield ratio and promote green, intelligent and high-quality development in the Qinba Mountain region.