Quantification and Spatial Differentiation of Ecosystem Service Value of Picea schrenkiana Forest in the Central Tianshan Mountains

SUN Yubo¹, ZHAO Shanchao², WANG Yapei², MA Xiaoli¹, CHEN Yuwen¹, HUANG Xuansheng¹, WANG Weixia¹

¹College of Forestry and Landscape Architecture, Xinjiang Agricultural University / Xinjiang Department of Education Key Laboratory of Arid Land Forestry Ecology and Industrial Technology, Urumqi 830052, Xinjiang, China
²Natural Forest Protection Center, Xinjiang Forestry and Grassland Bureau, Urumqi 830000, Xinjiang, China

Abstract: This study examines the Picea schrenkiana forest ecosystem in the central Tianshan Mountains. Based on the functional value method, we employed Getis-Ord Gi* hotspot analysis to quantitatively visualize the spatial patterns of ecosystem service values. The results demonstrate that the total ecosystem service value of Picea schrenkiana forests in the central Tianshan Mountains is 96.09×10⁸ Yuan·yr⁻¹, with water conservation, carbon sequestration and oxygen release, soil conservation, and biodiversity protection serving as the dominant functions. Ecosystem service values vary significantly across stand ages, with middle-aged forests exhibiting the highest value, followed by near-mature, mature, and young forests; over-mature forests show the lowest total value but the highest per-unit-area value. Among forest farms, Nanshan Forest Farm registers the highest value, followed by Hutubi, Banfanggou, and Miquan Forest Farms, with the Xinjiang Agricultural University Practice Forest Farm showing the lowest value. Significant spatial clustering characteristics are evident in ecosystem service values across forest farms, with hotspot areas concentrated in Hutubi and Nanshan Forest Farms in the southwest, while cold spot areas are concentrated in Miquan and Banfanggou Forest Farms in the northeast. These findings provide a scientific basis for optimizing regional ecological resource management and formulating ecological protection policies, while supporting the sustainable development of the Tianshan forest ecosystem.

Keywords: Picea schrenkiana forest; spatial pattern of ecosystem service values; ecosystem services; central Tianshan Mountains

Introduction

Ecosystem services refer to the various benefits that ecosystems provide directly or indirectly to human society while maintaining their own ecological balance, including functions such as water conservation, soil conservation, and carbon sequestration and oxygen release. Forests, as crucial components of terrestrial ecosystems, play vital roles in ecological security and socioeconomic sustainable development. In recent years, the classification, assessment, and accounting of ecosystem service functions have become important topics in global ecological economics research. Since the 1990s, Chinese researchers have gradually introduced relevant theories and established ecosystem service assessment systems based on China's actual conditions, including the functional value method and the equivalent factor method.

Currently, domestic and international research primarily employs functional value methods and equivalent factor methods for assessment. The functional value method offers high accuracy but requires substantial data, making it suitable for small-scale evaluations. The equivalent factor method is simple and efficient but insufficient in reflecting spatial heterogeneity and dynamic changes. Most existing studies have focused on global, national, or watershed scales, with relatively few investigations into the spatial distribution characteristics, regional differences, and synergistic or trade-off relationships among service functions at finer scales.

The Tianshan Mountain forest represents an essential component of Xinjiang's terrestrial ecosystem and plays an irreplaceable role in maintaining ecological security and promoting regional economic development. Picea schrenkiana, the most widely distributed and highest-volume tree species in Xinjiang's mountain forests, demonstrates significant ecological functions in water conservation, soil conservation, and climate regulation. Although previous studies have addressed multi-scale assessments of forest ecosystem services in Xinjiang, research on the spatial distribution characteristics and regional differences of forest ecosystem service functions in the central Tianshan Mountains remains insufficient, particularly regarding the aggregation characteristics and spatial synergies among different functions. Therefore, this study focuses on the Picea schrenkiana forest ecosystem in the central Tianshan Mountains, combining functional value methods with Getis-Ord Gi* hotspot analysis to quantitatively evaluate ecosystem service values and reveal their spatial distribution patterns and hotspot areas. The research aims to provide a scientific basis for regional ecological resource optimization management and ecological protection policy formulation, while offering theoretical support for the sustainable development of the Tianshan forest ecosystem.

1 Study Area Overview

The study area is located on the northern slope of the central Tianshan Mountains in Xinjiang (43°01′–43°97′ N, 86°46′–87°67′ E), primarily distributed in the mid-mountain zone at elevations of 1,500–2,200 m. The region belongs to a semi-arid coniferous forest climate zone, with an average annual temperature of 2.5°C and average annual precipitation of 600 mm. Precipitation distribution is uneven, concentrated in the same season as heat. Sunshine is abundant, with annual sunshine duration exceeding 2,040 hours. The terrain slopes from high in the south to low in the north. Soils are mountainous gray-brown forest soils (Fig. 1). Vegetation is dominated by Picea schrenkiana and grasses, with representative understory plants including Aegopodium alpestre, Polygonum viviparum, Poa nemoralis, Cicerbita azurea, and Stellaria soongorica.

Based on forest resource inventory data, the Picea schrenkiana forests in the central Tianshan Mountains were divided into five forest farms: Nanshan, Miquan, Banfanggou, Xinjiang Agricultural University Practice Forest Farm, and Hutubi. According to stand age, forests were classified as young, middle-aged, near-mature, mature, and over-mature. Required social public data included reservoir construction cost per unit capacity, water purification costs, fertilizer prices, and oxygen prices, obtained from government agencies, industry reports, and official websites, with data from 2022. Supplementary data were obtained from published research papers and professional journals. Field survey data on tree height, diameter at breast height (DBH), soil nutrient content, vegetation diversity, and canopy density were collected through sampling in 2022.

2 Methods

2.1 Assessment Indicators

Based on the "Specifications for Assessment of Forest Ecosystem Services in China" (hereinafter referred to as the "Specifications") and related formulas, this study adopted five specific assessment indicators: water conservation, soil conservation, carbon sequestration and oxygen release, nutrient accumulation, and biodiversity protection (Table 1).

2.2 Data Sources

Data sources included forest resource inventory data and vector data, social public data, literature and research results, and field monitoring data. Forest resource inventory data were used to classify the Picea schrenkiana forests in the central Tianshan Mountains into five forest farms and five age classes (young, middle-aged, near-mature, mature, and over-mature).

2.3 Calculation Methods

This study employed a distributed calculation method, dividing the central Tianshan Mountains into five primary calculation units: Nanshan Forest Farm, Miquan Forest Farm, Banfanggou Forest Farm, Xinjiang Agricultural University Practice Forest Farm, and Hutubi Forest Farm. Each primary unit was further divided into five secondary calculation units according to stand age. Calculations followed the formulas in the "Specifications" to generate homogeneous data for each unit, which were then aggregated for comprehensive analysis. Using the ArcGIS platform, we performed hotspot analysis (Getis-Ord Gi*) to identify statistically significant hotspot and cold spot areas.

3 Results

3.1 Physical Quantities and Values of Ecosystem Services

3.1.1 Physical Quantities of Ecosystem Services

The physical quantities of water regulation and water purification for the Picea schrenkiana forest ecosystem in the central Tianshan Mountains were 6.55×10⁸ m³·yr⁻¹ and 2.83×10⁸ m³·yr⁻¹, respectively. Carbon sequestration quantity was 1.21×10⁶ t·yr⁻¹, while oxygen release was 2.83×10⁶ t·yr⁻¹. Soil conservation quantity was 6.07×10⁶ t·yr⁻¹. Nutrient accumulation quantity was 2.79×10⁴ t·yr⁻¹, with different nutrients ranked as: nitrogen fixation > potassium fixation > phosphorus fixation. The number of negative ions provided was 2.79×10²⁴ ions·yr⁻¹. The physical quantities per unit area for each assessment indicator followed the same ranking as the total physical quantities (Table 3).

3.1.2 Ecosystem Service Values

The total ecosystem service value of Picea schrenkiana forests in the central Tianshan Mountains was 96.09×10⁸ Yuan·yr⁻¹. Water conservation value was highest at 34.06×10⁸ Yuan·yr⁻¹, followed by carbon sequestration and oxygen release at 31.87×10⁸ Yuan·yr⁻¹. Soil conservation value was 14.34×10⁸ Yuan·yr⁻¹, biodiversity protection value was 13.87×10⁸ Yuan·yr⁻¹, nutrient accumulation value was 1.75×10⁸ Yuan·yr⁻¹, and atmospheric purification value was lowest at only 0.20×10⁸ Yuan·yr⁻¹ (Table 4). Overall, water conservation, carbon sequestration and oxygen release, and soil conservation accounted for 35.45%, 33.16%, and 14.93% of total value, respectively, comprising 83.54% of the total ecosystem service value and demonstrating clear dominance (Fig. 2).

3.2 Ecosystem Service Values by Stand Age

Calculated values varied significantly across stand ages. Middle-aged forests exhibited the highest value at 386.84×10⁸ Yuan·yr⁻¹ (40.26% of total value), followed by near-mature forests at 347.87×10⁸ Yuan·yr⁻¹ (36.20%), mature forests at 177.98×10⁸ Yuan·yr⁻¹ (18.52%), young forests at 30.34×10⁸ Yuan·yr⁻¹ (3.16%), and over-mature forests at 17.91×10⁸ Yuan·yr⁻¹ (1.86%). Middle-aged and near-mature forests contributed 76.46% of the total value, playing crucial roles in the ecosystem. However, per-unit-area value showed a different pattern: over-mature > mature > near-mature > middle-aged > young forests (Table 5).

3.3 Ecosystem Service Values by Forest Farm

Both physical quantities and values varied among forest farms, showing positive correlation with forest area. Nanshan Forest Farm exhibited the highest value at 375.97×10⁸ Yuan·yr⁻¹ (39.13% of total), followed by Hutubi at 272.61×10⁸ Yuan·yr⁻¹ (28.37%), Banfanggou at 205.09×10⁸ Yuan·yr⁻¹ (21.34%), Miquan at 65.56×10⁸ Yuan·yr⁻¹ (6.82%), and Xinjiang Agricultural University Practice Forest Farm at 41.71×10⁸ Yuan·yr⁻¹ (4.34%). Nanshan Forest Farm showed advantages in carbon sequestration/oxygen release and water conservation, while Hutubi Forest Farm demonstrated particularly outstanding soil conservation value (Table 6).

Significant spatial clustering characteristics were evident across forest farms. Using vector data for the five forest farms and ArcGIS hotspot analysis tools, we mapped ecosystem service cold and hot spot distributions (Fig. 3). Hotspot areas concentrated in Hutubi and Nanshan Forest Farms in the southwest, particularly for water conservation, soil conservation, and carbon sequestration/oxygen release. Cold spot areas were mainly distributed in northeastern forest farms such as Miquan. Approximately 30% of the area comprised hotspots and 20% cold spots, reflecting regional imbalances in ecosystem service capacity (Table 7).

Overlay analysis of the various service function hotspot maps revealed strong spatial overlap. To further investigate ecosystem service capacity across spatial units, we superimposed the three most valuable services—water conservation, soil conservation, and carbon sequestration/oxygen release—identifying three hotspot categories (Fig. 4). Non-hotspot areas accounted for the largest proportion, while triple-hotspot areas were smallest. Triple-hotspot areas concentrated primarily in Hutubi and Nanshan Forest Farms (Table 8).

4 Discussion

This study calculated ecosystem service values for Picea schrenkiana forests in the central Tianshan Mountains using "Specifications" formulas and identified spatial patterns through hotspot analysis. The total ecosystem service value was 96.09×10⁸ Yuan·yr⁻¹, with water conservation, carbon sequestration/oxygen release, soil conservation, and biodiversity protection as the dominant functions. These findings align with Yang et al.'s summary of Chinese forest ecosystem service assessments, which identified water conservation, carbon sequestration/oxygen release, soil conservation, and biodiversity protection as dominant services in China's forest ecosystems. They also correspond with Li et al.'s theory that water conservation is primary in forest ecosystem services. Furthermore, studies by Sun et al. and Li et al. on Xinjiang's mountain forests and Altai Mountains showed similar dominant services, confirming Picea schrenkiana forests' crucial contributions to water conservation, carbon sequestration, and ecological security.

Notably, soil conservation function was relatively strong in our study area. Research by Li et al. and Huang et al. confirmed that natural forest protection programs significantly enhance soil conservation functions, with effectiveness becoming more pronounced over time. The per-unit-area ecosystem service value in our study area was lower than in regions such as Hunan, Chongqing, and Hebei, consistent with Qi et al.'s finding of "high in the east and low in the west, high in the south and low in the north" distribution patterns for China's forest ecosystem services. This pattern relates closely to regional climate conditions, forest types, and biodiversity. Our study area, located in western mountains with arid climate, simple forest types, and lower vegetation coverage and biodiversity, consequently shows lower values than eastern and southern forests.

The study also found atmospheric purification accounted for the smallest proportion of ecosystem services, differing somewhat from some previous conclusions. Yang et al. noted that mountain forests typically show weaker atmospheric purification than urban forests because natural forest areas are distant from urban and industrial pollution sources, involving fewer harmful gases and particulates. Therefore, this study only examined the negative ion provision indicator for atmospheric purification function, without calculating PM2.5, resulting in lower atmospheric purification values than other regions.

Ecosystem service values varied significantly across stand ages, with middle-aged forests highest, followed by near-mature, mature, and young forests, and over-mature forests lowest. However, over-mature forests showed the highest per-unit-area value. Middle-aged and near-mature forests, as the main components of Picea schrenkiana forests in the central Tianshan Mountains, occupy important positions in forest valuation. These stands are in rapid growth phases with high photosynthetic efficiency, superior carbon sequestration/oxygen release capacity, and vigorous transpiration that facilitates water conservation and regional microclimate regulation. Their rich species diversity and complex structure help maintain high biodiversity and possess good natural regeneration capacity. Over-mature forests, though lower in total value, exhibit the highest per-unit-area ecological value due to tall trees, well-developed root systems, abundant litter, and stable ecological structure, which significantly enhance per-unit-area effectiveness in water conservation and soil conservation. These results demonstrate significant differences in ecosystem service functions across stand ages, highlighting the importance of rational management and utilization of different-aged forest resources for improving regional ecosystem services and sustainable management.

Both physical quantities and values of ecosystem services varied among forest farms, with Nanshan Forest Farm showing the highest values, consistent with Li et al.'s findings that forest ecosystem service quantities and values correlate closely with forest area and volume. Values ranked as Nanshan > Hutubi > Banfanggou > Miquan > Xinjiang Agricultural University Practice Forest Farm, proportional to forest area. Cluster analysis revealed similar hotspot patterns across the five service functions, with hotspots concentrated in the largest-area Nanshan and Hutubi Forest Farms, and cold spots in Miquan and Banfanggou Forest Farms. Superimposition analysis showed triple-hotspot areas also concentrated in Nanshan and Hutubi Forest Farms, particularly in the largest Nanshan Forest Farm.

This study used forest inventory data from various forest farms in the central Tianshan Mountains to objectively reflect ecosystem service conditions. However, the assessment method relies heavily on forest area, tree height, and stand age, with some parameters difficult to obtain. Additionally, only five service functions were valued, which may affect result accuracy. Forest ecosystems are dynamic and constantly undergoing succession, making short-term monitoring data somewhat limited. Furthermore, substitute market prices used in the replacement cost method fluctuate with time and social demand, requiring adjustment of ecological value calculation coefficients to improve assessment precision. Quantifying forest ecosystem service values helps people intuitively understand the importance of forest ecosystems, enhances environmental awareness, and promotes more scientific and rational forestry development guidance.

5 Conclusions

1) The total ecosystem service value of Picea schrenkiana forests in the central Tianshan Mountains is 96.09×10⁸ Yuan·yr⁻¹, with water conservation, carbon sequestration/oxygen release, soil conservation, and biodiversity protection as the dominant functions.

2) Ecosystem service values vary significantly across stand ages, with middle-aged forests highest, followed by near-mature, mature, and young forests; over-mature forests show the lowest total value but highest per-unit-area value. Middle-aged and near-mature forests contribute most substantially and represent the core stage for Picea schrenkiana forest ecosystem services.

3) Nanshan Forest Farm exhibits the highest value, followed by Hutubi, Banfanggou, and Miquan Forest Farms, with Xinjiang Agricultural University Practice Forest Farm showing the lowest value. Significant spatial clustering occurs across forest farms, with hotspots concentrated in Hutubi and Nanshan Forest Farms in the southwest, and cold spots concentrated in Miquan and Banfanggou Forest Farms in the northeast.

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