Abstract:

Topography and geomorphology are key factors that influence changes in the urban land-use structure and spatial heterogeneity of carbon storage. In hilly areas, the influence of topography and geomorphology leads to significant vertical differences in vegetation cover, which in turn, leads to significant spatial heterogeneity in carbon storage. The Chang-Zhu-Tan urban agglomeration, with its typical hilly topography, is an important part of the urban agglomeration in the middle reaches of the Yangtze River, and an important hub for promoting the coordinated development of regional socioeconomic and ecological environmental protection. However, the rapid urbanization of the Chang-Zhu-Tan urban agglomeration has intensified, and the intensity of land development has accelerated, resulting in a serious decline in the overall carbon storage capacity, posing a major challenge to the sustainable development of the region's ecology and economy. Carbon storage are an important basis for the effective implementation of the ‘double carbon’ target by absorbing atmospheric carbon dioxide to mitigate climate change, as well as an important indicator for quantifying the total amount of carbon in the regional ecosystem and assessing the carbon sink capacity. Current research has not fully explored the spatial and temporal characteristics of carbon storage and its vulnerability in hilly areas from the perspective of topographic gradient. In addition, the study of carbon storage zoning based on topographic gradients is still insufficient to improve regional carbon storage and optimize spatial patterns. Therefore, this study evaluates the spatial and temporal characteristics of carbon storage and its vulnerability from the perspective of terrain gradient, and proposes a mechanism for carbon stock zoning and management based on terrain characteristics, which is of great significance in promoting the coordinated development of regional ecology and economy and the effective implementation of the “double carbon” goal. This study considers the Chang-Zhu-Tan city cluster with a typical hilly terrain, as an example. First, the carbon stock from 2000 to 2020 was quantified based on the carbon storage module of the InVEST model, and the spatial and temporal variations in carbon storage and its changes were investigated using ArcGIS. At the same time, the spatial correlation between the land-averaged carbon density of Chang-Zhu-Tan urban agglomeration and each terrain gradient was explored by GeoDa software. Second, the four dimensions of elevation, slope, topographic relief, and topographic position index were used to explore the effect of topographic gradient on carbon storage and evaluate the vulnerability of carbon storage. Carbon storage was zoned according to the topographic distribution index of elevation and slope, and corresponding management strategies were proposed. The results show that: 1) the spatial and temporal distribution of carbon storage is characterized by a declining trend of carbon storage in the Chang-Zhu-Tan Urban Agglomeration from 2000 to 2020, and trend of carbon storage loss is serious from 2000 to 2005, and then the trend of carbon storage decline slowed from 2010. The spatial distribution of carbon storage shows a pattern of “low in the center and high in the periphery”. Among them, the highest carbon storage is mainly concentrated in Liuyang City, Ningxiang City, and Youxian County, which are characterized by higher terrain and a better ecological environment. On the other hand, the districts of Changsha City are areas with frequent human activities and severe carbon storage depletion, indicating that human activities have a greater impact on spatial changes in carbon storage. 2) Spatial correlation between carbon storage and topographic factors: There is a significant positive correlation between the average land carbon density and various topographic factors. This shows that the average land carbon density increases with increasing topographic gradient, and there is a clear trend towards aggregation. 3) Topographic gradient of carbon storage. Carbon storage in the Chang-Zhu-Tan urban agglomeration is mainly concentrated in the areas with terrain gradient of 1-4. Overall, the average land carbon density increased with an increase in the terrain gradient in 1-3, while the average land carbon density showed a slight decreasing trend after the gradient in 4. Among them, areas with lower topography and gentle slopes had the lowest average land carbon density and the largest loss of carbon storage. Meanwhile, level 1 areas were dominated by cropland and woodland, while level 2-5 areas were dominated by woodland and grassland. The loss of cropland and forest was highest in Tier 1 areas, whereas the opposite was true for construction land. This suggests that the shift from high carbon sequestration land categories, such as woodland and cropland, to low carbon sequestration land categories, such as built-up land, is the main reason for the change. 4) Assessment of vulnerability of the topographic gradient of carbon storage. The land use change of the Chang-Zhu-Tan City Cluster from 2000 to 2020 showed a negative potential impact on the carbon storage service in general, but the negative potential impact weakened after 2010. The PI of the Chang-Zhu-Tan urban agglomeration is negative in the range of terrain gradients 1‒4, while it is positive in 2010‒2020 at elevation level 5 and 2000‒2010 at terrain position index level 5. The overall negative potential impact on carbon storage decreased with an increase in terrain gradient. This indicates that the greater the intensity of land use development in areas with lower and gentler topography, the more severe is the negative impact on carbon storage services. 5) Evaluation of carbon storage zoning in hilly areas. Based on the terrain distribution index, the Chang-Zhu-Tan urban agglomeration was divided into hilly intensive development, low hilly comprehensive management, medium hilly ecological protection, and high hilly ecological improvement areas. The hilly intensive development area is mainly located in the Changsha, Zhuzhou, and Xiangtan districts, and is the economic and development core of the urban agglomeration. The low-hill comprehensive management area is located on the periphery of the central urban area, and is an important hub for the coordination and management of human activities and ecological protection. The middle- and high-mountain ecological protection zones and the high-mountain ecological enhancement zone are located in the central city of the Chang-Zhu-Tan urban agglomeration, and the corresponding carbon storage capacity enhancement and management measures are proposed according to the characteristics of each region. From the perspective of topographic gradients in hilly areas, this study aimed to optimize the spatial structure of land use in each region, improve the carbon storage potential of hilly urban agglomerations, and provide a theoretical basis for promoting sustainable development.

Key words: hilly areas, carbon storage, gradient effect, vulnerability, InVEST model, zone evaluation

摘要：

丘陵地区有着复杂的生态特征。从地形梯度视角下对碳储量时空分异特征及脆弱性进行评价，并基于地形特征下提出碳储量分区与管理机制，对推动区域生态与经济协调发展，有效落实“双碳”目标具有重要意义。以典型丘陵地貌的长株潭城市群为例，基于InVEST模型量化2000-2020年的碳储量时空分异特征，通过高程、坡度、地形起伏度与地形位指数因素，探讨碳储量地形梯度效应及脆弱性，并依据地形分布指数进行碳储量分区并提出管理策略。结果表明，1）2000-2020年长株潭城市群碳储量呈现下降的趋势，空间上呈现“中间低四周高”的态势。2）地均碳密度与各类地形因素有着显著的正相关性，地势较低且坡度平缓的地区地均碳密度最低，且减损幅度最大。林地、耕地等高碳储地类向建设用地等低碳储地类转变是导致该变化的主要原因。3）2000-2020年长株潭城市群土地利用变化对碳储量服务总体表现为负向潜在影响，但随着地形梯度增加，负向潜在影响在不断减弱。4）基于地形分布指数，长株潭城市群划分为了丘陵集约发展区、低丘综合治理区、中高丘生态涵养区、高丘生态提升区。该研究从丘陵地区地形梯度视角出发，旨在优化各区域的土地利用空间结构、提高丘陵城市群的碳储潜力，为推动可持续发展提供理论依据。

关键词: 丘陵地区, 碳储量, 梯度效应, 脆弱性, InVEST模型, 分区评价