Abstract:

The Southwest region of China is a major karst area characterized by complex terrain, diverse landforms, and a variety of climates and vegetation types. It is the second largest natural forest area in China and plays a crucial role in ecological protection, biodiversity conservation, and water source preservation. In recent years, extreme climate events, such as high temperatures, rainstorms, and droughts, have occurred more frequently owing to global warming. In addition, urbanization and ecological projects, such as the conversion of farmland to forest, grassland, and other human activities, have had a profound impact on vegetation growth. Fractional vegetation cover (FVC) is an important indicator of the growth of surface vegetation communities, and serves as important data for describing ecosystems. It is an effective indicator for detecting regional ecological restoration and environmental changes and helps improve analysis accuracy by reducing uncertainties in spectral features, especially in non-vegetated areas. Therefore, the fractional vegetation cover is widely used. Fractional vegetation cover dynamics are influenced by climate change and human activities. Therefore, studying the relationship between vegetation coverage and extreme climates and quantifying the impact of these factors on vegetation change are of great significance for regional ecological protection. At present, it is widely recognized that climate dominates vegetation, and that climate change can affect the growth patterns of vegetation. The response of vegetation to climate is often complex, with a time-lag effect. Specifically, the response can be asymmetric, indicating that vegetation is not only affected by contemporaneous climate factors but also by the lagged and cumulative effects of climate factors. The time-lag effects represent vegetation growth being affected by the climatic conditions from the preceding month, whereas the time-accumulation effects indicate that vegetation growth is significantly affected by the cumulative climatic conditions from the previous months, as well as the current month. However, systematic analyses of time-lag and time-accumulation effects on vegetation growth, particularly concerning extreme climate indices, have rarely been conducted. In the karst region of southern China, most studies have focused on the effects of temperature and rainfall on vegetation, with only a few studies focusing on the correlation between vegetation and extreme climate indices. These studies qualitatively determined the effects of climate extremes on vegetation based on magnitude and correlation coefficients without considering the combined effects of time lag and cumulative effects. In addition, few studies have considered for extreme weather indices and have generally ignored the time lag and cumulative effects when quantifying the contributions of climate change and human activities to vegetation change. As a result, analyses using mainly contemporaneous vegetation and climatic data tend to underestimate the impact of climate on vegetation change, with some climate-induced impacts being misclassified as anthropogenic. This study, based on meteorological and MODIS data, uses methods such as pixel dichotomy, trend analysis, and time-delay and cumulative effect analysis to analyze the spatiotemporal evolution of vegetation coverage in Southwest China, as well as the time lag and cumulative effects of extreme climate change on vegetation. The study findings indicate that 1) from 2000 to 2022, the annual average fractional vegetation cover showed a significant upward trend, with an average interannual variation rate of 0.032/(10a) (p<0.01). However, some years, such as 2011 and 2012, saw a decrease in the fractional vegetation cover. Spatially, the linear change rates of the annual fractional vegetation cover range from −0.30 to 0.26/(10a). Approximately 75.6% of the vegetated areas in the study area had a postive linear slope with a fractional vegetation cover. Regarding the significance of fractional vegetation cover trends, the proportions of vegetated areas with significantly increasing and decreasing fractional vegetation cover trends were 51.1% and 7.4%, respectively. Overall, the fractional vegetation cover in the southwestern region showed an increasing trend. 2) The region with non-temporal effects only accounted for 3.1%‒23.5% of the studied area. The accumulation effects of the average temperature on fractional vegetation cover were 1.18±0.87 months, while it for precipitation were 1.63±0.97 months. The response of fractional vegetation cover to the time lag and cumulative effects of extreme climate indices mainly reflected the cumulative effect. The cumulative effect of rainfall in to 1-3 months ranges from 50.9% to 71.5%, whereas the cumulative effect of temperature ranged from 43.6% to 64.1%. The time lag effect was relatively concentrated, mainly in Yunnan, with a significant proportion of areas exhibiting a three-month time lag. 3) After considering the optimal effect of time delay accumulation, the explanatory power of climate change on vegetation in Southwest China increased by 8.3%. Climate change, human activities, and other factors contributed to 54.5% and 45.5% of the changes in vegetation coverage in the region, respectively. Climate change positively contributed to the fractional vegetation cover in 73.7% of the area, mainly in Sichuan, Chongqing, Guizhou, and Guangxi. The negative contribution area ratio was 26.3%, and was mainly distributed in the western regions of Sichuan and Yunnan. The area with a climate contribution greater than 80% accounted for approximately 26.7%, and was concentrated in the Hengduan Mountains and high-altitude primitive forests in the western part of the study area. Therefore, in the ecological restoration and management of Southwest China, it is necessary to implement appropriate ecological construction and management measures according to the characteristics of the regional climate. The contribution of human activities and other factors to the change in vegetation coverage in Southwest China was also predominantly positive, with 75.5% of the area showing a positive contribution and 24.5% showing a negative contribution. The area where human activities contribute more than 70% accounts for approximately 29.7% and is mainly concentrated in the Sichuan Basin, Chongqing, and densely populated low-altitude areas. Based on the consideration of temporal effects and extreme climate, as well as modified residual analysis, underestimations of the effect of climate change on vegetation could be avoided, and the accuracy of the estimation of vegetation change contributed by climate change and human activities could be improved. We suggest that these effects should be incorporated into dynamic vegetation models to better understand vegetation growth under accelerated climate change. This study provides a scientific basis for vegetation monitoring and ecological protection in the region.

Key words: extreme climate, fractional vegetation cover, time-lag and cumulative effects, climate change, human activities, southwest region

摘要：

气候变化对植被的影响存在滞后和累积效应。探究西南区域极端气候变化对植被生长的影响，为中国西南区域植被恢复和生态安全提供数据支撑。基于气象数据和MODIS数据，采用像元二分法、趋势分析和时滞与累积效应分析等方法，分析了西南地区植被覆盖度时空演变特征以及极端气候变化对植被覆盖度的时滞与累积效应影响。研究结果表明，1）时间上，植被覆盖度年均值呈显著上升趋势，年际变化率均值为0.032/(10a)（p<0.01）；空间上，75.6%区域的植被呈改善趋势，其中51.1%区域植被呈现出显著改善趋势。2）植被覆盖度对极端气候指数时滞与累积效应的响应主要表现累积效应，其中降雨指数1－3个月累积效应面积占比在50.9%－71.5%之间，温度指数累积效应面积介于43.6%－64.1%之间；而时滞效应影响较为集中，主要分布于云南地区，时滞3个月的面积占比相对较大。3）气候变化和人类活动及其他因素分别对西南地区植被覆盖度的变化贡献了54.5%和45.5%，气候贡献度大于80%的区域面积约占26.7%，主要集中研究区西部的横断山脉及高海拔的原始森林地区。人类活动贡献度超过80%的区域主要集中在四川盆地、重庆以及人口密集的低海拔区域。西南地区气候变化对植被覆盖度变化的影响大于人类活动。

关键词: 极端气候, 植被覆盖度, 时滞与累积效应, 气候变化, 人类活动, 西南地区