[Quantitative Assessment of the Impact of Climate Change on the Growing Season of Vegetation Gross Primary Productivity in the Middle and Lower Reaches of the Yangtze River].

Quantitatively determining the direct, indirect, and comprehensive effects of climatic factors on the growing season of the vegetation GPP (GPP) in the middle and lower reaches of the Yangtze River at the regional and vegetation type scales can provide a scientific basis for the management and restoration of regional vegetation resources under the background of global climate change. Using MODIS GPP data, meteorological data, and vegetation type data, combined with Theil-Sen Median trend analysis and the Mann-Kendall significance test, the spatiotemporal characteristics of the GPP in the middle and lower reaches of the Yangtze River were investigated at different temporal and spatial scales. Path analysis was used to further reveal the direct, indirect, and comprehensive effects of climate factors on GPP variation in different vegetation types. The results showed that:① from 2000 to 2021, the vegetation GPP in the middle and lower reaches of the Yangtze River showed a fluctuating upward trend, with a rising rate (in terms of C, same below) of 2.70 g·(m·a) (<0.01). The GPP of different vegetation types all showed a significant upward trend (<0.01), with shrubs having the highest upward rate of 3.31 g·(m·a) and cultivated vegetation having the lowest upward rate of 2.54 g·(m·a). ② The proportion of the area with an upward trend in GPP in the middle and lower reaches of the Yangtze River was 88.11%. The proportion of the area with an upward trend in GPP was greater than 84% for all different vegetation types, with shrubs (49.76%) and cultivated vegetation (44.36%) having significantly higher proportions of the area with an upward trend than that in other vegetation types. ③ The path analysis results showed that precipitation and the maximum temperature had a significant positive direct effect on vegetation GPP (<0.05), whereas solar radiation had a non-significant positive effect ( ≥ 0.05). The indirect effects of maximum temperature, precipitation, and solar radiation on vegetation GPPGS were all non-significantly negative ( ≥ 0.05). Under the combined effects of direct and indirect influences, precipitation and maximum temperature had a non-significant positive effect on vegetation GPP ( ≥ 0.05), whereas solar radiation had a non-significant negative effect on vegetation GPP ( ≥ 0.05). Among different vegetation types, precipitation was the main climate factor affecting the changes in GPP of cultivated vegetation, whereas the maximum temperature was the main climate factor affecting the changes in GPP of coniferous forests, broad-leaved forests, shrubs, and grasslands. ④ The changes in vegetation GPP in the middle and lower reaches of the Yangtze River were mainly influenced by the direct effects of maximum temperature, precipitation, and solar radiation, with the direct effect of precipitation dominating 56.72% of the changes in GPP. The research results can provide a reference for quantifying the carbon sequestration potential of vegetation in the middle and lower reaches of the Yangtze River and formulating ecological restoration governance policies tailored to local conditions under the background of global climate change.