[Analysis of Multiple Drivers of Fractional Vegetation Cover Evolution in Beijing, Tianjin, and Hebei Based on the Optimal Parameters Geographical Detector].

A clear understanding of the driving mechanisms behind spatial and temporal vegetation dynamics is of great significance in guiding the protection and restoration of ecological environments. The spatial and temporal distribution pattern of fractional vegetation cover （FVC） and its multiple driving mechanisms in Beijing, Tianjin, and Hebei were investigated using long-term NDVI data from 2002 to 2020. A total of 25 specific variables were selected to represent the influencing factors of topography, climate, geotechnical type, and human activities. The selected variables were combined with the methods of metric dichotomies, trend analyses, biased correlation analyses, and optimal parameter geodetectors. The results indicated that： ① The FVC in Beijing, Tianjin, and Hebei over the past 20 years exhibited a general downward trend, with an average decline rate of approximately 0.002 4 per annum. The FVC levels in the northern and southern regions were relatively high, while those in the western and central regions were relatively low. Approximately 28.73% of the areas exhibited a significant decreasing trend （<0.01）, with the greatest decline observed in the southern, eastern, and central regions of Beijing, Tianjin, and Hebei. ② The main factors driving the evolution of FVC （>0.15） were identified as natural factors, including evapotranspiration, cumulative precipitation, soil type, topography, and human activities, such as financial income, GDP per capita, nighttime light intensity, and population density. These factors were found to interact with each other. The enhancement effect on FVC was found to be nonlinear, with a bilinear enhancement effect observed when the combination of LST and ET was considered. This was followed by the combination of solar radiation and ET, which demonstrated the highest explanatory power （>0.81）. The FVC was highest for the combination of LST and ET （>0.83） and second highest for the combination of solar radiation and ET （>0.81）. ③ Evapotranspiration ranged from 572 to 772 mm, cumulative precipitation ranged from 461 to 514 mm, and radiation ranged from 1 980 to 2 030 W·m. When LST was between 5.5 and 12 ℃, average temperature was between -0.902 and 3.74 ℃, altitude was between 1 560 and 2 180 m, terrain type was large undulating middle mountain, soil type was leaching soil, and vegetation type was forest land, with FVC performing the best. The results of this study can assist the regional government in making accurate policy decisions regarding the protection and restoration of the vegetation ecosystem.