| 摘 要： |
Historical forest and grassland cover changes not only are critical indicators for quantifying ecological and environmental change processes but also serve as fundamental data for long-term climate change simulations and terrestrial ecosystem carbon emission assessments. However, because of limitations in historical data, quantitative estimations and spatially gridded reconstructions of these changes remain challenging, necessitating further methodological exploration. This study focused on China's present-day land area over the past millennium, objectively capturing the characteristics and drivers of forest and grassland cover changes. On this basis, using the forest transition theory and the space-for-time substitution method, we depicted the historical deforestation process as an inverted S curve and developed a model to reconstruct historical forest area changes based on the functional relationship between the forest area and population size dynamics. Subsequently, a gridded forest allocation model was established on the basis of deforestation tendencies. For the grassland cover, we implemented region-specific methods, such as the cropland area deduction method and the habitat constraint method, to quantitatively reconstruct historical changes. Consequently, we obtained provincial forest and grassland area changes over the past millennium and mapped 10-km-resolution gridded data of forest and grassland cover. The results indicated the following. (1) The methods developed using population data as a proxy objectively reproduced the spatiotemporal evolution of forest and grassland cover in China over the past millennium. These feasible methods offer a novel pathway for the quantitative reconstruction of historical forest and grassland cover changes. (2) The data indicated that China's forest area generally decreased over the past millennium, characterized by a decrease-then-increase pattern. The forest area experienced three distinct phases: a slow decline (AD 1000-1650), a rapid decline (AD 1650-1960), and a gradual recovery (AD 1960-2000). The area decreased from 298 million hectares (Mha) in AD 1000 to 89 Mha in AD 1960 before increasing to 153 Mha in AD 2000. Spatially, deforestation began in the middle-lower reaches of the Yellow River and gradually expanded to the middle-lower reaches of the Yangtze River, the southern coastal areas of China, southwest China, and northeast China, with the forest cover declining by 27%, 40%, 58%, 55%, and 35% in these regions, respectively. (3) China's grassland area has shown a continuous decline over the past millennium with three phases: stable fluctuation (AD 1000-1600), slow decline (AD 1600-1900), and rapid decline (AD 1900-2000). The grassland area decreased from 305 Mha in AD 1000 to 277 Mha in AD 2000. Notably, zonal grassland areas in Northeast China, Inner Mongolia, Gan-Ning, Qinghai, Xinjiang, and Xizang decreased by 28 Mha over the millennium, whereas nonzonal secondary grassland areas in the hilly and mountainous areas of eastern and southern China increased by 0.3 Mha. |
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