Historical forest coverage (HFC) dataset

We reconstructed a historical forest cover dataset for China from 1900 to 2020 at five-year intervals and 1 km spatial resolution (Fig. 1). Evaluation of the Mixed-Cell Cellular Automata (MCCA) model indicated an overall accuracy of 0.97, F1-score of 0.91, and Kappa coefficient of 0.89 for the 2000s (SI Appendix, Table S1). Subsequently, we used historical topographic maps surveyed in the 1950s to further validate the dataset, which yielded an overall accuracy of 0.97, F1-score of 0.82, and Kappa coefficient of 0.81 (SI Appendix, Fig. S1, Table S2). To assess historical accuracy, we used the locations of 283,707 georeferenced old trees, each over 100 years old, as indicators of early 20th-century forest fragments (SI Appendix, Fig. S2). To validate the forest cover maps, we identified a 1 km2 pixel as a ‘hit’ if it contained both at least one old tree and a forested area ≥1 hectare (i.e., ≥1% forest cover). This threshold provides a conservative yet ecologically justified benchmark for forest presence, capturing the minimal extent of a forest patch likely to contain ancient trees, given their average density (~0.36 trees/km²) (39), while reducing misclassification risks from isolated pixels or geolocation uncertainty. Based on this criterion, 93.86% (n = 266,294) of old trees in 2020 and 78.66% (n = 223,152) of those estimated to be present in 1900 fell within qualifying pixels. Additionally, when using a stricter 10-hectare threshold, 84.21% (n = 238,912) in 2020 and 70.05% (n = 198,742) in 1900 still met the criterion. These high concordance rates across spatial thresholds and time periods underscore the reliability of our forest reconstruction and the robustness of the multi-source validation strategy.

本研究重建了一套1900年至2020年中国历史森林覆盖数据集，时间间隔为5年，空间分辨率为1千米（图1）。对混合元胞自动机（Mixed-Cell Cellular Automata, MCCA）模型的评估结果显示，2000年代的总体精度为0.97，F1分数为0.91，Kappa系数（Kappa coefficient）为0.89（补充材料附录表S1）。随后，我们利用1950年代测绘的历史地形图对该数据集开展进一步验证，得到总体精度为0.97，F1分数为0.82，Kappa系数为0.81（补充材料附录图S1、表S2）。为评估该数据集的历史精度，我们以283707棵经地理配准（georeferenced）的百年以上古树的点位，作为20世纪早期森林斑块的指示标志（补充材料附录图S2）。为验证森林覆盖图，我们将同时满足以下两个条件的1平方千米像素定义为“命中”：该像素内至少包含1棵古树，且森林覆盖面积≥1公顷（即森林覆盖率≥1%）。该阈值为森林存在性提供了一个保守但符合生态学逻辑的基准：考虑到古树的平均密度约为0.36棵/平方千米（文献39），该阈值可捕捉到极有可能包含古树的最小森林斑块范围，同时降低孤立像素或地理定位不确定性带来的误分类风险。基于该标准，2020年的古树中有93.86%（n=266294），以及1900年估算存在的古树中有78.66%（n=223152），均落在符合要求的像素范围内。此外，当采用更严格的10公顷阈值时，2020年仍有84.21%（n=238912）的古树、1900年有70.05%（n=198742）的古树符合该标准。上述不同空间阈值与时间跨度下的高一致性率，均证明了本次森林覆盖重建工作的可靠性，以及多源验证策略的稳健性。