Knowing the extent and environmental drivers of forests is key to successfully restore degraded ecosystems, and to mitigate climate change and desertification impacts using tree planting. Water availability is the main limiting factor for the development of forests in drylands, yet the importance of groundwater resources and palaeoclimate as drivers of their current distribution has been neglected. Here we report that mid-Holocene climates and aquifer trends are key predictors of the distribution of dryland forests worldwide. We also updated the global extent of dryland forests to 1,283 million hectares and showed that failing to consider past climates and aquifers has resulted in ignoring or misplacing up to 130 million hectares of forests in drylands. Our findings highlight the importance of a wetter past and well-preserved aquifers to explain the current distribution of dryland forests, and can guide restoration actions by avoiding unsuitable areas for tree establishment in a drier world.

了解森林的范围和环境驱动因素是成功恢复退化生态系统以及通过植树缓解气候变化和荒漠化影响的关键。水的可用性是旱地森林发展的主要限制因素，但地下水资源和古气候作为其当前分布驱动因素的重要性却被忽视了。在这里，我们报告说，全新世中期气候和含水层趋势是全球旱地森林分布的关键预测因素。我们还将旱地森林的全球范围更新为 12.83 亿公顷，并表明未能考虑过去的气候和含水层导致忽视或错放了多达 1.3 亿公顷的旱地森林。