A poor understanding of the fraction of global plant biomass occurring belowground as roots limits our understanding of present and future ecosystem function and carbon pools. Here we create a database of root-mass fractions (RMFs), an index of plant below- versus aboveground biomass distributions, and generate quantitative, spatially explicit global maps of RMFs in trees, shrubs and grasses. Our analyses reveal large gradients in RMFs both across and within vegetation types that can be attributed to resource availability. High RMFs occur in cold and dry ecosystems, while low RMFs dominate in warm and wet regions. Across all vegetation types, the directional effect of temperature on RMFs depends on water availability, suggesting feedbacks between heat, water and nutrient supply. By integrating our RMF maps with existing aboveground plant biomass information, we estimate that in forests, shrublands and grasslands, respectively, 22%, 47% and 67% of plant biomass exists belowground, with a total global belowground fraction of 24% (20–28%), that is, 113 (90–135) Gt carbon. By documenting the environmental correlates of root biomass allocation, our results can inform model projections of global vegetation dynamics under current and future climate scenarios.

对作为根存在于地下的全球植物生物量的比例知之甚少，限制了我们对当前和未来生态系统功能和碳库的理解。在这里，我们创建了一个根质量分数 (RMF) 数据库，这是植物地下与地上生物量分布的索引，并生成了树木、灌木和草中 RMF 的定量、空间明确的全球地图。我们的分析揭示了植被类型之间和植被类型内 RMF 的大梯度，这可归因于资源可用性。高 RMFs 出现在寒冷和干燥的生态系统中，而低 RMFs 在温暖和潮湿的地区占主导地位。在所有植被类型中，温度对 RMF 的定向影响取决于水的可用性，这表明热量、水和养分供应之间存在反馈。通过将我们的 RMF 地图与现有的地上植物生物量信息相结合，我们估计在森林、灌木丛和草地中，分别有 22%、47% 和 67% 的植物生物量存在于地下，全球地下总比例为 24%（20– 28%)，即 113 (90–135) Gt 碳。通过记录根生物量分配的环境相关性，我们的结果可以为当前和未来气候情景下全球植被动态的模型预测提供信息。