Changes in above- versus belowground biomass distribution in permafrost regions in response to climate warming

Permafrost regions contain approximately half of the carbon stored in land ecosystems and have warmed at least twice as much as any other biome. This warming has influenced vegetation activity, leading to changes in plant composition, physiology, and biomass storage in aboveground and belowground components, ultimately impacting ecosystem carbon balance. Yet, little is known about the causes and magnitude of long-term changes in the above- to belowground biomass ratio of plants (η). Here, we analyzed η values based on 3,013 plots and 26,337 plant-specific measurements representing eight sites across the Tibetan Plateau from 1995 to 2021. Our analysis revealed distinct temporal trends in η for three vegetation types: a 17% increase in alpine wetlands, and a decrease of 26% and 48% in alpine meadows and alpine steppes, respectively. These trends were primarily driven by temperature-induced growth preferences rather than shifts in plant species composition. Our findings indicate that in wetter ecosystems climate warming promotes aboveground plant growth, while in drier ecosystems, such as alpine meadows and alpine steppes, plants allocate more biomass belowground. Four process-based biogeochemical models failed to simulate the observed changes in η, which highlights the importance of improved process understanding of the processes driving the response of biomass distribution to climate warming, which is crucial for predicting the future carbon trajectory of permafrost ecosystems.永久冻土（Permafrost）区域储存了陆地生态系统中约一半的碳，且其升温幅度至少是其他任何生物群系的两倍。该区域的升温已对植被活动产生影响，引发植物群落组成、生理特性以及地上、地下部分生物量储存的变化，最终干扰生态系统碳平衡。然而，学界对植物地上-地下生物量比值（η）长期变化的成因与幅度仍知之甚少。本研究基于1995年至2021年间青藏高原（Tibetan Plateau）8个研究站点的3013个样地与26337份植物特异性测量数据，对η值展开分析。分析结果显示，三类植被的η值呈现显著的时间变化趋势：高寒湿地的η值上升17%，高寒草甸与高寒草原的η值分别下降26%与48%。这些趋势主要由温度诱导的生长偏好驱动，而非植物物种组成的转变。本研究结果表明，在水分充足的生态系统中，气候升温会促进植物地上部分生长；而在高寒草甸、高寒草原等水分受限的生态系统中，植物会将更多生物量分配至地下。四个基于过程的生物地球化学模型均未能模拟出观测到的η值变化，这凸显了深入理解驱动生物量分配响应气候升温的过程的重要性——这对预测永久冻土生态系统未来的碳动态轨迹至关重要。