Department of Biology, Stanford University, Stanford, CA, USA.
Department of Environmental Systems Science, ETH Zürich, Zürich, Switzerland.
Nature. 2019 May;569(7756):404-408. doi: 10.1038/s41586-019-1128-0. Epub 2019 May 15.
The identity of the dominant root-associated microbial symbionts in a forest determines the ability of trees to access limiting nutrients from atmospheric or soil pools, sequester carbon and withstand the effects of climate change. Characterizing the global distribution of these symbioses and identifying the factors that control this distribution are thus integral to understanding the present and future functioning of forest ecosystems. Here we generate a spatially explicit global map of the symbiotic status of forests, using a database of over 1.1 million forest inventory plots that collectively contain over 28,000 tree species. Our analyses indicate that climate variables-in particular, climatically controlled variation in the rate of decomposition-are the primary drivers of the global distribution of major symbioses. We estimate that ectomycorrhizal trees, which represent only 2% of all plant species, constitute approximately 60% of tree stems on Earth. Ectomycorrhizal symbiosis dominates forests in which seasonally cold and dry climates inhibit decomposition, and is the predominant form of symbiosis at high latitudes and elevation. By contrast, arbuscular mycorrhizal trees dominate in aseasonal, warm tropical forests, and occur with ectomycorrhizal trees in temperate biomes in which seasonally warm-and-wet climates enhance decomposition. Continental transitions between forests dominated by ectomycorrhizal or arbuscular mycorrhizal trees occur relatively abruptly along climate-driven decomposition gradients; these transitions are probably caused by positive feedback effects between plants and microorganisms. Symbiotic nitrogen fixers-which are insensitive to climatic controls on decomposition (compared with mycorrhizal fungi)-are most abundant in arid biomes with alkaline soils and high maximum temperatures. The climatically driven global symbiosis gradient that we document provides a spatially explicit quantitative understanding of microbial symbioses at the global scale, and demonstrates the critical role of microbial mutualisms in shaping the distribution of plant species.
森林中优势根际共生微生物的身份决定了树木从大气或土壤库中获取限制养分、固碳以及抵御气候变化影响的能力。因此，描述这些共生关系的全球分布并确定控制这种分布的因素，对于理解森林生态系统的现状和未来功能至关重要。在这里，我们利用一个包含超过 110 万森林清查样地的数据库，生成了一个具有空间分辨率的全球森林共生状态图，这些样地总共包含了超过 28000 个树种。我们的分析表明，气候变量——特别是受气候控制的分解速率变化——是主要共生关系全球分布的主要驱动因素。我们估计，外生菌根树木（代表所有植物物种的 2%）构成了地球上约 60%的树木茎干。外生菌根共生关系在季节性寒冷和干燥的气候抑制分解的森林中占主导地位，是高纬度和高海拔地区的主要共生形式。相比之下，内生菌根树木在无季节性、温暖的热带森林中占主导地位，并且在季节性温暖和湿润的气候促进分解的温带生物群落中与外生菌根树木共同存在。由外生菌根或内生菌根树木主导的森林在大陆之间的过渡相对突然地发生在气候驱动的分解梯度上；这些过渡可能是植物和微生物之间的正反馈效应造成的。共生固氮生物——与菌根真菌相比，它们对分解的气候控制不敏感——在具有碱性土壤和最高温度较高的干旱生物群落中最为丰富。我们记录的这种气候驱动的全球共生梯度提供了一个具有空间分辨率的全球微生物共生的定量理解，并证明了微生物共生在塑造植物物种分布方面的关键作用。
