Mutualisms are ubiquitous in natural systems, but less is known about how these positive interactions influence species distributions compared with antagonistic interactions, such as competition and predation. The niche concept is one useful approach for thinking about factors that shape species ranges, which we apply here towards understanding how the nature of plant-mycorrhizal symbioses change across large environmental gradients. We used a continuous niche mapping approach to examine how two ectomycorrhizal fungi (Thelephora terrestris, Suillus pungens) impact pine seedling growth across a two-dimensional soil nitrogen (N) and phosphorus (P) gradient. We found that ectomycorrhizal fungi improved plant growth most in nutrient addition treatments with highly imbalanced N:P ratios, demonstrating that mycorrhizal benefits depend on interactions between niche axes. However, T. terrestris (highN:lowP) and S. pungens (lowN:highP) benefited plants most at opposite ends of the resource ratio spectrum, consistent with niche partitioning and functional specialization. While ectomycorrhizal fungi are often thought of as being most beneficial for nitrogen uptake, our results suggest that members of the Thelephoraceae may specialize in improving plant P uptake. Ectomycorrhizal colonization by a single fungus increased plant niche volume (calculated as convex hull volumes of plant growth response surfaces across N and P gradients) compared to non-mycorrhizal control plants and shows the overall positive effects of mutualisms on plant niche volume. Despite plant host benefits in S. pungens and T. terrestris single species treatments, the presence of both fungi together decreased plant niche volume. The lack of functional complementarity, despite functional specialization, indicates that in some environments, either fungal competition or the cost of maintaining a suboptimal mycorrhizal partner can limit the benefits of a higher quality partner. The niche mapping approach we present has the potential to answer fundamental questions about the dimensions of functional diversity in ectomycorrhizal fungi and the distributions of mycorrhizal symbioses.

互惠在自然系统中无处不在，但与竞争和掠夺等拮抗作用相比，这些积极的相互作用如何影响物种分布还鲜为人知。生态位概念是一种用于思考影响物种范围的因素的有用方法，在此我们将其应用于了解植物菌根共生物的性质如何在较大的环境梯度中变化。我们使用连续的生态位映射方法来研究两种外生菌根真菌（Thelephora terrestris，Poillus pungens）的情况。）在二维土壤氮（N）和磷（P）梯度上影响松树幼苗的生长。我们发现，在营养比增加的情况下，N：P比例极不平衡的情况下，外生菌根真菌最多能改善植物生长，这表明菌根的益处取决于利基轴之间的相互作用。然而，T.蒺藜（highN：lowP）和S. pungens（lowN：highP）在资源比率谱的相反两端受益最多的植物，这与利基分配和功能专业化相一致。人们通常认为外生菌根真菌对氮的吸收最有益，但我们的结果表明，菊科的成员可能擅长改善植物对磷的吸收。与非菌根对照植物相比，单一真菌的菌根定植增加了植物的生态位体积（计算为跨越N和P梯度的植物生长响应面的凸壳体积），显示了共生对植物生态位体积的总体正面影响。尽管在植物宿主好处S. pungens和T.蒺藜单种处理，两种真菌的存在共同降低了植物的生态位体积。尽管有功能专长，但缺乏功能互补性表明在某些环境中，真菌竞争或维持次优菌根伴侣的成本可能会限制高品质伴侣的利益。我们目前的利基定位方法有可能回答有关外生菌根真菌的功能多样性和菌根共生体分布的基本问题。