The identity of the dominant microbial symbionts in a forest determines the ability 70 of trees to access limiting nutrients from atmospheric or soil pools 1,2 , sequester 71 carbon 3,4 and withstand the impacts of climate change 1-7 . Characterizing the global 72 distribution of symbioses, and identifying the factors that control it, are thus integral to 73 understanding present and future forest ecosystem functioning. Here we generate the first 74 spatially explicit map of forest symbiotic status using a global database of 1.2 million forest 75 inventory plots with over 28,000 tree species. Our analyses indicate that climatic variables, 76 and in particular climatically-controlled variation in decomposition rate, are the primary 77 drivers of the global distribution of major symbioses. We estimate that ectomycorrhizal 78 (EM) trees, which represent only 2% of all plant species 8 , constitute approximately 60% of 79 tree stems on Earth. EM symbiosis dominates forests where seasonally cold and dry 80

Summary 1.We hypothesized that tropical plant species with different mycorrhizal associations reduce competition for soil phosphorus (P) by specializing to exploit different soil organic P compounds. 2. We assayed the activity of root/mycorrhizal phosphatase enzymes of four tree species with contrasting root symbiotic relationships -arbuscular mycorrhizal (AM) (angiosperm and conifer), ectomycorrhizal (EM) and non-mycorrhizal -collected from one of three soil sites within a montane tropical forest. We also measured growth and foliar P of these seedlings in an experiment with P provided exclusively as inorganic orthophosphate, a simple phosphomonoester (glucose phosphate), a phosphodiester (RNA), phytate (the sodium salt of myo-inositol hexakisphosphate) or a no-P control. 3. The EM tree species expressed twice the phosphomonoesterase activity as the AM tree species, but had similar phosphodiesterase activity. The non-mycorrhizal Proteaceae tree had markedly greater activity of both enzymes than the mycorrhizal tree species, with root clusters expressing greater phosphomonoesterase activity than fine roots. 4. Both the mycorrhizal and non-mycorrhizal tree species contained significantly greater foliar P than in no-P controls when limited to inorganic phosphate, glucose phosphate and RNA. The EM species did not perform better than the AM tree species when limited to organic P in any form. In contrast, the non-mycorrhizal Proteaceae tree was the only species capable of exploiting phytate, with nearly three times the leaf area and more than twice the foliar P of the no-P control. 5. Our results suggest that AM and EM tree species exploit similar forms of P, despite differences in phosphomonoesterase activity. In contrast, the mycorrhizal tree species and nonmycorrhizal Proteaceae appear to differ in their ability to exploit phytate. We conclude that resource partitioning of soil P plays a coarse but potentially ecologically important role in fostering the coexistence of tree species in tropical montane forests.

Findings of immense microbial diversity are at odds with observed functional redundancy, as competitive exclusion should hinder coexistence. Tradeoffs between dispersal and competitive ability could resolve this contradiction, but the extent to which they influence microbial community assembly is unclear. Because fungi influence the biogeochemical cycles upon which life on earth depends, understanding the mechanisms that maintain the richness of their communities is critically important. Here, we focus on ectomycorrhizal fungi, which are microbial plant mutualists that significantly affect global carbon dynamics and the ecology of host plants. Synthesizing theory with a decade of empirical research at our study site, we show that competition-colonization tradeoffs structure diversity in situ and that models calibrated only with empirically derived competition-colonization tradeoffs can accurately predict species-area relationships in this group of key eukaryotic microbes. These findings provide evidence that competition-colonization tradeoffs can sustain the landscape-scale diversity of microbes that compete for a single limiting resource.

Evolutionary theory predicts that mutualisms based on the reciprocal exchange of costly services should be susceptible to exploitation by cheaters. Consistent with theory, both cheating and discrimination against cheaters are ubiquitous features of mutualisms. Several recent studies have confirmed that host species differ in the extent that they are able to discriminate against cheaters, suggesting that cheating may be stabilized by the existence of susceptible hosts (dubbed "givers"). We use an evolutionary gametheoretical approach to demonstrate how discriminating and giver hosts associating with mutualist and cheater partners can coexist. Discriminators drive the proportion of cheaters below a critical threshold, at which point there is no benefit to investing resources into discrimination. This promotes givers, who benefit from mutualists but allow cheater populations to rebound. We then apply this model to the plant-mycorrhizal mutualism and demonstrate it is one mechanism for generating host-specific responses to mycorrhizal fungal species necessary to generate negative plant-soil feedbacks. Our model makes several falsifiable, qualitative predictions for plant-mycorrhizal population dynamics across gradients of soil phosphorus availability and interhost differences in ability to discriminate. Finally, we suggest applications and limitations of the model with regard to coexistence in specific biological systems.

Aim Ectomycorrhizal fungi (ECMF) are partners in a globally distributed tree symbiosis implicated in most major ecosystem functions. However, resilience of ECMF to future climates is uncertain. We forecast these changes over the extent of North American Pinaceae forests. Location About 68 sites from North American Pinaceae forests ranging from Florida to Ontario in the east and southern California to Alaska in the west. Taxon Ectomycorrhizal fungi (Asco‐ and Basidiomycetes). Methods We characterized ECMF communities at each site using molecular methods and modelled climatic drivers of diversity and community composition with general additive, generalized dissimilarity models and Threshold Indicator Taxa ANalysis (TITAN). Next, we projected our models across the extent of North American Pinaceae forests and forecast ECMF responses to climate changes in these forests over the next 50 years. Results We predict median declines in ECMF species richness as high as 26% in Pinaceae forests throughout a climate zone comprising more than 3.5 million square kilometres of North America (an area twice that of Alaska state). Mitigation of greenhouse gas emissions can reduce these declines, but not prevent them. The existence of multiple diversity optima along climate gradients suggest regionally divergent trajectories for North American ECMF, which is corroborated by corresponding ECMF community thresholds identified in TITAN models. Warming of forests along the boreal–temperate ecotone results in projected ECMF species loss and declines in the relative abundance of long‐distance foraging ECMF species, whereas warming of eastern temperate forests has the opposite effect. Main Conclusions Our results reveal potentially unavoidable ECMF species‐losses over the next 50 years, which is likely to have profound (if yet unclear) effects on ECMF‐associated biogeochemical cycles.