Colin Averill, Jennifer M. Bhatnagar,   Michael C. Dietze, William D. Pearse, and Stephanie N. Kivlin

PNAS November 12, 2019 116 (46) 23163-23168

ECOLOGY

Significance

Most plants on Earth form a symbiosis with root-associated (i.e., mycorrhizal) fungi, trading plant photosynthate for fungal-acquired soil nutrients. Ecologists have long thought that different types of mycorrhizal fungi—arbuscular vs. ectomycorrhizal—represent adaptations to high vs. low soil nutrient availability. Here we show that these different mycorrhizal associations are linked to differences in a suite of plant traits related to nutrient economic strategies. Ectomycorrhizal plant species are more nutrient use-conservative than arbuscular mycorrhizal plant species, an effect that is robust to controlling for plant growth form and evolutionary history. These findings bolster emerging theories in ecosystem ecology that leverage the ecology of mycorrhizal fungi to better predict ecosystem carbon-nutrient cycle interactions.

Abstract

Mycorrhizal fungi are critical members of the plant microbiome, forming a symbiosis with the roots of most plants on Earth. Most plant species partner with either arbuscular or ectomycorrhizal fungi, and these symbioses are thought to represent plant adaptations to fast and slow soil nutrient cycling rates. This generates a second hypothesis, that arbuscular and ectomycorrhizal plant species traits complement and reinforce these fungal strategies, resulting in nutrient acquisitive vs. conservative plant trait profiles. Here we analyzed 17,764 species level trait observations from 2,940 woody plant species to show that mycorrhizal plants differ systematically in nitrogen and phosphorus economic traits. Differences were clearest in temperate latitudes, where ectomycorrhizal plant species are more nitrogen use- and phosphorus use-conservative than arbuscular mycorrhizal species. This difference is reflected in both aboveground and belowground plant traits and is robust to controlling for evolutionary history, nitrogen fixation ability, deciduousness, latitude, and species climate niche. Furthermore, mycorrhizal effects are large and frequently similar to or greater in magnitude than the influence of plant nitrogen fixation ability or deciduous vs. evergreen leaf habit. Ectomycorrhizal plants are also more nitrogen conservative than arbuscular plants in boreal and tropical ecosystems, although differences in phosphorus use are less apparent outside temperate latitudes. Our findings bolster current theories of ecosystems rooted in mycorrhizal ecology and support the hypothesis that plant mycorrhizal association is linked to the evolution of plant nutrient economic strategies.

See https://www.pnas.org/content/116/46/23163

Figure 2: Arbuscular vs. ectomycorrhizal trait means as estimated by calculating a latitude-specific posterior and its 95% credible interval: N green (A), N senescent (B), N roots (C), P green (D), P senescent (E), P roots (F). Posterior estimates account for differences in mean climate values across latitudes and the effect of climate predictors on trait values. Because N-fixation status, leaf habit, and plant growth form also affect the trait intercept value, all plotted values are normalized to the angiosperm plant growth form, evergreen leaf habit, and non–N-fixing status.