Mutualism as a Plant Functional Trait: Linking Variation in the Mycorrhizal Symbiosis to Climatic Tolerance, Geographic Range, and Population Dynamics
Plant functional traits, commonly defined as anatomical, physiological, and morphological characters that influence individual fitness and population growth, and thus ecological distribution, have been studied for more than a century. Plant interactions with microbial mutualists could also be functional traits because they influence how plants respond to environmental variation, but tests of this hypothesis are rare. I explored whether variation in the plant mutualism with arbuscular mycorrhizal (AM) fungi is associated with climatic tolerances, geographic range size, and fitness components (survival, reproduction, individual growth) that influence population growth. Analyses were done with categorical and quantitative metrics of mutualism variation to determine whether outcomes were consistent. Plants that are always colonized by AM fungi (AM state) occurred in warmer climates than either plants that are never colonized by AM fungi (NM state) or plants that vary in whether they are colonized (AMNM state), whereas AMNM plants had larger geographic ranges than either AM or NM plants. The effect of variation in individual growth rate on population growth rate was significantly higher in AMNM and NM plants than in AM plants, whereas the effect that variation in survival and reproduction had on population growth rate did not differ among mycorrhizal states. Quantitative variation in the mutualism, expressed as mycorrhizal growth response (MGR; biomass ratio of AM fungal–inoculated plants to noninoculated controls) rarely predicted climate tolerances and geographic range size. These findings suggest that mycorrhizal state is a functional trait based on traditional definitions but that MGR may not necessarily reflect functionally meaningful variation in the mutualism, at least not beyond the context of the conditions in which it was measured. Nonetheless, these findings provide the first evidence linking variation in mycorrhizal mutualism to the fitness components that influence population growth, implying that variation in plant interactions with mycorrhizal fungi has consequences for plant population dynamics.