Mycorrhizal symbiosis affects the realized niches of plant species

Mechanisms of coexistence has fascinated ecologists for a long time and one of the proposed ways is minimizing competition by niche differentiation. According to this, to coexist, species must differ in their realized niches (i.e. coexisting species must have distinct resource and habitat requirements). Traditionally, the realized niches are thought to be affected by competitors, but recent hypotheses state that symbiotic relationships could also be important.

Therefore, in a paper published in Journal of Ecology, we investigated whether and how the associations with symbiotic mycorrhizal fungi alter the realized niches of vascular plant species, using plant species co-occurrence data from the Netherlands and plant mycorrhizal trait data. We found that, indeed, plants with different mycorrhizal statuses and types had distinct environmental preferences. In addition, the ranges of environmental conditions which plant species tolerate (niche widths), depend on the mycorrhizal status and mycorrhizal type. Specifically, facultatively mycorrhizal plant species had wider niches compared to obligately and non-mycorrhizal plants, indicating that the ability of the faculatively mycorrhizal plants to regulate the presence of the symbiosis plays a key role in determining the range of habitats these plants can occupy. Regarding mycorrhizal types, ecto- and ericoid mycorrhizal plants had wider niches than plant species with other mycorrhizal types. For the ectomycorrhizal plants the underlying mechanisms can possibly be the higher diversity of ectomycorrhizal fungal symbionts and positive plant-soil feedbacks, whereas for the ericoid plants the mechanisms remain unclear.

The differences in plant niches among distinct groups of plants indicate that mycorrhizal symbiosis is an important contributing factor to plant coexistence, and this information could also help predicting vegetation change due to climate change or human impact.

Realized niche volume for obligately (OM) mycorrhizal, facultatively mycorrhizal (FM) and non-mycorrhizal (NM) status, for arbuscular (AM), dual (AEM), ecto- (EcM), ericoid (ErM) and orchid (OrM) mycorrhizal type, and for flexible (FL) and inflexible (IFL) plants. (Graph 2b from the paper.)

Citation: Gerz, M., Bueno, C. G., Ozinga, W. A., Zobel, M., & Moora, M. (2017). Niche differentiation and expansion of plant species are associated with mycorrhizal symbiosis. Journal of Ecology, DOI: 10.1111/1365-2745.12873 (link to full text)

Abstract: 

1. Mycorrhizal symbiosis is a widespread association between plant roots and mycorrhizal fungi, which is thought to contribute to plant niche differentiation and expansion. However, this has so far not been explicitly tested.
2. To address the effect of mycorrhizal symbiosis on plants’ realized niches, we addressed how mycorrhizal status (i.e. the frequency of occurrence of mycorrhizal symbiosis), flexibility (i.e. the ability to grow both with and without mycorrhizal symbiosis) and type of a plant species affect the realized niche optima, widths and volumes. For this, we used co-occurrence data from the flora of the Netherlands along soil fertility, moisture, pH, salinity, light and temperature gradients. Phylogenetic dependency of the species was taken into account using phylogenetic generalized least squares models.
3. We show that facultatively and flexibly mycorrhizal plants have the widest niches compared to non-mycorrhizal and obligately mycorrhizal, and inflexible plants respectively. Among obligate plant symbionts, ecto-and ericoid mycorrhizal plants exhibited the widest niches compared to plants with other mycorrhizal types. Also, plants with different mycorrhizal statuses and types differed in their realized niche optima.
4. Synthesis. Our results indicate that mycorrhizal symbiosis mediates plant niche differentiation and expansion, facilitating the understanding of current distribution patterns of plant species, as well as predicting shifts in plant distribution and dominance due to environmental changes.