Mutualist limitation; the strong influence mycorrhizal arrival has on interspecific competition

Mycorrhizae represents one of Earth’s first mutualisms to take place on terrestrial habitats. The union of plant roots and fungal hyphae has allowed both parties to live from the dynamic, ever changing forest floor for millennia. We are aware of the direct, positive services these fungi provide its plant partner, but there is still so much to learn. New research suggests that the timing of mycorrhizal inoculation is a critical part of the plant’s life and depending on what species your looking at, can be more important than competition with other species. Since the genesis of modern ecology, we have understood that competition is a main driver of species distribution. This still holds true, but now we are finding additional driving forces that point to the fungal kingdom. 

Two mycorrhizal types. From Bonfante & Genre 2010.

Kabir Peay wanted to test the differences between the fitness of Pinus muricata seedlings in the face of interspecific competition and varying timing of mutualist arrival. As you know, pine trees form mutualistic relationships with higher, ectomycorrhizal fungi. To adequately test interspecific competition without soil mutualists complicating interactions, Pinus muricata was grown with the arbuscular mycorrhizal plant competitor Baccharis pilularis. Because the plants used don’t share the same mutualist type, the symbiote of one competitor will not colonize the other competitor during the experiment which would ultimately lessen the impact of competition. Pine seedlings that grew in the absence of the plant competitor represent the control parameters in this first experiment.

Pinus muricata.

In the other experiment, Kabir Peay inoculated sterile seedlings of Pinus muricata with the ectomycorrhizal fungus, Suillus pungens at 10, 41, or 144 days after planting. The controls in this experiment were never inoculated with the plant’s symbiote. Together, these two experiments paint a more detailed picture of what’s going on beneath the forest floor and show not only the importance of fungal availability, but the timing of fungal arrival.

The results in this paper show that when ectomycorrhizae inoculation is delayed, plant seedling growth is reduced 4-8-fold. Competition too reduced pine growth, but the availability of the pine’s fungal mutualist was actually more significant. These interactions combine, with Pinus muricata seedlings being more impacted by plant competitor when it had delayed or absent mycorrhizal inoculation. This author clearly shows that for these specific biotic systems, fungal mutualisms are a greater driver of ecological interactions than interspecific competition. 

Pines that had later arriving fungal mutualists were more easily outcompeted by plant competitor. Abiotic factors such as soil content (two different colors) also had a significant influence. From Peay 2017. 

Although higher fungi are widely known for dispersing insane quantities of fungal spores over vast distances, newer research is showing that fungal communities are actually quite patchy. The mycorrhizal fungi that is available to seedlings have a much more important influence than we previously thought. The arrival time of mycorrhizal fungi can make or break seedling success, especially when other plant competitors with differential fungal mutualists are growing nearby. This temporal variability in fungal mutualists is likely to continue driving forest community assemblages, and most likely has shaped plant phenology. Some species like Pinus muricata have such a dependence on their fungal mutualist, it would make sense that the production of seeds from mature plants coincides with the fruiting of their compatible mycorrhizal fungi.

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