Understanding The Impact of Arrival Order on Ectomycorrhizal Competition and Host Plant Growth

Ectomycorrhizal (ECM) fungi form partnerships with roughly 60% of tree species, trading nutrients, disease defense, and water for photosynthetic sugars. Different species of ECM fungi will compete for these photosynthetic sugars, and their success is highly dependent on when they colonize relative to other competitors —a phenomenon known as “priority effects”. Previous studies have shown changes in ECM fungal arrival timing are crucial in shaping ECM fungal communities. However, it is not well documented how these arrival timing changes affect plant host growth. This study aims to elucidate how different fungal arrival scenarios affect ECM fungal competitive success and host plant growth by inoculating Pinus muricata seedlings with two ECM fungal species, Rhizopogon salebrosus and Suillus pungens in different timing scenarios. S. pungens was the competitive dominant species, and consistently outcompeted R. salebrosus in all arrival timing scenarios. Both S. pungens and R. salebrosus colonized more root tips when they arrived first, underscoring the importance of priority effects in ECM fungal competition. Staggered arrival treatments, where one fungus was inoculated 31 days before the other, had similar plant growth to simultaneous co-inoculation treatments. In contrast, all combinations of two-species inoculation treatments (both staggered and co-arrival) had greater plant growth than single-species inoculation treatments, suggesting that the presence of more mutualists was more important to plant growth than arrival synchrony. These results support the idea that symbiotic diversity enhances plant host fitness, and that competition between ectomycorrhizal partners does not negatively impact the host. Overall, the findings in this project improve current understanding of how ectomycorrhizal fungal communities assemble and are relevant to developing more holistic strategies for protecting forest ecosystems.