Tree seedling shade tolerance may be governed by not only physiological carbon balance, but also responses to soil fungal pathogens and mycorrhizae. This is important because the seedling-establishment phase is a major demographic bottleneck, thereby influencing tree species diversity. Several factors could constrain seedling establishment, but their interactions are not well known. Soil fungal pathogens may reduce seedling populations through distance- and density-dependent mortality. Mycorrhizae may ameliorate or exaggerate pathogen effects, depending upon environmental conditions. Also, species may have differential survivorship under low light, which may be associated with trade-offs between growth and defense against fungal pathogens. Despite the importance of pathogens, mycorrhizae, and irradiance in seedling establishment, they are rarely studied together. To investigate these interactions, I conducted a greenhouse experiment, controlling pathogen/mycorrhizae presence and irradiance, across three Acer species differing in shade tolerance.
Results/Conclusions
When grown in sterile conditions without mycorrhizae or pathogens, species did not significantly differ in mortality. Under high light with the addition of fungi, species did not differ in mortality, except Acer rubrum experienced greater mortality than A. negundo and A. saccharum with both pathogens and mycorrhizae present. In general, mortality due to addition of fungi was limited to low-light conditions, suggesting that plant-soil feedbacks may influence light gradient partitioning among species. Across species, seedlings species experienced differential mortality under low light with the addition of pathogens and/or mycorrhizae, with A. negundo and A. rubrum experiencing higher mortality than A. saccharum (considered a more shade tolerant species). Under low light, seedlings experienced similar mortality with the addition of mycorrhizae to the addition of pathogens, suggesting that mycorrhizae act parasitically under low light. These interactions may exaggerate differences among tree seedling species in understory mortality and provide opportunities for species coexistence.