Seedling abundance response to increased inorganic soil nutrients depends on mycorrhizal associations

Background/Question/Methods

In the temperate forests of eastern North America, most tree species are associated with arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. These two mycorrhizal types have some broad functional differences that influence their ability to access certain pools of nutrients. In general, AM fungi are hypothesized to mostly utilize inorganic nutrients while ECM fungi have a greater capacity to produce extracellular enzymes to degrade soil organic matter, allowing them to utilize organic nutrients. Leveraging a 10-year soil manipulation experiment designed to promote inorganic nutrient availability (via raising pH with lime), we discovered the treatment increased growth rates in AM associated trees, but decreased growth rates in ECM associated trees. However, to understand the long-term impacts on forest composition and dynamics, we need to consider effects on recruitment. Thus, this study quantified how seedling abundance and composition responded to the same altered soil conditions. In 2019, we collected data on seedlings (DBH < 0.1 cm) and saplings (DBH between 0.1 – 4.9 cm) in both the control and elevated pH treatments. All seedlings and saplings were identified to species and assigned to a mycorrhizal type.

Results/Conclusions

In total, we recorded 22,270 seedlings and 561 saplings. There were 38 woody species, however the most abundant species was Acer rubrum, followed by Acer saccharum, Ulmus rubra, Fagus grandifolia, and Fraxinus americana. Overall, there was a significant difference between mycorrhizal types when comparing seedling abundance (P < 0.001), with many more AM associated seedlings than ECM associated seedlings. There was not a significant effect of treatment on seedling abundance (P = 0.19), however there was a significant interaction between treatment and mycorrhizal association (P = 0.007). Overall, AM seedling abundance tended to be higher, but ECM seedlings were less abundant, in the elevated pH treatment when compared to the controls. However, the response to the treatments was not always consistent within a mycorrhizae type. For example, Quercus spp. followed the pattern of reduced ECM seedling abundance in the elevated pH treatment, however Fagus grandifolia (also an ECM) was not sensitive to treatment. Overall, it appears that increasing soil pH and inorganic nutrient availability could have long-term consequences for species composition based on their mycorrhizal association however, as with everything in ecology, there are caveats.