Over the last 50 years, the eastern deciduous forest biome has experienced substantial climate and atmospheric fluctuations. The Clean Air Acts (CAA, 1970 & 1990) have decreased sulfur and nitrogen deposition (increasing soil pH) in the northern Ohio region. However, the Emerald Ash Borer (EAB) invasion has caused rapid and dramatic forest change, alongside increased monthly mean temperatures and precipitation at our 8 ha deciduous forest study site. We are using permanent plot surveys to examine relationships between tree growth (size, density, and distribution), forest habitat types (floodplain, sloped, and upland), environment (temperature, precipitation, NO3, SO4), and mycorrhizal associations (arbuscular mycorrhizal (AM) & ectomycorrhizal (EM)). The surveys (1974, 1986, 1998, & 2015) dovetail with the CAA and amendments, and they bracket pre- and post-EAB eras. The forest preserve contains 33 AM and 17 EM tree species, important for understanding changes in forest successional trends. Since atmospheric pollution - particularly N and S compounds – can impact many attributes of mycorrhizal fungi, we reasoned that tree successional dynamics may have been indirectly influenced by changes in their mycorrhizal associations. Multivariate analyses revealed that the effects of decreased SO4 and NO3 (and increased pH) resulted in marginally significant (p < 0.10) forest compositional responses. Overall, the forest experienced significant (p < 0.01) declines in the effective number of tree species for both density and basal area measures. Basal area growth was the highest between 1974 and 1986 for all three habitats, but greatest in the floodplain (p < 0.001) followed by declining growth in all three habitats in subsequent sample years (p < 0.001). Different forest layers (canopy, understory, and sapling) revealed AM dominance across all plots, with fewer EM associations in plots containing younger trees (understory and sapling). The understory showed the most AM dominance, with only one tree species, Ostrya virginiana, having an EM association. The sapling and understory layers had statistically significant correlations with SO4 and pH (p < 0.01). We question if the decrease in EM associations are partially attributable to the lag time of the CAA’s effects on SO4 and NO3 pollution, as reflected in the forest composition changes. Our analyses so far indicate that sample year, forest layer, and habitat differences result from interactions of pollution reduction and climate change.
