Soil biotic and abiotic thresholds in sugar maple and American beech seedling establishment in forests of the northeastern United States

Aims: Climate change is expected to shift climatic envelopes of temperate tree species into boreal forests where unsuitable soils may limit range expansion. We studied several edaphic thresholds (mycorrhizae, soil chemistry) that can limit seedling establishment of two major temperate tree species, sugar maple (arbuscular mycorrhizal, AM) and American beech (ectomycorrhizal, EM). Methods: We integrate two field surveys of tree seedling density, mycorrhizal colonization, and soil chemistry in montane forests of the Adirondack and Green Mountains (Mtns) in the northeastern United States. We conducted correlation and linear breakpoint analyses to detect soil abiotic and biotic thresholds in seedling distributions across edaphic gradients. Results: In the Green Mtns, sugar maple seedling importance (an index of species relative density and frequency, IV) declined sharply with low pH (< 3.74 in mineral soil) and low mycorrhizal colonization (< 27.5% root length colonized). Sugar maple importance was highly correlated with multiple aspects of soil chemistry, while beech was somewhat sensitive to pH only; beech mycorrhizal colonization did not differ across elevation. Mycorrhizal colonization of sugar maple was positively correlated with soil pH and conspecific overstory basal area. In the Adirondacks, sugar maple importance, but not beech, plateaued above thresholds in soil calcium (~ 2 meq/100 g) and magnesium (~ 0.3 meq/100 g). Conclusions: The establishment of sugar maple, but not beech, was impeded by both biotic and abiotic soil components in montane conifer forests and by soil acidity in temperate deciduous forests. These differences in species sensitivity to edaphic thresholds will likely affect species success and future shifts in forest composition.