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

Abstract 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 and American beech. Methods We integrate two field surveys of tree seedling density, mycorrhizal colonization, and soil chemistry in temperate deciduous and montane conifer forests of the Adirondack and Green Mountains in northeastern United States. We conducted correlation and linear breakpoint analyses to detect soil abiotic and biotic thresholds in species seedling distributions across edaphic gradients. Results In the Green Mtns, sugar maple seedling relative importance (IV) declined sharply at low pH (<3.74 in upper mineral soil) and low mycorrhizal colonization (<27.5% of root length colonized). Sugar maple IV was highly correlated with soil chemistry, while beech was generally insensitive to soil variables. Mycorrhizal colonization of sugar maple was strongly positively correlated with soil pH and conspecific overstory basal area. In the Adirondacks, sugar maple IV plateaued above thresholds in soil calcium (~2 meq/100g) and magnesium (~0.3 meq/100g) where these nutrients were no longer limiting. Sugar maple IV declined steeply with increasing aluminum and decreasing pH. Conclusions The establishment of sugar maple, but not beech, was impeded by both biotic and abiotic soil components in boreal 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.