PS 40-118 - Mycorrhizal-associated nutrient economies are modulated by the density of small stems in temperate forest understories

Abstract: Forest composition is shifting due to changes in climate and disturbance regimes, but the consequences of such shifts are unclear. In eastern forests of the US, trees that associate with arbuscular mycorrhizal (AM) fungi are becoming increasingly dominant relative to trees that associate with ectomycorrhizal (ECM) fungi. However, most estimates of shifts in tree-mycorrhizal dominance have been based on tree basal area (sensu the “mass ratio hypothesis”), with less consideration given to the density of small stems, which may enhance or diminish the magnitude of tree effects on soils owing to trait dissimilarity between understory and overstory trees (sensu the “trait dissimilarity” hypothesis). To understand how interactions between tree basal area and stem density influence soil characteristics and processes, we synthesized data from five large (16-25 ha) Forest Global Earth Observatory (ForestGEO) plots in the eastern US that contained a mixture of AM and ECM trees ( >150,000 stems). At each site, all stems greater than 1cm diameter were measured and soil properties known to track mycorrhizal dominance - soil pH, soil C:N and net nitrification rates – were quantified. Soils data were matched to tree census data within 20x20m quadrats, and relationships between soils and mycorrhizal dominance based on basal area were compared to those between soils and mycorrhizal dominance based on stem density.

In general, basal area and stem density exhibited similar relationships to soils and followed expected patterns (e.g., decreasing pH and nitrification, and increasing C:N, with increasing ECM dominance). However, at some sites and for certain variables, stem dominance had equal or better predictive power than basal-area dominance. These effects were strongest when all stems (down to 1cm diameter) were included and weakest when the small stems were excluded. In plots where there were mismatches in the mycorrhizal dominance of stems vs basal area (e.g., AM stems dominating in ECM basal-area dominated plots and vice versa), the effects of the understory stems weakened the predicted relationship between mycorrhizal dominance and soils. Collectively, our results indicate mycorrhizal-associated nutrient economies are not solely due to the largest trees in a plot, but can be influenced by smaller, understory trees. Basal area measures alone may therefore be insufficient for assessing relationships between tree-mycorrhizal dominance and soils, at least in eastern temperate forests.