Acidic atmospheric deposition and soil acidification have suppressed regeneration of important tree species in the northeastern United States (e.g., sugar maple). However, with declining SOx and NOx emissions, regional soils have started to recover at a time when regional ecosystems increasingly experience the effects of changing climate. It is unknown how these simultaneous environmental changes may shape forest regeneration and thus future forest composition. We studied how tree seedling composition changed over time across a well-defined gradient of soil acidification in the Adirondacks and if the observed changes could be explained by changing climate or soil-chemical characteristics. In 2009 and 2015 we counted tree seedlings in the same 50 northern hardwood plots (20 × 50 m). We used the Wilcoxon signed-rank test to assess changes in count, frequency, and relative density of sugar, striped, and red maple, and American beech. Importantly, we analyzed if changes on acidified soils (with base saturation – BS – in the upper B horizon < 12%) differed from changes on soils with BS >12%. We used generalized linear mixed models to link the observed changes to a suite of regional DAYMET climate variables and soil-chemical data collected on our plots.
Results/Conclusions
Between 2009 and 2015 sugar and red maple counts declined by 20% and 43% whilst striped maple and beech increased by 50% and 74%. Red maple declined significantly (p < 0.05) in count, frequency, and relative density across all plots and on the subset with BS < 12%. Sugar maple increased significantly in frequency and relative density on plots with BS < 12% and decreased where BS > 12%. Striped maple increased significantly in relative density on all plots and on those with BS <12%. Beech increased significantly in count and relative density on plots with BS < 12%. Mixed models revealed several significant predictors of changes in sugar and red maple. Change in the 5-year means of annual precipitation (mm) and growing degree days positively predicted change in red maple counts and negatively predicted change in sugar maple counts. Base saturation in the upper B horizon (measured in 2009) positively predicted change in sugar maple counts. The pseudo-R2 for this model’s fixed effects was 0.45. We conclude that both soil characteristics and changing climate may impact patterns of Adirondack hardwood regeneration and thus future overstory composition.