Relationship of understory diversity to soil nitrogen, topographic variation, and stand age in an eastern oak forest, USA

Nitrogen (N) availability is a primary limiting factor in many temperate deciduous forests. However, increased atmospheric N deposition over recent decades has dramatically altered nutrient cycles in many eastern forests. Given the variability of ecosystem responses to N deposition and the sensitivity of herbaceous layer vegetation to edaphic and microenvironmental conditions, changes in nutrient dynamics could have important implications for forest diversity and productivity. To better understand variations in soil N relative to understory dynamics, we sampled herbaceous layer composition and diversity across topographic gradients in managed (10-year-old aggrading) and mature (>125 years) mixed-oak stands in southeastern Ohio. Vegetation was sampled in spring and summer to capture variations in vernal and late season herb communities. Edaphic and microenvironmental conditions were characterized during these same periods, including analyses of upper mineral soil samples for total C, N, and C/N ratio. Aggrading stands showed significantly lower soil N than mature forest stands (spring = 0.145% versus 0.165%; summer = 0.146% versus 0.197%; P < 0.001). Topography influenced soil N, with greater availability on lower and north-facing slopes (P < 0.05). Across all stands, C/N was strongly correlated with herb layer composition (spring r = 0.606; summer r = 0.449) and, in mature stands, was a strong predictor of understory richness (linear regression; r2 = 0.634; P < 0.001), particularly on poorer sites. These results emphasize that changes in soil and vegetation with increased N deposition are likely to be site-specific, even within relatively uniform systems. Understory diversity patterns on less fertile sites or in more mature forests, those systems exhibiting strongest correlations with soil C/N ratios, appear most likely to be affected, whereas edaphic limitations in vigorously growing, aggrading vegetation may be less impacted. Because herbaceous layer interactions are tightly linked to ecosystem-level nutrient dynamics and to woody seedling success, these influences have the potential to significantly alter overstory recruitment patterns and broader ecosystem responses to N deposition.