PARENT SESSION
Oral Session #104: Plant Communities: Vegetation Analysis.
Presiding: S. Will-Wolf
Friday, August 9. 9:00 AM to 12:00 PM. Grand Ballroom West, Radisson.
The relationship of soil nitrogen to herb layer dynamics and diversity patterns in an eastern oak forest, USA.
Small, Christine*,1, McCarthy, Brian2, 1 Department of Botany, New London, CT2 Department of Environmental and Plant Biology, Athens, OH
ABSTRACT- Understory herbaceous vegetation is a vital component of eastern deciduous forests, typically comprising the largest proportion of forest diversity. In these systems, understory composition is generally correlated with microenvironmental conditions such as topography, light availability, and soil fertility. Soil nitrogen is often a primary limiting factor, strongly correlated with species composition and diversity. However, dramatic increases atmospheric nitrogen deposition in recent decades have caused some eastern forests to exhibit symptoms of nitrogen saturation, nitrogen availability in excess of biological demand. To better understand the relationship of soil nitrogen to understory dynamics, we examined patterns of soil carbon and nitrogen relative to herb-layer composition and diversity in managed (10-yr-old aggrading) and mature oak stands of southeastern Ohio. Understory vegetation was sampled in April and June to capture peak growth of vernal and summer herbs. Upper mineral soil samples were collected during these periods and analyzed by dry combustion for organic carbon and nitrogen contents. Managed stands showed significantly lower soil nitrogen than mature forests (spring = 0.145% vs. 0.165%, summer = 0.146% vs. 0.197%, P <0.001). Topography also influenced soil nitrogen, with greater availability on lower and north-facing slopes (P <0.05). Soil C:N ratio was strongly correlated with herb-layer composition in all stands (CCA ordination; spring r = 0.606; summer r = 0.449) and, in mature forests, was a strong predictor of understory richness (r2 = 0.634, P <0.001), particularly on more xeric sites. This strong relationship suggests that changes in nitrogen availability could have important implications for understory diversity patterns in eastern forests. Further, since herb-layer interactions may influence woody seedling success in reaching the forest canopy, changes in understory dynamics may play a significant role in overstory recruitment patterns and broader ecosystem responses to nitrogen deposition.
KEY WORDS: understory herbaceous stratum, herb-layer diversity, nitrogen saturation