Single-tree effects of savanna trees and the influence of invasive species on soil nitrogen cycling

Tree species composition is thought to be a major controlling factor in nutrient cycling beneath tree canopies. Savanna-like ecosystems may offer the best opportunities for testing hypotheses regarding the relationship between above-ground tree species and below-ground communities and their relationship to nutrient cycling, because individual trees and their canopies are typically non-overlapping. Understory plant species can also exert an influence on small-scale spatial patterns of nutrient cycling and decomposition processes. In cases where understory plants are invasive species, such influences can represent an alteration of function and possibly soil biota. The objective of this study was to simultaneously examine the influence of three tree species, Fraxinus quadrangulata, Quercus muehlenbergii, and Carya ovata, and an invasive shrub, Lonicera maackii, on nutrient cycling in a savanna ecosystem in central Kentucky. Assessments of net N mineralization were used to examine the effects of savanna tree species and an invasive shrub on N availability. Among species, N mineralization was highest under F. quadrangulata and lowest under Q. muehlenbergii. A significant interaction between sampling location and tree species suggests an intriguing interaction between the effects of L. maackii and canopy trees on nitrogen cycling, such that L. maackii increases N mineralization and nitrification under C. ovata but not under the other two species. If the effects of invasive species on certain ecosystem processes are strongly influenced by over-story species, this could suggest a novel approach to understanding ecosystem vulnerability to invasions of L. maackii and potentially other invasive species.