Spatial Patterning of the Effects of Savanna Trees and Exotic Species on Soil Nutrient Cycling.

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.  This non-overlapping characteristic of the spatial arrangement of canopy trees permits evaluation of the spatial extent of the influence, radiating out from the trunk of isolated, individual trees, or ‘tree-islands.’  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 increasing spread of invasive species thus has the potential to greatly alter ecosystem processes, including the influence that native plant and tree species have on decomposition and nutrient cycling.  The objective of this study was to simultaneously examine the influences 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 with the goal of identifying spatial patterning that will inform: 1) soil nutrient and biotic influences with the potential to constrain regeneration of savanna tree species, and 2) attributes of invasive species (particularly L. maackii) that may provide insights to effective control of further invasion.  A spatially explicit assessment of nutrient cycling, utilizing autocorrelation and semivariograms, was used to examine the effects of savanna tree species and an invasive shrub on N availability. Spatial relationships beneath the canopies of the three dominant savanna trees emerged at a smaller sampling distance.  While no consistent patterns emerged among tree species, L. maackii appeared to alter the spatial structure of observations under a number of trees, with C. ovata trees most often affected by L. maackii.