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

Background/Question/Methods Litter quality, litter decomposition rates, and soil nitrogen (N) dynamics are strongly linked, and tree species composition is thought to be a major controlling factor in nutrient cycling and decomposition processes beneath tree canopies. Savanna-like ecosystems may offer the best opportunities for testing hypotheses regarding the relationships between above-ground tree species and these processes, 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 from that found in the native habitat. 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 and decomposition in a savanna ecosystem in central Kentucky.  Sampling was conducted under savanna trees, with and without the influence of the invasive shrub, and in the surrounding savanna vegetation matrix.  Assessments of nitrogen availability and litter decomposition rates were used to examine the effects of savanna tree species and an invasive shrub on N cycling over a period of three years.

Results/Conclusions

Trends varied among sampling periods, but a relatively consistent interaction between sampling location (tree vs. tree+shrub) and tree species suggests an intriguing interaction between the effects of L. maackii and canopy tree species on nitrogen cycling, such that L. maackii increases N mineralization and nitrification under C. ovata but not under the other two species.  Preliminary decomposition data show that L. maackii litter breaks down much more rapidly than the litter of all three tree species, generally as expected based on initial lignin concentration.  Q. muehlenbergii and C. ovata litter had lower decay constants than F. quadrangulata, reflecting differences in initial lignin content among the litter of the tree species.  While all three tree species’ litter appears to break down more rapidly in the presence of L. maackii, this effect is especially pronounced with C. ovata litter, again suggesting an interaction between canopy species and L. maackii.  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 the vulnerability of ecosystem processes to invasions of L. maackii and potentially other invasive species.