There goes the neighborhood: The impact of the invasive shrub Lonicera maackii on nutrient cycling, decomposition dynamics, and the decomposer community

Background/Question/Methods   Invasive plants may have variable effects within a given environment depending on their interactions with the dominant native species, adding another layer of complexity to our understanding of invasive species impacts. Little research has examined such species-species interactions within a site. Savanna trees with non-overlapping canopies offer a unique opportunity to assess changes within the ‘islands of fertility' that occur under individual trees as a result of interactions between invasive species and different native trees species. The objective of this study was to examine the impact of the invasive shrub Lonicera maackii on soil, nutrient cycling, and decomposition processes, and the decomposer community under the three dominant tree species, Fraxinus quadrangulata, Quercus muehlenbergii, and Carya ovata, in a savanna in central Kentucky. A four year assessment of soil nutrient availability conducted from 2005-2008 was coupled with measurements of soil physical parameters (pH, moisture, bulk density), and the soil microbial community was examined using phospholipid fatty acid analysis (PLFA). Single- and mixed-species litterbags (L. maackii + individual tree species litter) were placed under savanna trees and in the surrounding savanna vegetation matrix, and litter decomposition rates, litter C and N, and microarthropod abundance were assessed over a period of two years.

Results/Conclusions   Data from the nutrient cycling study suggest an intriguing interaction between the effects of L. maackii and the native savanna trees such that L. maackii seasonally increases N mineralization (p=0.002) and nitrification (p=0.012) and alters bulk density (p=0.0332) under C. ovata but not under the other two species. Preliminary PLFA data indicate that the presence of L. maackii may affect microbial biomass. Decomposition data show that litter from L. maackii breaks down more rapidly than the litter of all three tree species. The rate at which L. maackii decomposed varied depending on its location, with L. maackii breaking down more rapidly under C. ovata (p=0.0021). While all tree species' litter appears to break down more rapidly in the presence of L. maackii, this effect was again more pronounced with C. ovata litter, further highlighting the variable interaction between canopy species and L. maackii. The greatest abundance of litter microarthropods was also found in the mixed C. ovata + L. maackii litter. 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.