The Role of Allelopathy in Cogongrass (Imperata cylindrica) Invasion.

We conducted a greenhouse study to assess the effects of cogongrass allelopathy on a suite of species native to southeastern pine savanna ecosystems. We hypothesized that rhizosphere “leachate” collected from cogongrass invaded soils would adversely affect the growth, root morphology and mycorrhizal colonization of these species (relative to leachate collected from mixed natives). Additionally, we hypothesized that compounds not present in a native savanna rhizosphere would be present in the cogongrass rhizosphere. Our results indicated an apparent allelopathic effect from cogongrass, although it varied by species. A ruderal grass (Andropogon arctatus Chapm.) and ericaceous shrub (Lyonia ferruginea (Walter) Nutt.) were unaffected by the cogongrass leachate, while a mid-successional grass (Aristida stricta Michx. var. beyrichiana (Trin. and Rupr.) D.B.Ward) and tree (Pinus elliottii Engelm.) were negatively affected. For A. stricta, we observed a 35.7% reduction in aboveground biomass, a 21.9% reduction in total root length, a 24.6% reduction in specific root length and a 23.5% reduction in total mycorrhizal root length, relative to the native leachate treatment. For P. elliottii, there was a 19.5% reduction in percent mycorrhizal colonization and a 20.1% reduction in total mycorrhizal root length. Comparisons with a DI water control in the second season support the possibility that the observed treatment effects were due to the negative effects of cogongrass leachate, rather than a facilitative effect from the mixed native species. Chemical analyses identified 12 putative allelopathic compounds (mostly phenolics) in the cogongrass leachate. The concentrations of most of these compounds were significantly lower, if they were present at all, in the native leachate. One compound was a novel alkaloid. The speculated structure was hexadecahydro-1-azachrysen-8-yl ester (C₂₃H₃₃NO₄) and it appeared to be present at fairly high levels in cogongrass leachate. This compound was not found in the native leachate. We propose that the observed treatment effects are attributable, at least partially, to these qualitative and quantitative differences in leachate chemistry.