Influence of native bunchgrass and invasive forb litter on plant growth in a semi-arid bunchgrass prairie

Litter can influence plant species diversity. I investigated plant litter effects for density, cover, and biomass on common St. Johnswort Hypericum perforatum L., a nonnative- species-group, and a native-species-group. The following hypotheses were tested: litter source would favor species dominant in the litter; the high amount of litter (908 grams) would decrease density more than the moderate amount (454 grams); coarse size litter would decrease density more than fine size; and the effect of litter would depend upon interactions of all three variables. Above-ground plant material was removed from two adjacent sites, one a native bunchgrass prairie (native site), the other infested with St. Johnswort (invasive site). Three-way litter treatments were applied in October 1999 and reapplied in July 2000 in combinations of high- or moderate-amount of litter, native or St. Johnswort species by source, and fine or coarse texture by size. Sampling occurred at peak standing crop July 2000 and 2001. Analysis of variance in 2001 data results follow. St. Johnswort was decreased by its own litter (all P <.06). St. Johnswort and native species were detrimentally influenced more by the high amount of litter than by the moderate amount (all P <.05). Although litter size did not influence St. Johnswort, native species biomass was more detrimentally influenced by coarse size litter than by fine (P <.05). When we added more fine size litter, native species were more detrimentally influenced than when we increased the amount of coarse litter (P <.06). In the invasive site, moderate amounts of coarse native litter decreased St. Johnswort. High amounts of coarse St. Johnswort litter decreased it even more (both P <.05). The opposite effects were found for native species (both P <.05). In the native site, the only treatment that reduced St. Johnswort more than no litter was the high amount of fine St. Johnswort litter (P <.05). The same fine St. Johnswort litter in moderate amounts was the only treatment that did not decrease native species (P <.05). The complexity of litter influence through interactions of amount, source, and size detrimentally and non-detrimentally caused changes to plant species diversity at each site (all P <.05).