EFFECTS OF COLLEMBOLA (ARTHROPODA) AND RELATIVE GERMINATION DATE ON COMPETITION BETWEEN MYCORRHIZAL PANICUM-VIRGATUM (POACEAE) AND NONMYCORRHIZAL BRASSICA-NIGRA (BRASSICACEAE)

In this experiment the separate and interactive effects of grazing of vesicular-arbuscular mycorrhizal hyphae by collembola and of relative germination date of competition between the mycorrhizal perennial grass Panicum virgatum and the non-mycorrhizal annual crucifer Brassica nigra were investigated. In the absence of competition, P. virgatum mass and P uptake were not affected by collembola grazing; grazing did reduce tissue N concentration and root:shoot ratio. Competition from B. nigra plants of the same age/size ("simultaneous competition") significantly reduced P. virgatum total, root, and shoot mass relative to control plants not subject to competition. In contrast, when in competition, B. nigra plants did not differ in biomass from B. nigra controls grown without competition. Simultaneous competition also reduced N and P uptake by P. virgatum, but not by B. nigra. Grazing by collembola during simultaneous competition increased the differences in nutrient uptake and tended to shift the competitive balance further toward the non-mycorrhizal B. nigra. When P. virgatum plants were subjected to competition from B. nigra plants which germinated three weeks later ("offset competition") the situation was reversed: offset B. nigra plants were negatively affected by competition while the larger, older P. virgatum plants were not. Thus, relative germination date is important in determining the relative competitive ability of these two species. Grazing by collembola did not affect offset competition. The grazing of VAM hyphae by collembola appears to increase N availability in this experimental system. Under simultaneous competition, this N is taken up by the more extensive root system of B. nigra; under offset conditions, the root system of the smaller B. nigra plants is insufficient to take advantage of the added resources. Thus, we suggest that grazing-induced transient changes in nutrient availability and the differential abilities of the two species to make use of these added resources constitute the mechanisms by which relative germination date and collembola grazing influence competition.