PARENT SESSION
Oral Session #96: Plant Competition II.
Presiding: K. Suding
Thursday, August 8. 1:00 PM to 3:45 PM. Mesquite Room, Radisson.
Contingent competitive relationships in a heavily invaded oak grassland.
MacDougall, Andrew*,1, Turkington, Roy1, 1 University of British Columbia, Vancouver, British Columbia, Canada
ABSTRACT- Invasive plants are often assumed to be competitively superior to native flora, based on high levels of exotic inundation. However, given substantial levels of habitat alteration and seed introduction following European settlement, historical factors could also explain present-day patterns. We tested these alternatives with field and glasshouse experiments, examining competitive relationships in an invaded Quercus garryana grassland. We asked: 1) what are the mechanisms that maintain exotic dominance? 2) do dominance patterns persist with manipulations of disturbance, eutrophication, and establishment timing? Results indicate that the invasive dominants (Poa pratensis, Dactylis glomerata) are adequate competitors, but not to the degree that explains invasion levels. Under control conditions, these grasses form stable swards that restrict light and suppress seed recruitment; the sward is maintained by vegetative spread. Disturbance, however, reduced exotic cover significantly. Successional trajectories indicate that a different suite of dominants will form with perturbation, though this includes new exotics. Glasshouse work demonstrated that competitive outcomes were inconsistent among treatments of fertility, density, and establishment timing; field-observed competitive hierarchies were not maintained. Although the interaction of fertility and density magnified some competitive advantages, these effects were nullified by establishment timing. Even the weakest competitor was resistant to neighbour competition if given a head start. Our results suggest that current dominance patterns are not determined by outright competitive superiority of the exotics. Instead, it appears to be contingent on site history combined with recruitment-based feedback loops. However, these conditions can be de-stabilized, and native flora may be resistant to displacement if allowed to re-establish.
KEY WORDS: plants, competition, invasion, disturbance