A six-year study of fire-related flowering cues and coexistence of two perennial grasses in a wet longleaf pine (Pinus palustris) savanna

Integrating population and community ecology can improve our understanding of the impacts of natural disturbances. Fire-stimulated flowering occurs in many long-lived herbaceous species of fire-maintained grasslands and savannas. Coexistence of these long-lived species may be affected in part by interspecific differences in the effect of fire-stimulated flowering on resource conservation, clonal growth, and longevity. This study comprised two parts. The first part investigated the effectiveness of different fire-related cues on fire-stimulated flowering in two cooccurring dominant grass species in a wet longleaf pine (Pinus palustris) savanna in southeastern Mississippi, USA. The second part investigated the immediate effects of the most important of these cues (damage and removal of aboveground vegetation and surface litter in May) along with nutrient addition on several measures of fitness and abundance over 6 years. Despite being a very effective inductive cue, when repeated frequently over 6 years, clipping plus surface litter removal significantly reduced flowering in both species. This negative effect was reduced to some extent by nutrient addition in Muhlenbergia expansa (which exhibited higher reproductive investment following clipping and fire than did Ctenium aromaticum). Frequent clipping resulted in an increasing numerical advantage of C. aromaticum over M. expansa with time. There was evidence of a modest release of C. aromaticum from competition with M. expansa in response to annual clipping. Responses suggest that suppressing flowering until after fire reduces the cost of flowering and maintains shoot densities, at least in M. expansa. Differences in the responses of these two species to repeated clipping and nutrient addition suggest that, despite their both exhibiting fire-stimulated flowering, each species is favored by slightly different fire frequencies. Moderate variation in fire frequency could maintain their coexistence in the long term.