PARENT SESSION
Contributed Oral Session 138: Fire and Vegetation Dynamics
Thursday, August 11, 1:30 PM - 5:00 PM, Meeting Room 519 B, Level 5, Palais des congrès de Montréal

Long-term oscillations in grassland driven by fire.

MacDougall, Andrew^{1, 2}, Turkington, Roy ², ¹ Department of Biology, Regina, SK, Canada² Department of Botany, Vancouver, BC, Canada

ABSTRACT- The impacts of fire on the stability of grasslands remain poorly understood. Fire may prevent competitive exclusion by dominant species and thus slow population destabilization. Conversely, fire could be as destabilizing as competition so that persistence of subordinate species requires unburned refugia. Habitat loss in contemporary landscapes means the latter is unlikely to occur, so that the re-introduction of fire may have unexpected consequences. We test the impact of fire on population and community stability using a six-year burn experiment in a fragmented fire-suppressed oak grassland. Empirical data were used to model longer-term trends, and examine how local burning affects stability at regional levels. In the absence of fire, the primary determinant of species persistence and community structure was competition by several dominant C3 grasses. Annual burning significantly increased species turnover, favoring a suite of fire-tolerant species dominated by perennial forbs. Although fire altered the composition and structure of the grassland, there was no mechanism to maintain its occurrence. The fire-tolerant flora were significantly less productive, so that fuel levels dropped, fire frequency decreased, and the grasses eventually re-emerged as dominants. Repeated burning, therefore, caused a continuous oscillation between grass-dominated and forb-dominated communities at local scales over time. These results support regional-based meta-stability models that integrate the sum total of all local equilibrium and non-equilibrium processes affecting local grasslands. Fire suppression has eliminated the grass-forb oscillation, and the vulnerability of species to fire in small remnants means that the link between population and community-level processes has become decoupled. Restoration efforts must, therefore, balance the reduction of competition by burning with the vulnerability of subordinate species to its occurrence.

Key words: fire, grasslands, species composition, stability