Questions: How does the time interval between subsequent stand-replacing fire events affect post-fire understorey cover and composition following the recent event? How important is fire interval relative to broad- or local-scale environmental variability in structuring post-fire understorey communities?

Location: Subalpine plateaus of Yellowstone National Park (USA) that burned in 1988.

Methods: In 2000, we sampled understorey cover and Pinus contorta density in pairs of 12-yr old stands at 25 locations. In each pair, the previous fire interval was either short (7–100 yr) or long (100–395 yr). We analysed variation in understorey species richness, total cover, and cover of functional groups both between site pairs (using paired t-tests) and across sites that experienced the short fire intervals (using regression and ordination). We regressed three principal components to assess the relative importance of disturbance and broad or local environmental variability on post-fire understorey cover and richness.

Results: Between paired plots, annuals were less abundant and fire-intolerant species (mostly slow-growing shrubs) were more abundant following long intervals between prior fires. However, mean total cover and richness did not vary between paired interval classes. Across a gradient of fire intervals ranging from 7–100 yr, total cover, species richness, and the cover of annuals and nitrogen-fixing species all declined while the abundance of shrubs and fire-intolerant species increased. The few exotics showed no response to fire interval. Across all sites, broad-scale variability related to elevation influenced total cover and richness more than fire interval.

Conclusions: Significant variation in fire intervals had only minor effects on post-fire understorey communities following the 1988 fires in Yellowstone National Park.

Nomenclature: Dorn (1992).

Abbreviations: YNP = Yellowstone National Park.