Ecological - Second Order
Fire Return Intervals
The time interval between stand-replacing fires can influence patterns of initial postfire succession if the abundance of postfire propagules varies with prefire stand age. We examined the effect of fire interval on initial postfire lodgepole pine (Pinus contorta var. latifolia Engelm.) density in Yellowstone National Park (YNP) following the 1988 fires. We asked whether postfire propagule abundance, measured as prefire percent serotiny, varied with fire interval and could explain patterns in postfire succession. The response of lodgepole pine density to variation in fire interval was explained by spatial and temporal variation in prefire serotiny. At low elevations, postfire lodgepole pine recruitment correlated strongly with prefire percent serotiny, which varied nonlinearly with prefire stand age. As a result, postfire lodgepole pine densities varied nonlinearly with fire interval. In contrast, at high elevations serotiny was low, varied little with stand age and did not influence postfire lodgepole pine densities, although, fire interval was still a significant predictor of postfire densities. At high elevations, fire interval varied nonlinearly with postfire lodgepole densities, presumably due to the temporal variation in propagule abundance from open cones in adjacent unburned stands. Temporal variation in stand-level serotiny at low elevations was best explained by age of individual trees. Logistic regression indicated that trees expected to be serotinous had a low probability of exhibiting serotiny at a young age, with increasing probability as trees matured up to 140 yr. This increase in serotiny with tree age likely accounts for the initial increase in stand-level percent serotiny with stand age at low elevations. The spatial variation in serotiny was correlated with variation in historical fire regimes. Fire interval models derived from lower elevations in YNP indicate that fire occurred historically at 135-185-yr intervals, whereas at higher elevations fires occurred at 280-310-yr intervals. The spatial patterns of serotiny appear to have been influenced by variability in historical fire regimes across the Yellowstone landscape, which has conditioned contemporary successional responses to disturbance.