Vegetation responses to changing fire regimes in a rocky mountain forest

Abstract

In western North America, subalpine forests experience fires that vary greatly in terms of severity and extent. However, beyond observational and dendroecological records, little is known about past fire severity and magnitude. This is because metrics used to identify fire and ecological impacts in the deep past (i.e., sedimentary charcoal and pollen data) are coarse tools for examining fine-scale environmental responses. Yet large fires should result in changes in pollen abundance and composition, which in turn can be used to identify event types, or severity of past events. We compare pollen spectra changes from a subalpine forest following identified fire events, to pollen spectra from periods between fires. From the pollen data, two types of fire events may be inferred. Fire events that affect understory plant composition (low to mid-severity) result in increases in canopy pollen. Fire events that consume both understory and canopy plants (high severity) result in decreases in canopy pollen relative to understory pollen and likely reflect differences in the recovery rates of trees compared to shrubs, grasses, and forbs. Inferred fire type showed no relationship to the size of charcoal peaks, suggesting that the quantity of charcoal observed in a sample does not provide information on fire severity. These results reveal the potential of examining fire regimes through time by combining sedimentary charcoal and pollen data, extending our interpretations of fire regimes deeper into the past.