FIRE-DRIVEN VEGETATION DYNAMICS IN CENOZOIC FOREST ECOSYSTEMS, WESTERN NORTH AMERICA

Stratigraphic palynology has proven to be a useful tool in the recognition of natural fire cycles. Aperiodic fire cycling played an important role in shaping the structure of the source vegetation mosaic in a range of Tertiary floras studied, including the Thunder Mountain (Eocene) of Idaho, the late Paleogene Florissant (Colorado), and the Neogene Succor Creek assemblage of the Oregon-Idaho boundary region. Recognition of fire events is limited by the geographic scale of the event and the temporal resolution inherent in closely-spaced, stratigraphically sequential palynological sampling. Fire event horizons typically have elevated levels of charcoal micro-fragments, but such fragments may be lost or obscured as a result of certain sampling or processing procedures. The palynomorph spectra derived from post-fire recovery seres, particularly forest communities late in the recovery sequence, are more readily resolved than a fire event horizon or subsequent, but transient, communities dominated by herbaceous plants and shrubs. In comparison with typical fluvial/deltaic sedimentary sequences, floras preserved in volcaniclastic rocks appear to have inherently better temporal resolution. Distinctive post-fire vegetation elements can easily be misinterpreted as vegetation response to climate change if the role of fire is not recognized.