CRETACEOUS ECOLOGICAL SUCCESSIONS AND THE SCALE OF K/T POST-IMPACT VEGETATION DISTURBANCE

In extant terrestrial ecosystems, disturbances of sufficient magnitude to alter the nature of the standing vegetation typically trigger an ecological phenomenon known as succession. Where the disturbance agent (flooding, fire, volcanism, or even extraterrestrial impact) is of sufficient magnitude to destroy pre-existing vegetation, the earliest or pioneering elements of extant recovery successions tend to be dominated by herbaceous flowering plants. In contrast, during the Mesozoic and early Cenozoic, ferns and other free-sporing plants served as the "weeds" that characterized the pioneering stage of terrestrial successions. Subsequent to the disturbance, depending on scale and moisture availability, the affected areas appear to have been dominated by "fern prairies". In such cases, spores dominate the local spore/pollen rain, resulting in a "fern spike" that persists in the palynological record until the fern-dominated assemblage is incrementally replaced by other taxa in the later successional sequence. Such "fern spikes" are ubiquitous ecological markers that attest to large-scale disturbance by a range of possible agents. In the case of the western interior of North America, the fern spike that occurs immediately above the iridium-rich K/T impact fall-out layer almost certainly marks the widespread destruction of vegetation. However, North American palynological data suggest that the geographic extent of such destruction was limited, possibly corresponding with the postulated distribution of an post-impact incandescent ejecta plume. Vegetation outside of the plume footprint does not appear to have suffered significant impact-related alteration. The relatively few examples of K/T fern spike occurrences in other parts of the world are all equivocal to some degree. Most current models of the immediate post-K/T world invoke pervasive destruction and a worldwide ecological catastrophe. An understanding of Cretaceous terrestrial vegetation dynamics suggests that the impact may have had regional, not global, ecological consequences.