The evolution of fire-dependent flowering in goldenasters (Pityopsis spp.)1

Abstract

Gowe, A. K. and J. S. Brewer (Department of Biology, University of Mississippi, University, Mississippi, 38677), The evolution of fire-dependent flowering in goldenasters (Pityopsis spp.), J. Torrey Bot. Soc. 132: 384– 400. 2005.—Although prescribed fire is thought to benefit fire-adapted species, unequivocal evidence for the existence of fire-adapted species is lacking in many systems. We present the results of a comparative ecological study of fire-dependent flowering in the genus Pityopsis (Asteraceae). We constructed a phylogeny of the genus based on published morphological data and then mapped fire-dependent flowering on the resulting topology. We examined relationships between floral induction, tree and groundcover canopy coverage, and presettlement fire frequency within each species' preferred habitat. Results of our investigations supported the following hypotheses: 1) Fire-dependent flowering occurred in one infrageneric taxonomic group (sect. Graminifoliae) but not in the other (sect. Pityopsis); 2) Fire-dependent flowering occurred in habitats or microsites that contained a dense groundcover canopy during years without fire, and 3) Fire-dependent flowering was more likely to occur in areas of relatively high fire and lightning frequency in the United States. The phylogenetic analysis suggested that the two sections of Pityopsis did not form separate monophyletic groups. Furthermore, the character used to distinguish sections in previous taxonomic treatments (i.e. basal rosette present at the time of flowering) may, in fact, be equivalent to fire-dependent flowering. We hypothesize that fire-dependent flowering is a form of adaptive phenotypic plasticity that was retained in the Graminifoliae section and permitted exploitation of a fluctuating competitive environment within a variety of different fire-prone savannas in the southeastern United States.