EFFECTS OF FIRE ON CLONAL GROWTH AND DYNAMICS OF PITYOPSIS GRAMINIFOLIA (ASTERACEAE)

The rhizomatous perennial Pityopsis graminifolia was studied in a Florida sandhill community in an annually burned site, a periodically burned site, and a site that has been protected from fire since 1965. These different fire regimes significantly affected the demography and life histories of both plants and plant parts in this clonal species. Fires resulted in reductions in ramet biomass and height, and an increase in the (root + rhizome)/shoot biomass ratio. Burning also decreased the total number of flower heads and new rhizomes produced per ramet. However, the survivorship of initiated rhizomes was greater in burned sites and resulted in a larger number of established daughter ramets per clone. As a result, in burned sites there was a shift in clone structure toward larger numbers of smaller ramets, but there were no significant reductions in seed or rhizome production on a per genet basis. The results showed that the responses to fire in P. graminifolia are different when measured at the genet vs. ramet level and that the effects of fire on clones can be explained by demographic responses of plant parts. Population regeneration in the study sites was dependent on successful clonal ramet production because no seedling recruitment was observed. This suggests that disturbances other than fire are important for new genet recruitment in these clonal populations. DISTURBANCE is a pervasive influence in natural communities, and its effects on the structure and dynamics of many plant communities have been well studied. However, the influences of disturbance on individual plant populations or on plant life histories have received less attention. Characteristics of disturbance such as its frequency, intensity, duration, timing, and heterogeneity may all influence plant population responses. Plant species with a clonal growth form are common in many disturbed habitats (Noble, Bell, and Harper, 1979; Abrahamson, 1980; Fetcher and Shaver, 1983). Because of their multiple modes of shoot production (seedling establishment and clonal spread) and multiple levels of population structure and dynamics (i.e., vegetatively produced shoots or ramets, and whole multistemmed clones or genets), clonal plants may respond in multiple and complex ways to disturbance. Recruitment, growth, and mortality may be altered differentially at the genet and ramet population levels, and these responses in turn influence population structure, genetic variation, persistence, and spatial patterning. I Received for publication 26 November 1986; revision accepted 2 March 1987. The technical assistance of Loretta Hodgdon is greatly appreciated. This work was supported by a Faculty Research and Creative Scholarship Award from the University of South Florida. 2 Current address: Division of Biology, Kansas State University, Ackert Hall, Manhattan, KS 66506. A few investigators have begun to examine responses of clonal plants to disturbance (Noble et al., 1979; Fetcher and Shaver, 1983; Goldberg and Werner, 1983; Hartnett and Bazzaz, 1985). Several studies have focused on shifts in the balance between sexual reproductive effort and vegetative growth in disturbed and undisturbed habitats (Gadgil and Solbrig, 1972; Sarukhan, 1974; Abrahamson, 1980; Grace and Wetzel, 1981). However, in many clonal species the maintenance of local populations relies heavily or almost exclusively on ramet production (Fetcher and Shaver, 1983; Hartnett and Bazzaz, 1985; Pitelka, Hansen, and Ashmun, 1985). Therefore, clonal growth patterns and the demography of ramets and genets may be more important than simply the relative biomass allocation to sexual reproduction in explaining local poulation dynamics and persistence or responses to disturbance. Fire is an important natural disturbance in many communities. In the upland longleafpine and slash pine sandhill communities of the southeastern United States the understory grasses and forbs are influenced by frequent fires (Lemon, 1949; Laessle, 1958). Responses to burning including increased vegetative growth and plant nutrient content, increased flowering, or altered phenologies have been reported for some of these species (Wahlenberg, Greene, and Reed, 1939; Biswell and Lemon, 1943; Lemon, 1949, 1967; Hilmon and Lewis, 1962; Lewis and Hart, 1972; Davis and Platt, 1984; Evans and Platt, 1984). However, the