POPULATION BIOLOGY OF BONAMIA GRANDIFLORA (CONVOLVULACEAE): EFFECTS OF FIRE ON PLANT AND SEED BANK DYNAMICS

The effects of disturbance on reproduction and plant and seed bank dynamics in the perennial herb Bonamia grandiflora were studied by comparing populations in recently burned, mechanically disturbed, and undisturbed habitats in central Florida over a 3-year period. Plant densities, seed production, and the occurrence of herbivory and predispersal seed predation varied considerably between sites and between years, with recently disturbed sites supporting the densest and most dynamic populations. Death of established plants was rare in all sites. In each site, the soil seed bank was several-fold larger than single season seed rains suggesting that B. grandiflora seeds are long-lived and accumulate in the soil. There was no evidence that postdispersal predation or pathogens have any significant influence on the seed bank dynamics. Fire resulted in large increases in stem densities due to both increased clonal stem production and new genet recruitment from seed. Burning also caused significant increases in the percentage of flowers producing seed and the numbers of capsules and seeds per plant. The seed rain was ten to thirty times greater in the burned site relative to adjacent unburned site during the 3 years after burning. However, additions to the seed bank from the postfire seed rain were balanced by equivalent losses due primarily to seed mortality during fire, and to a much lesser extent due to germination and new genet establishment. As a result, the subsequent densities of seeds stored in the soil in these two sites were similar, indicating that fire results in a significant turnover in the seed bank population but no immediate change in its size. These effects on seed bank dynamics, in addition to new genet recruitment, suggest that periodic fires may play an important role in the maintenance of genetic variability as well as the size of these populations. IT HAS BECOME increasingly recognized that natural disturbance, occurring at varying temporal and spatial scales, plays an important role in many types of vegetation (White, 1979; Pickett, 1980; Pickett and White, 1985). Many studies have focused on the effects of disturbance on species composition, diversity, or other aspects of plant community structure. Theoretical frameworks such as the intermediate disturbance hypothesis and patch dynamics models have provided a basis for many empirical studies (Connell, 1978; Miller, 1982; Runkle, 1982; Pickett and White, 1985). Few studies, however, have examined the influence of disturbance regimes on the demographic and life history traits of individual species (e.g., Platt, 1975). The type, frequency, intensity, duration, timing, and spatial heterogeneity of distur' Received for publication 24 February 1988; revision accepted 1 August 1988. This research was supported by a grant from The Nature Conservancy. We thank D. J. Gibson, E. E. Evans, and 0. J. Reichman for valuable comments on the manuscript. 2 Please address all correspondence and requests for reprints to this author. bance may influence a variety of plant life history and demographic traits (Bazzaz, 1984). Disturbance can alter the size and structure of a plant population by influencing growth, survivorship, seed production, and/or seedling recruitment. In populations where recruitment occurs primarily via vegetative reproduction rather than by seed, disturbance may play an important role in rhizome dynamics and ramet production (Fetcher and Shaver, 1983; Hartnett, 1987). In some cases, disturbance may cause significant changes in the size or structure of a plant population by influencing the structure and dynamics of the seed bank rather than the adult plants. Several previous studies on seed pools have focused on the role of animals in the depletion of stored seeds (Nelson and Chew, 1977; Borchert and Jain, 1978; Brown, Reichman, and Davidson, 1979; Reichman, 1979) and the spatial and temporal patterns of seed distribution within a habitat (e.g., Reichman, 1984), but comprehensive studies on the dynamics of particular species' seed pools are rare (Sarukhan, 1974; Harper, 1977) and the roles of various types of natural disturbance in the dynamics of buried seed populations are unknown.