Seedling establishment in natural and experimental Florida scrub gaps

Journal of the Torrey Botanical Society, Apr-Jun 2003 by Petru, Martina, Menges, Eric S

PETRU, AND E. S. MENGES (Archbold Biological Station, P.O. Box 2057, Lake Placid, FL 33862). Seedling establishment in natural and experimental Florida scrub gaps. J. Torrey Bot. Soc. 130: 89-99. 2003.-Gaps are often important microhabitats for recruitment of herbaceous plants. In the Florida scrub, fires create gaps by top-killing resprouting shrubs. These natural gaps are aboveground gaps (without aboveground vegetation) that may also be complete gaps (if belowground roots and rhizomes are absent). Within three types of gaps (natural gaps, experimental aboveground and experimental complete gaps), we studied seedling recruitment and survival of spontaneous species and seven sown herbs for two years. We created aboveground gaps by clipping vegetation and complete gaps by removing shrub roots and rhizomes. These gaps occurred in two Florida scrub habitats: rosemary scrub and scrubby flatwoods. We also explored factors that can affect recruitment patterns in gaps, such as soil moisture, root water retention and light availability. In addition, we tested effects of supplemental watering on sown seeds in the field since drought in 2000 and 2001 suppressed natural seedling recruitment. The watering treatment increased recruitment in all types of gaps. Over half of all gaps were colonized, but there were few seedlings. Recruitment was highest in watered, complete gaps in scrubby flatwoods, where the soil moisture levels were higher and light levels lower than in the other gap types. Seedlings of a single spontaneously recruited species (Lechea deckertii) were more abundant than sown species. Locally common gap specialists were more abundant than generalists. Because many of these gap specialists are endangered plant species, understanding how disturbances create microsites for their regeneration is critical for conservation and management of the Florida scrub ecosystem. In small-scale conservation projects, artificially created gaps may substitute for post-fire gaps when prescribed burning is impossible.

Key words: artificial gaps, drought, endemic plant species, fire ecology, mechanical treatments.

Disturbance is an important component of many ecosystems, affecting their spatial and temporal heterogeneity, relative species abundances (e.g. White 1979; Sousa 1984) and population distribution and dynamics (e.g. Pickett and White 1985; Bowles et al. 1990; Quintana-Ascencio and Morales-Hernandez 1997; Lennartsson and Oostermeijer 2001). Many plant species are dependent on disturbances, including fires, ice storms, floods, droughts, landslides, or human-maintained management regimes, to create open spaces for colonization and microsites for regeneration (Grubb 1977; Petraitis et al. 1989; Hobbs and Huenneke 1992; Pavlovic 1994).

Fires are large-scale disturbances in shrub-dominated ecosystems such as the Florida scrub. Florida scrub plants have developed various post-fire recovery mechanisms. Most dominant shrubs resprout and spread clonally, while many herbaceous species, including many endangered narrow endemics, recruit seedlings (Menges and Kohfeldt 1995). Fires create open patches (gaps) in the shrub matrix providing recruitment opportunities for obligate seeders (e.g. Hawkes and Menges 1996; Menges and Hawkes 1998; Young and Menges 1999).

Past studies have defined gaps in Florida scrub based on the distribution of the matrix shrubs. Gaps are defined as openings larger than 1 m^sup 2^ in the matrix of dominant shrubs taller than 0.5 m (oaks Quercus chapmanii, Q. geminata and Q. inopina, palmettos Sabal etonia and Serenoa, repens, lyonias Lyonia lucida, L. ferruginea and L. fruticosa or Florida rosemary Ceratiola ericoides (Menges and Hawkes 1998; Young and Menges 1999; nomenclature follows Wunderlin 1998). Post-fire gaps are large aboveground gaps, gradually becoming filled by tall shrubs between fires. Long-term Ore exclusion may cause gaps to become assimilated into the matrix. These aboveground gaps extend from one shrub canopy to the other but usually include herbs and lichens. In addition, shrub roots and rhizomes grow into aboveground gaps, making belowground gaps smaller than aboveground gaps. We are defining 'complete gaps' as the coexistence of above- and belowground shrub-free spaces. Complete gaps have been documented in several studies from forest ecosystems (e.g. Wilczynski and Pickett 1993; Ehrenfeld et al. 1995; Connell et al. 1997; Ostertag 1998), however in Florida scrub, complete gaps and their relationships to fire have not been studied. Our investigation helps to establish the significance of gaps for Florida scrub plant populations.

Edges of sandy roads, firelanes and plow lines within the scrub are essentially anthropogenic complete gaps. Roadsides have been shown to host higher population densities of some herbaceous scrub species, compared to the population densities found in post-fire gaps in the scrub (Hartnett and Richardson 1989; Myers 1990; Hawkes and Menges 1995; Lambert and Menges 1996).