Burning creates contrasting demographic patterns in Polygala lewtonii (Polygalaceae): a cradle-to-grave analysis of multiple cohorts in a perennial herb

Abstract

Fire drives the population dynamics of many plants. By following successive cohorts of Polygala lewtonii Small (Polygalaceae), a short-lived herb endemic to fire-maintained Florida sandhills, in both burned and unburned microsites, we investigated how fire affected vital rates throughout cohort lifetimes. We followed cohorts from before to 6 years after a prescribed fire in 220 25-cm-radius quadrats, recording survival and seedling recruitment quarterly, and growth and fecundity annually. Fire effects were most pronounced in the first 2 post-burn quarterly censuses, when cohorts in burned (v. unburned) quadrats had seven-fold higher seedling recruitment, significantly higher seedling survival, and a 16.7% gain (v. 1.2% loss) in quadrat occupancy. Plants in burned (v. unburned) quadrats also flowered earlier, were more likely to survive to reproduce and had longer lifespans. The negative effects of density on survival were relaxed in burned quadrats for the first 2 censuses. Burning creates contrasting demographic trajectories for burned v. unburned cohorts. In burned microsites, higher seedling recruitment and survival, earlier flowering and longer lifespans combine to produce a greater contribution to the seedbank and, thus, to population viability. The present study documents the pyro-demographic mechanisms linking the life history of a perennial herb with a frequent fire regime.