Savanna ecosystems depend on periodic fires to prevent conversion to a closed forest state. However, invasion by exotic species can alter the natural fire regime in savannas. The endangered pine rockland savanna ecosystem of south Florida, USA, is dominated by the canopy pine Pinus eliotti var. densa, and depends on frequent understory fires to maintain a diverse herbaceous flora and to limit understory competition from hardwoods. Stand dynamics in these savannas are threatened by the exotic invasive shrub Brazilian pepper (Schinus terebinthifolius). A multi-stemmed shrub from South America, Brazilian pepper has high fuel moisture and can reduce herbaceous ground cover at large sizes and high densities, creating the potential for a reduction in fire temperature and extent. We conducted field experiments in a pine rockland savanna within Everglades National Park, Florida, to test: 1) the effects of fire on Brazilian pepper mortality, growth and fecundity, and 2) the potential for Brazilian pepper to initiate a fire-inhibiting feedback.
We showed that prescribed burns in 2006 and 2007 caused significant (30-50%) mortality of Brazilian pepper, especially at small sizes (< 2cm basal diameter) and low densities (< 2 plants per 5 meters). Plants surviving fire tended to re-sprout, grow rapidly, and recover to reproductive maturity within 2 years. Using temperature-sensitive paints, we showed that a high density of Brazilian pepper caused a reduction in fire temperature of up to 200°C, and a corresponding reduction in probability of mortality as large as 90%. While frequent fire may be effective at controlling low-density populations of Brazilian pepper, there exists the potential for fire suppression to interact with exotic plant populations and significantly alter the disturbance regime, facilitate invasion, and convert the savanna ecosystem to an exotic-dominated forest.