This study addressed the effects of salinity and pot size on the interaction between leguminous plant hosts and arbuscular mycorrhizal fungi in four pine rockland soils using a shade house trap-plant experiment. Little is known about the belowground diversity of pine rocklands and the interactions between aboveground and belowground biota – an increased understanding of these interactions could lead to improved land management decisions, conservation and restoration efforts. Following twelve weeks of growth, plants were measured for root and shoot dry biomass and percent colonization by arbuscular mycorrhizal fungi. Overall, arbuscular mycorrhizal fungi had positive fitness effects on the four legume species (Cajanus cajan, Chamaecrista fasciculata, Tephrosia angustissima and Abrus precatorius), improving their growth rate, shoot and root biomass; pot size influenced plant-fungal interactions; and percent colonization by arbuscular mycorrhizal fungi was influenced by soil type as well as salinity.

Abstract Exotic plant species may facilitate their invasion into native communities through the modification of ecosystem disturbances such as fire regimes. Where frequent fires are common, invasive plants that suppress fire may induce a positive feedback which further suppresses fire and promotes their continued invasion. In the pine rockland savanna ecosystem of south Florida, the frequent understory fire regime may be altered by the fire-resistant invasive shrub Brazilian pepper (Schinus terebinthifolius). In this thesis, I document the interaction of Brazilian pepper and fire in these savannas. I show that fire causes significant (30%-50%) mortality among low-density populations of Brazilian pepper. However, Brazilian pepper exhibits rapid growth and reproduces quickly following fire, and in the absence of fire it has a low mortality rate. Furthermore, Brazilian pepper can cause a reduction in fire temperature from 47° C at low densities, to almost 200° C at high densities, where it can completely impede fire spread. This creates the potential for Brazilian pepper to initiate a fire-suppressing feedback if it can reach a density threshold during extended fire-free intervals. At a landscape scale, I analyzed digital aerial photographs to show that fire frequency correlates with the extent of Brazilian pepper invasion into pine savanna fragments in southern Florida. In savannas where fire is frequent, Brazilian pepper does not heavily invade, but savannas that are heavily invaded tend to be unburned for more than 20 years. This supports both the regulation of low-density Brazilian pepper populations by fire, and the potential for high-density Brazilian pepper populations to suppress fire and facilitate further invasion.