Two important questions regarding invasive species are: 1) What makes some communities resistant to invasion? 2) What can make communities once resistant to invasion vulnerable to invasion? Previous studies have suggested that Florida rosemary (Ceratiola ericoides) uses allelopathy to limit invasion of scrub habitats by highly flammable species (e.g., grasses). Florida rosemary, not adapted for frequent fires, thus limits the accumulation of proximate flammable fuels and reduces the expected fire frequency. Following Hurricane Katrina, which caused near-total mortality in Florida rosemary shrubs on two barrier islands off the Mississippi coast, torpedograss (Panicum repens), a non-native, facultative wetland grass on the fringes of the scrub began invading the scrub communities. An increase in competition or fire frequency due to torpedograss invasion could prevent recovery by Florida rosemary and perhaps other associated fire-sensitive or gap-dependent native plant species, drastically altering community composition in the rosemary scrub. To test the hypothesis that Florida rosemary previously prevented torpedograss spread via allelopathy, ramets of torpedograss were grown in soil collected from within torpedograss swards, near torpedograss swards, bare dunes, beneath dead Florida rosemary, beneath live Florida rosemary, and beneath live Florida rosemary with weekly litter addition. Treatments were grown with and without activated carbon.

Results/Conclusions:                                                                 

Rosemary litter addition inhibited torpedograss root biomass, but not rhizome or aboveground biomass, relative to root biomass in soil from beneath live rosemary without litter addition, which was itself not significantly different from root growth in soil from bare dunes. However, this effect was not reversed by the addition of activated carbon (a commonly accepted requirement to assert allelopathy as the cause of inhibition). Root biomass was higher when grown in soil from within torpedograss swards, which contained more organic material and perhaps more available nutrients. Shoot and rhizome growth were unresponsive to litter addition or soil occupation by Florida rosemary. The presence of rosemary litter may be an important factor inhibiting torpedograss growth when moisture or nutrients are scarce, but insufficient to completely repress the spread of this invasive grass. If so, the increase in moisture and nutrient-rich detritus associated with Hurricane Katrina, compounded by the simultaneous removal of most Florida rosemary shrubs, may help account for the spread of torpedograss after the hurricane. Understanding the potential causes of torpedograss spread into the scrub following Hurricane Katrina may yield valuable insights into the interactions of native and non-native species before and after catastrophic disturbances.