Twenty years of vegetation change in five long‐unburned Florida plant communities

Abstract

Abstract. Rates and directions of change over a 20‐yr interval in five long‐unburned (> 60 yr) plant communities were studied using multivariate analyses and compositional vectors. The study sites were located in fire and summer‐drought adapted, xerophytic vegetation with many endemics on acidic, nutrient‐poor, sandy soils in south‐central peninsular Florida. Sizes of individual stems from 72 sets of nested permanent quadrats were measured in 1969, 1979, and 1989.Patterns of vegetation change differed by community. Flatwood and bayhead quadrats showed rapid increases in densities and basal areas of Persea borbonia (red bay). In the southern ridge sandhill community, evergreen clonal Quercus species (oaks) and Pinus clausa (sand pine) increased in dominance and grasses declined. Oaks (especially Q. geminata) also increased in importance in scrubby flatwoods. Sand pine scrub was relatively stable in composition, but experienced marked structural changes due to substantial sand pine mortality (18% during 1969–1979, 39% during 1979–1989). Compositional changes in the absence of fire were greatest whereas structural changes were least in southern ridge sandhill and scrubby flatwoods, both communities which normally receive frequent, recurrent fire. Compositional changes were lowest in sand pine scrub, which is normally infrequently burned.Classic successional patterns such as species replacement, decreases in density, and increases in basal area were generally lacking. Tree densities increased in two of four community types (southern ridge sandhill, scrubby flatwoods); while basal area declined in the flatwoods/bayhead and sand pine scrub sites. Directions of compositional vectors included divergent, opposing, and complex patterns, suggesting vegetation change in the absence of fire has a strong stochastic component.