Species composition and spatiotemporal pattern of the seed bank and vegetation in native and degraded Florida rosemary scrub

Background/Question/Methods Disturbance plays an influential role in determining species composition and spatial structure. We evaluated how species composition and spatiotemporal pattern of the vegetation and soil seed bank changed along a disturbance gradient in Florida rosemary scrub. We hypothesized that changes in species composition would reflect contrasting abilities of species to cope with disturbance. We expected that increasing anthropogenic disturbance would favor stress tolerant species and would reduce habitat spatial heterogeneity. During the summers and winters of 2007-2009, we assessed percent vegetation cover and soil seed bank species composition in nine permanent grid plots located in three community types: (1) native Florida rosemary scrub which has been subjected to natural fire and controlled burns, (2) degraded scrub which has undergone mechanical disturbance and light cattle grazing, and (3) semi-improved pasture which has been subjected to introduction of non-native forage grasses, nutrient additions, hydrological changes, and heavy cattle grazing. During May-June 2007 and February 2009, ocular estimates of percent vegetation and ground cover were assessed. Using the seedling emergence method, we evaluated species composition of the seed bank from soil samples collected in August 2008 and January 2009.

Results/Conclusions Across all sites, aboveground vegetation was dominated by long-lived perennials while the seed bank was dominated by short-lived species. Pasture and native scrub had the greatest differences in composition and spatial structure above- and belowground. Species richness and composition were similar between degraded and rosemary scrub but relative species abundances differed. Lower shrub cover and higher subshrub cover was observed in degraded scrub. The vegetation was aggregated across all sites and larger more homogenous patch sizes were observed in species with relatively high abundance in the degraded scrub. Abundance of scrub herbs above- and belowground was highest in native scrub, lower in the degraded scrub, and absent in the pastures. Spatial aggregation was less frequent in the seed bank than the vegetation, especially among species that were not present aboveground. In native scrub the seed banks of scrub plants were spatially aggregated and were positively associated with both conspecific species aboveground and litter cover. These patterns were not observed for the same scrub species in degraded scrub. In this study, increased human disturbance first disrupted the spatial structure in the degraded scrub and more severe disturbance in the pastures eventually led to elimination of many of the component scrub species.