Effect of time since burn on soil seedbanks in the jarrah forest of Western Australia

Bauxite is mined in the jarrah (Eucalyptus marginata Sm.) forest of south-western Australia and 700-800 ha each year are rehabilitated by mining companies. As well as broadcasting seeds and planting, the mine restoration relies heavily on the natural soil seedbank as a means of reinstating plant diversity after mining. The natural soil seedbank is a large but highly variable resource, both spatially and temporally. In the present study, we used 32 sites paired across burn boundaries to determine changes in soil seedbanks due to time since burn. At each site, we took one hundred 100-cm(2) soil samples down to 5-cm depth and recorded germinable seeds. The mean total seedbank over all sites was 472 seeds m(-2) (90-1210 seeds m(-2)). Five months after a spring burn, there was a significant decrease in the seedbank of legumes, compared with unburnt sites, probably due to germination in spring immediately after burning. Non-legumes (mostly non-heat-responsive species) did not show this decrease. One year after burning, there was a significant reduction in the total seedbank, presumably due to germination of most species in the winter following the burn. Legumes and non-legumes both showed this decrease. There was a recovery of the soil seedbank by 3 years after burning. This recovery was associated with higher densities of species from the families Apiaceae, Rhamnaceae (Trymalium ledifolium Fenzl), Stylidiaceae, Rutaceae, Centrolepidaceae and Sterculiaceae (Lasiopetalum floribundum Benth.). These families represent annuals, herbaceous species and small perennial shrubs. The recovery was likely due to vigorous flowering, and seed set by seedlings and sometimes resprouts following the burn. Once recovered, the soil seedbank stayed reasonably consistent up to 22 years after burning. Soils stripped 1 year after burning may contribute less plant density and diversity to rehabilitated areas. There is no evidence that the soil seedbank significantly decreases in the longer term with increasing time since burning up to 22 years. The community composition of the soil seedbank was strongly affected by geographical location, with sites closer to each other having similar composition. Only sites 1-year-since burn showed changes in the community composition due to burning.