Evaluating emergence, survival, and assembly of Banksia woodland communities to achieve restoration objectives following topsoil transfer

The science of restoration ecology seeks ways to advance the understanding of how to restore native ecosystems that have been degraded or destroyed. Ecological theory suggests that environmental filters influence the outcome of ecological restoration and ultimately long-term restoration success. In this study, three types of environmental filters: dispersal, abiotic, biotic were manipulated to improve understanding of how to successfully re-establish native plant communities. The abiotic filter was manipulated by decreasing soil compaction (ripping) and evaporation (shade). The biotic filter was addressed with control of herbivory (fencing) and weeds (herbicide). The dispersal limitation was examined by altering the application depth of the transferred topsoil (deep and shallow topsoil volume) and application of germination cues (smoke and heat). This study was located in Banksia woodland - a Mediterranean-type ecosystem restricted to the Swan Coastal Plain in Western Australia that is diminishing due to rapid urban expansion. Topsoil from Banksia woodland vegetation contains a large native soil seed bank. Here, topsoil from a newly cleared site was stripped, transferred and applied to six recipient sites within two months of vegetation clearing. The recipient sites had been grazed for about 80 years prior to purchasing for conservation as part of a biodiversity offset program. Following topsoil transfer, a fully factorial combination of three filter manipulation treatments was applied across the six sites to identify successful restoration techniques. The dispersal filter was tested by altering the volume of topsoil seed bank applied. The abiotic filter experimental manipulation was performed using topsoil ripping. The biotic filter was examined by installing herbivore exclosures. Emergence and survival of Banksia woodland species were quantified in spring and autumn for two consecutive years after topsoil transfer. Manipulation of the abiotic filter in soil ripping treatment reduced the densities of the emerging native perennials significantly (t = 4, P < 0.001). Overall, the most successful technique was the application of a high volume of unripped topsoil, with resulting mean densities of native perennials of 15.9 ± 0.2 (SE) m-2 in the first year. Similarly, high volume of unripped topsoil resulted in the highest mean densities of native perennials of 7.6 ± 0.1 (SE) m-2 in the second year after topsoil transfer. Application of plot-scale heat treatments in the second year recorded 4.5 % increase in the emergence densities of native perennials compared with site-scale control plots (t = 11.4, P < 0.001). Mean seedling survival over the 2-year sampling period was 2.44% ± 0.2 (SE). The highest survival through the first summer drought occurred within topsoil ripping treatment in combination with artificial shade (mean survival of 27.3 % ± 5.6 (SE), t=7.8, P<0.001). High mortality occurred during the second summer drought and overall mean seedling survival over the 2-year sampling period was 2.44% ± 0.2 (SE). Breaking plant species into key functional groups, the number of non-resprouters oscillated around 70% in both years. Nitrogen-fixers comprised 50% of total native flora richness in the first year after topsoil transfer and decreased markedly to 20% in the second year. Plant assemblages in the second year after topsoil transfer comprised mostly of non-native perennial grasses and perennial, small-seeded native woody shrubs. The transferred topsoil seed bank contained a close-to-reference species richness of native species propagules. Deep topsoil returned the highest mean densities of native plant species and second highest number of native plant taxa (163 on deep topsoil, 166 on fenced topsoil, of total 171 plant species recorded in this study). The recorded plant species richness comprised about 25% of total species pool recorded in Banksia woodland ecosystems in its natural distribution on Swan Coastal Plain, Western Australia and about 105% of total plant taxa recorded in the reference site before clearing. These plant taxa were mostly understorey species that suggests a high potential for mitigating environmental barriers on restoration sites with the use of transferred topsoil, but more research needs to focus on improving survival of native seedlings in their early stages of establishment.