Banksia woodlands are a taxonomically rich native vegetation community found in south-western Western Australia. Currently < 20% of the woodlands remains, the result of clearing for extensive urban development and to establish commercial plantations of Pinus pinaster in the 1960s. Government policy has dictated that the pines are removed, and a significant percentage of the land be restored back to Banksia woodlands. Our knowledge of the abiotic and biotic limitations to restoration success in post-pine environments is limited, and without this, conservation of this community is unlikely. Experiments were established to understand native plant x soil interactions, to inform restoration activities. The impact of post-pine soils on key Banksia woodland species was assessed by (a) germinating over 28 days, and (b) growing seedlings over 22 weeks across a soil chronosequence (time since pine removal: 1 year, 5 years, 8 years) and a reference undisturbed Banksia woodland soil. Soil abiotic factors such as nutrient status and water retention capacity were identified as potential drivers of restoration success. In subsequent experiments, the impact of the biotic factor of weed presence, in particular perennial veldt grass (Ehrharta calycina), on seedling growth and susceptibility to drought (stomatal conductance and survival) were investigated.
Soil nutrient levels and pH at post-pine sites were not significantly different (P > 0.05) from undisturbed Banksia woodland soil or across the chronosequence. Although this suggests post-pine soil shouldn’t limit plant growth in terms of nutrient availability, germination responses varied between species and across soils. For example, seeds of the Fabaceae (Gompholobium tomentosum, Jacksonia floribunda, Kennedia prostrata) displayed higher germination in one year post-pine soil compared to older sites (1.53-fold increase in maximum germination percentage), whereas soil age had no effect on germination in seeds of the Proteaceae (Banksia attenuata, B. menziesii, Hakea trifurcata) (P > 0.5202). Preliminary analysis of plant growth and morphology indicates that seedling growth responses vary between species and across soil types, suggesting that soil nutrient status alone doesn’t explain plant performance. Impacts of abiotic x biotic interactions have also been examined, including the impact of weeds (in particular E. calycina) on seedling growth and drought susceptibility in native species, and this is likely to be a significant factor in ultimately defining restoration outcomes.