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What potential is there for regeneration of native species from the soil seed bank in Coast Tea Tree-dominated scrub?



Shrub encroachment generally causes the loss of native species in herbaceous-dominated communities. The ability of the original ecosystem to return to its pre-encroachment state (i.e. its ecological resilience) will be partially contingent on the capacity of these species to regenerate from soil-stored seed. Coast Tea Tree (Leptospermum laevigatum) has formed a dense scrub in many areas previously dominated by grassy woodland, and hence, managers need guidance about the effectiveness of strategies designed to recover the pre-encroachment vegetation. In this context, we ask: what is the potential of species stored in the soil seed bank to return following Tea Tree removal? A germination experiment was undertaken using soil collected from dense stands of Tea Tree that had been long established. Heat/smoke was applied to soils to simulate the effects of a fire on the soil seed bank, while leaf litter treatments were used to mimic both undisturbed stands and stands where shrubs have been slashed where litter creates a physical barrier to emergence. We found the soil seed bank was dominated by exotic forbs (83% of all germinants) and contained few grasses. Heat and smoke decreased total species density but increased species diversity through the suppression of common exotics. Our data suggest that slashing would result in germination being dominated by exotic flora, but using fire would likely reduce that dominance. However, we conclude that recovery by much of the original flora after site occupation by Coast Tea Tree may be contingent on mechanisms other than soil-stored seeds.