Soil seed bank dynamics in post-fire heathland succession in south-eastern Australia

Abstract

Soil seed banks can exert a strong influence on the path of vegetation succession following fire, with species varying in their capacity to persist in the seed bank over time, leading to changes in seed bank composition and propagules available for post-fire colonisation. This study examined the effect of time since fire on soil seed bank dynamics in a chronosequence of seven sites spanning 26 years in a south-eastern Australian sand heathland. No significant change was evident in the species richness and density of the germinable soil seed bank, but species composition differed significantly among young (0-6 years since fire), intermediate (10-17 years since fire) and old-aged (24-26 years since fire) sites (using presence/absence data). No significant trend was observed in the similarity between the extant vegetation and the soil seed bank with time since fire. A total of 32% of the species recorded in the soil seed bank were not present in the above-ground vegetation at the same site, which suggests that species requiring fire for germination may be present in the seed bank. Most species present in the extant vegetation were not recorded (63%) or were in very low abundances in the soil seed bank (29%). The mode of regeneration appears to be the major determinant of species absence in the soil seed bank, as 66% of species occurring in the extant vegetation but not in the seed bank have the capacity to regenerate by resprouting. This study shows that a major shift in the successional pathway after fire due to altered seed bank composition is unlikely in this vegetation; most species not recorded in the seed bank are either resprouters (obligate or facultative) or serotinous, suggesting that they will readily regenerate following fire. Unless fire frequencies are high and kill fire-sensitive obligate seeders before they reach maturity, the chance that the soil seed bank could substantially alter vegetation composition within the study area after fire is low. However, it is unclear how successional pathways may alter in response to severe fires with the potential to kill both seeders and resprouters.

Transition from heathland to scrub in south-eastern Tasmania: extent of change since the 1970s, floristic depletion and management implications

“…The low similarity between the species composition of seed bank and above ground vegetation has been observed in other Australian heathlands (see Wills and Read 2007), and other vegetation types (Leck et al 1989;Davies et al 2013). A possible explanation for the low levels of similarity may be that long-lived seeds are bioturbated down the soil profile over many decades, and become isolated from the effects of fire unless fire is preceded by erosion.…”

“…Ozothamnus ferrugineus, present only in the soil seed bank, is a wind-dispersed shrub of disturbed ground. However, Wills and Read (2007) suggest that the majority of species not recorded in the seed bank may be serotinous or regenerate by resprouting, and that the capacity of the soil seed bank to change vegetation composition after fire is relatively limited. In accordance with this finding, our study also shows that resprouters are less frequently present in the seed bank than the above ground vegetation at both time periods.…”

“…For example, of the species that occurred more frequently in the heat treatment, two were not present in the above ground vegetation in 2013, but were recorded in 1976, and five species (Acacia genistifolia, Astroloma humifusum, Boronia parviflora/pilosa, Dillwynia glaberrima and Pimelea linifolia) are commonly found in Tasmanian heathlands (Kirkpatrick and Harris 1999). All these genera, excluding Boronia, include obligate seeders (Wills and Read 2007). In addition, graminoids that were present only in the seed bank were more common in nonheat treated samples, and those that were present in the above ground vegetation were more frequent in the heat treatment.…”

“…However, it has also been shown that the similarity between standing vegetation and seed bank can be as little as 11-38 % (Wills and Read 2002). A later study of Australian heathland by the same authors also showed a low similarity, with 32 % of species in the soil seed bank not present above ground, 63 % of above ground species not recorded and 29 % of species found in very low abundance in the seed bank (Wills and Read 2007). Therefore, although seed banks may be valuable in maintaining some heathland species through a period without fire, they cannot be relied on to maintain all.…”

“…About 67 % of Australian heathland species are known to regenerate after fire by resprouting (Pausas et al 2004). Furthermore, the mode of regeneration is often the most important factor determining species absence in the seed bank, and species found in the above ground vegetation but not the soil seed bank are more likely to have the capacity to resprout after fire (Wills and Read 2007). However, although this trait may be useful given appropriate fire intervals, it may make it difficult for such heath species to persist once they have been lost from the above ground vegetation.…”