Deforestation strongly affects soil seed banks in eucalypt forests: Generalisations in functional traits and implications for restoration
Highlights
► Deforestation of eucalypt communities results in a poor soil seed store of native trees and shrubs. ► Soil seed bank species shared traits associated with the ability to respond rapidly to disturbance. ► Excluded species shared traits associated with the ability to withstand environmental stress. ► Eucalypt communities can not be restored from the native soil seed bank alone.
Introduction
With increasing worldwide focus on restoration of degraded ecosystems (Chazdon, 2008, Choi et al., 2008) and improvement of biodiversity values in production landscapes (Brockerhoff et al., 2008, Scherr and McNeely, 2008), soil seed banks offer the potential to make a significant contribution to natural regeneration of vegetation communities (Bakker et al., 1996, Thompson et al., 1997, Grime, 2001). Whether the soil seed bank can assist restoration efforts depends, among other factors, on the richness and density of native species represented across successional groups, and of non-target or invasive species (Halpern et al., 1999, Willson and Traveset, 2000, Lang and Halpern, 2007). Thus, knowledge of the composition of soil seed banks and shifts in plant traits due to land-use change and associated disturbance regimes can help to guide management strategies designed to promote the germination of target species over non-target species. In turn, this will identify the need for more direct approaches to native species return such as broadcast seeding or planting (Yates and Hobbs, 1997, Standish et al., 2007, Prober and Smith, 2009).
Many forest species produce small quantities of short-lived seed. As such, the conditions in typical forest environments select for traits associated with high rates of seedling establishment at the expense of dispersal in space and time (Leishman and Westoby, 1994, Warr et al., 1994, Thompson et al., 1997, Hermy et al., 1999, Bossuyt and Hermy, 2001, Bossuyt and Honnay, 2008). This pattern is consistent with reports of low species similarity between the seed bank and extant vegetation of forested systems (Hopfensperger, 2007). In contrast, some authors have reported a relatively high component of late-successional species in the soil seed bank of certain forest types, explained by seed dispersal and dormancy characteristics of constituent species (Leckie et al., 2000) or disturbance regimes that provide for increased light availability, germination, and seed return to the soil (Mayer et al., 2004).
Soil seed banks associated with land-use change from native forest or with site degradation are often characterised by severe depletion of richness and abundance of native (especially woody) species. There is also a dominance of early successional species that persist in the soil seed bank or accumulate via dispersal (Warr et al., 1994, Halpern et al., 1999, Willson and Traveset, 2000, Roovers et al., 2006, Bossuyt and Honnay, 2008). This scenario is typical of soil seed banks in abandoned farmlands (Standish et al., 2007, Prober and Smith, 2009) and coniferous plantations that have replaced native forest (Halpern et al., 1999, Moles and Drake, 1999, Onaindia and Amezaga, 2000, Augusto et al., 2001). Moreover, early successional species are often dominated by introduced species so that the return of native species is contingent on establishment by other means (Amezaga and Onaindia, 1997, Moles and Drake, 1999, Wunderle, 1997, Hérault et al., 2005, Standish et al., 2007, Prober and Smith, 2009).
This study aimed to identify generalisable patterns in plant functional traits (sensu Violle et al., 2007) in response to land-use change and implications for the potential role of soil seed banks in restoration of native sclerophyll forest. We investigated two land-use change scenarios: clearing of native eucalypt forest for grazing and subsequent abandonment, and for establishment of Pinus radiata plantation. Earlier work examining the response of understorey vegetation to these two land-use change scenarios revealed a significant decline in the richness of native species, particularly ant-dispersed species (Meers et al., 2008, Meers et al., 2010b). Further, for abandoned farmland there was an increase in introduced annuals and vertebrate-dispersed species and a decrease in resprouters and clonal, non-rosette herbs relative to native forest (Meers et al., 2008). Both outcomes suggest that the soil seed bank may be compromised in terms of restoration potential.
We examined both the response of individual traits to land-use change and groups of species exhibiting correlations between a set of plant traits via emergent groups (sensu Lavorel et al., 1997). The application of emergent groups recognises that local and regional management variables apply to the whole plant, i.e. the combination of traits, rather than each trait separately. This approach provides results that are readily applicable to other regional areas and supporting different pools of species (Hérault et al., 2005). We address the following broad questions:
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Are there differences in the seed density and composition of the germinable soil seed bank following land-use change and, if so, what are they?
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How does the soil seed bank compare to extant vegetation?
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What is the potential of soil seed banks to contribute to the regeneration of native forest species?
Section snippets
Study site
The Delatite Peninsula is located on the northern slopes of the Great Dividing Range in north-eastern Victoria, Australia (145° 58′ E, 37° 8′ S) and has a steeply dissected topography with altitude ranging from 300 to 500 m above sea level. Annual rainfall is approximately 850 mm and falls mainly in winter and spring (Bureau of Meteorology, 1887–2003). Mean monthly maximum temperature ranges from 12.0 °C in July to 29.0 °C in February, with corresponding mean monthly minimum temperatures ranging from
General trends in soil seed bank composition
Soil seed bank density ranged 20-fold across the four land-use types (Fig. 1). The high density of germinants in FNF and AF is due to introduced species that comprised over 75% of germinable seeds (Fig. 1). From the 101 species that emerged, Aira caryophyllea (34% of total seedlings) and Centaurium tenuiflorum (9%) were the most abundant (Appendix A).
The density of germinants in heat-treated soils was significantly greater than in controls across all land-use types (increase: FNF 46%; AF 17%;
Soil seed bank response to land-use change
Abandoned pastures and farmlands typically have large soil seed banks and the seedling density for abandoned farmland in this study (approximately 7500 seed m−2) approached the magnitude of densities reported elsewhere. For example, soil from grazed woodland in Queensland had more than 13,000 seed m−2 (Navie et al., 1996) and soil from grazed grassland in Victoria had approximately 18,500 seed m−2 (Morgan, 1998). In Europe, seed density was similar for former farmland (12,500 seed m−2; Bossuyt et al.,
Acknowledgements
This research was funded by the Department of Sustainability and Environment (Victoria) and through an Australian Postgraduate Award (T. Meers). Soils and plant material were collected under permit number 10502 issued by the Victorian Government Department of Sustainability and Environment. We thank two anonymous reviewers for comments that improved the manuscript.
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Current address: Greening Australia, 333 Bennetts Road, Norman Park, Queensland 4170, Australia.
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Current address: School of Environmental Science, Murdoch University, Murdoch, Western Australia 6150, Australia.
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Current address: Faculty of Agriculture, Food and Natural Resources, University of Sydney, Redfern, New South Wales 2015, Australia.