Factors affecting seedling regeneration of woody species in a northern Australian tropical savanna

  • Samantha Setterfield

    Student thesis: Doctor of Philosophy (PhD) - CDU


    Savanna vegetation covers approximately 40% of the surface of the tropics. Tropical savannas contain a large proportion of the world's population and are experiencing increasing land use pressures, including vegetation clearance, grazing and the creation of pastures. A fundamental climatic characteristic of tropical savannas is rainfall seasonality, with annual alternation of rainy and dry seasons. Regular fire is also a characteristic feature of tropical savannas, and fire is a widely used management tool. Fire is considered to be an important determinant of savanna structure and growth, and has been demonstrated to have an important effect on plant population dynamics. The effective use fire as a management tool requires a thorough understanding of its ecological effects on the savanna plant community.

    The tropical savannas of northern Australia cover approximately one-fifth of the continent. Land-use pressures are increasing in the region, and there is an increasing need to understand the regeneration ecology of savanna species so that sustainable land management practices (including vegetation rehabilitation) can be undertaken. This thesis examines factors affecting seedling regeneration of three common woody species in undisturbed and disturbed savanna in northern Australia. Fire is a predominant force in this region and particular attention is paid to the effects of fire on seedling regeneration.

    Patterns of seed production and seed fall were investigated for the three species: Eucalyptus miniata, Eucalyptus tetrodonta and Acacia oncinocarpa. For all species, there were substantial temporal and spatial variation in seed production. Ovule production in these species occurred during the dry season, coinciding with a period of frequent (often annual) fire. Fire reduced seed production in all species by at least an order of magnitude compared to the unburnt trees, and the results indicated that both the fire intensity, and its timing in relation to phenological events, are important determinants of seed supply for regeneration.

    Investigations on the post-dispersal fate of seed showed that seedling regeneration of E. miniata and A. oncinocarpa are limited by both seed supply and microsite availability. The experimental application of seed to sites in three fire regimes showed that burning reduced seedling emergence success compared to unburnt sites. This suggests that annual fire regimes (the predominant management regime in the region) can have a dramatic impact on regeneration, by both reducing seed supply and seedling emergence. The latter effect may be partly attributed to the loss of seeds to harvester ants, although investigations showed that the major limit to E. miniata establishment is caused by seed-bed conditions. Seed predation by ants appears to be a significant factor limiting recruitment in E. miniata by reducing the chance of seedling establishment from low (<10%) to virtually none.

    A study of patterns of vegetation recolonisation on gravel pits in Kakadu National Park showed that despite the completion of physical rehabilitation to the pits, the re-establishment of overstorey vegetation, particularly Eucalypts, was poor. Like the undisturbed savanna, the re-establishment of woody vegetation on these severely disturbed areas is limited by both the availability of seed and the availability of suitable microsites. The primary supply of seed to the gravel pits in Kakadu is natural seedfall from the surrounding undisturbed vegetation. The work in thesis suggests that this supply will be low for several common woody species. Minor soil scarification substantially increased seedling emergence success, but the study suggests that seed supply must immediately follow the physical soil treatment. Therefore, due to the unreliability of an adequate seed supply from the vegetation surrounding the pits, seed must be artificially broadcast to ensure the re-establishment of woody vegetation. In addition, the rehabilitated site must be protected from fire for at least one year following seedling emergence to allow the seedlings to develop sufficient resources to survive burning.
    Date of AwardFeb 1997
    Original languageEnglish
    SupervisorAlana Anderson (Supervisor) & Richard Williams (Supervisor)

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