Ecology of seed germination for broad-acre restoration of native vegetation on cracking clay vertosols

Abstract

In Australia, substantial ecological restoration of farmland is undertaken in conjunction with community-based natural resource management agencies, with the objective of balancing agricultural land use and biodiversity conservation through revegetation with native species. Across the North-West Plains of New South Wales (NSW), both large-scale and small-scale restoration efforts are frequently required. However, fragmented agricultural landscapes, the lack of sufficient scientific information on the seed ecology of native species and the environmental conditions associated with the region's vertosol soils are major challenges for revegetation. This thesis investigated germination traits in 73 plant species that are important components of endangered vegetation communities in north-western NSW to produce information useful for plant propagation and ecological restoration. In order to identify the optimum combination of environmental conditions to maximise the percentage and rate of germination in 14 'Eucalyptus' seedlots of ten species from north-western NSW, germination responses to seasonal temperature regimes and light were examined, and the relationship between these factors and seed size investigated. The effect of three alternating day/night temperature treatments (spring, summer, winter) and two light treatments (light/dark, dark) was investigated in growth cabinets. Germination patterns varied between species and seedlots. In general, the presence of light and winter and spring temperatures resulted in higher average germination than darkness and summer temperatures. However, some seedlots germinated consistently well under all treatments. Germination of small seeds was higher in the presence of light while larger seeds germinated better under continuous darkness. Time to germination was about threefold faster in response to summer and spring temperatures than winter temperatures. The seeds of many acacias, which are important in ecosystem regeneration due to the ability of 'Acacia' species to fix nitrogen, have a physical dormancy that must be broken prior to their use. In order to identify convenient methods to break seed dormancy in ten 'Acacia' species from north-west NSW, seed response was investigated to three dormancy-breaking treatments and two incubation temperatures. Mechanical scarification (two intensities) and hot water were applied as seed pre-treatments and seeds incubated under two temperature/light-controlled treatments in germination cabinets. The results varied with species but the three dormancy-breaking treatments significantly increased germination percentage or reduced the time to germination in all but one species. Temperature had an effect on only one species. Germination was greater or more rapid after mechanical scarification than after hot-water treatment. North-western NSW is one of Australia's biodiversity 'hotspots' due to the number of endemic plants and diversity of species, some of which lack seed ecology information for restoration and conservation purposes. Seeds of 49 species were investigated to identify limitations to germination, the pre-treatments needed to overcome such limitations, and determine the suitability of these species for direct seeding or propagation from seed. Seed viability, germination under different seasonal temperatures (winter, spring/autumn, summer) and a requirement for seed pretreatment to promote germination were all assessed experimentally in germination cabinets. Seed viability varied widely among the 49 species; temperature determined germination success in 27 species and various seed pre-germination treatments were effective in increasing germination percentage in 22 species. The results will be useful in propagating these species in the nursery and for direct seeding in the field. The decline in eucalypt-dominated woodlands across the wheat–sheep belt of southern and inland eastern Australia is of concern, and revegetation targets have been set to restore woody vegetation cover in over-cleared landscapes. In order to provide guidelines for direct-seeding eucalypts in large-scale revegetation of cracking clay soils in north-western NSW, seedling emergence was investigated in relation to moisture regime, sowing depth and seed size in six species of 'Eucalyptus' in a glasshouse experiment. Seedling emergence was low despite high seed viability and provision of optimum temperatures and soil moisture. All six species exhibited greatest emergence when sown at 0–6-mm depth, with seed size being less important than moisture (except under dry conditions) and proximity to the surface. Species responded differently to the three watering treatments. Success in direct-seeding these species in vertosol soils in the region may be unreliable. The findings of this research should contribute to seed management and direct seeding in large-scale revegetation projects on cracking clay soils in the agricultural districts of north-western NSW. The research variously examined the seed viability, quality, persistence and germination response of seeds in a range of species to light, temperature and sowing depth. It aimed to determine where release of germination constraints was necessary, as well as cost-effective techniques that can be applied to large quantities of seed required in broad-acre revegetation.