Seed addition and soil scarification increases Darwin woollybutt Eucalyptus miniata and wattle Acacia oncinocarpaseedling establishment at the CSIRO Kapalga Research Station, Kakadu National Park, Northern Territory, Australia

Published source details

Setterfield S.A. (2002) Seedling establishment in an Australian tropical savanna: effects of seed supply, soil disturbance and fire. Journal of Applied Ecology, 39, 949-959.

Background

The use of fire as a land management tool has resulted in debate over the ecological consequences of imposed fire regimes. Australia's savannas are typically burnt every 1–3 years despite concerns about the effect of such frequent fire on recruitment of savanna plants. This research aimed to determine whether seed or microsite availability limits seedling recruitment of the overstorey tree Darwin woollybutt Eucalyptus miniata and the midstorey shrub Acacia oncinocarpa, and in a second experiment (see Case 635) if seed or microsite availability is affected by frequent fire, and consequent effect on seedling recruitment.

Action

Study site: The study was conducted at the CSIRO Kapalga Research Station in Kakadu National Park, Northern Territory, Australia. It was undertaken in tall (up to 25 m) open forest dominated by Eucalyptus miniata and Eucalyptus tetrodonta. The mid-layer (4–10 m) included several wattle Acacia species. The climate is monsoonal, characterized by high temperatures throughout the year and highly seasonal rainfall. Approximately 90% of the 1,300 mm annual rainfall falls between December and March.

The study area was subjected to annual or biennial fire until 1987, and then remained unburnt until the commencement of a fire experiment in 1990.

Microsite and seed supply as limiting factors: Quadrats (24, 75 × 75 cm) were established at each site in unburnt areas. Experimental manipulations were addition of seed and/or disturbance of the soil surface to increase the number of microsites suitable for germination.

In November 1993, microsites were manipulated by removing the leaf litter from quadrats, scarifying the soil surface with a hand cultivator to a depth of 5 cm, then replacing the leaf litter. Seed was added at low (20 seeds) and high density (200 seeds), scattered evenly within the quadrat. The germinability of seed was 95% for E.miniata and 51% for A.oncinocarpa.

Treatments (applied to three replicates for both species at each of the three sites) were:

1. soil and litter disturbed, low density of seed applied

2. soil and litter disturbed, high density of seed applied

3. soil and litter undisturbed, low density of seed applied

4. soil and litter undisturbed, high density of seed applied

Quadrats were 'fenced' (four strips of galvanized wire mesh 5 cm tall covered in flywire screen) to ensure that seed was not washed away by rainfall. Sticks and leaf litter were pushed against the fences to encourage access to ants. Seedlings were initially counted and marked (with a wooden skewer) on 10 January 1994. The quadrats were rechecked in February, March, April, May, July and October 1994 to determine cumulative total seedling emergence, and dry season survival.

Consequences

Effects of seed addition and microsite creation on seedling emergence: Both increasing the number of seeds and manipulating microsites increased seedling emergence. Increasing seed density from 20 seeds to 200 seeds on unscarified soil resulted in approximately 20 times more E.miniata seedlings and seven times more A.oncinocarpa seedlings. Scarification resulted in a 2-4 times increase in the number of seedlings establishing. A combination of high density seed addition and scarification resulted in around 75 times the number of E.miniata and 20 times the number of A.oncinocarpa seedlings compared to the undisturbed soil, low seed density treatment. The higher number of E.miniata seedlings establishing compared with A.oncinocarpa is probably due to higher germinability.

Conclusions: In unburnt areas, seedling regeneration was limited by both seed supply and microsite availability.

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