Seedling establishment in an Australian tropical savanna: effects of seed supply, soil disturbance and fire

Summary

Study 1

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.

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.

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.

Study 2

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 if seed or microsite availability is affected by frequent fire, and consequent effect on seedling recruitment of two common species, the overstorey tree Darwin woollybutt Eucalyptus miniata and the midstorey shrub Acacia oncinocarpa. A second experiment (see Case 634) looked at whether seed or microsite availability limits seedling recruitment.

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. Fire treatment units were 15–20 km² catchments, each based on seasonal creek-lines. This study investigated three fire regimes (three replicate catchments each) that had been applied since 1990: i) 'unburnt'; ii) 'early', burnt annually early in the dry season (May/June); and iii) 'late', burnt annually late in the dry season (September). Fires lit early in the dry season (April–June) were typically low in intensity and patchy, those late in the dry season (July–November) usually more intense often scorching the canopy completely and burning extensive areas. These fire regimes are typical of those occurring across the mesic savanna region.

Effects of fire regime on seedling emergence: The experiment was undertaken in three replicate catchments of each of three experimental fire regimes: unburnt, early and late. Two experimental sites were located within each catchment, and six 75 × 75 cm quadrats were randomly located at each. In November 1993, 200 Eucalyptus miniata seeds were sown into each of three quadrats, and 200 Acacia oncinocarpa seeds sown in the remaining three quadrats at each site.

Seedlings was initially counted on 10 January 1994 (each was marked with a wooden skewer). Seedling emergence was monitored in February, March, April, May, July (post-early burn) and October 1994 (post-late burn). Sites were censused again in June 1995 (i.e. at the end of the second wet season). In September 1994, a wildfire entered one of the unburnt catchments, killing all marked seedlings. Therefore, data on seedling survival in October 1994 and June 1995 are from the two remaining unburnt catchments.

The ground flora in the savanna changes dramatically throughout the year, dependent on rains. As this could affect seedling emergence and establishment, the grass, forb, woody sprout and leaf litter cover in each quadrat were estimated at each census. Canopy cover can effect germination and seedling establishment, so canopy cover over each quadrat was measured. Differences in canopy cover are more likely to be attributable to differences in soil moisture availability and therefore germination success. Differences in the canopy and ground cover between regimes were therefore analysed.

Seed loss due to ants: To determine whether differences in seedling establishment could be due to differences in seed lost to ants, a seed removal experiment was established and losses to seed removal were assessed by recording the loss of seeds from seed caches located at four random sites within each replicate catchment.

Seedling emergence: All E.miniata seedlings that emerged did so in the first wet season following sowing in 1994, whereas some A.oncinocarpa seedlings (< 1% of seed sown) emerged from the seedbank during the second wet season. The total number of seedlings that established decreased with increasing intensity of the fire season. Both burning regimes reduced seedling emergence, possibly because the reduced canopy cover caused unfavourable microclimate, the increased grass and forb ground cover increased competition for resources.

Seed loss due to ants: The number of seeds removed from caches after 72 h varied significantly between fire regimes for both E.miniata and A.oncinocarpa with more than double the number of seeds removed from the two burnt regimes compared with the unburnt regime. About 18% of E.miniata seeds were removed from the unburnt catchments compared with approximately 40% and 50% in the early and late regimes, respectively. For A.oncinocarpa, 35% of seeds were removed from the unburnt regime compared with c. 70% in the burnt regimes.

Conclusions: The results indicate that sexual regeneration of i>Eucalyptus miniata and the midstorey shrub Acacia oncinocarpais disadvantaged by current burning practices because both seed supply and the number of microsites are reduced. Thus, long-term changes in savanna floristic structure seem likely unless fire managers aim to increase the fire-free intervals. The relative abundance of species able to reproduce vegetatively may increase under frequent fire regimes. Such a change may take a long time to detect given the fire-resistant and long-lived nature of overstorey species, and the capacity for vegetative regeneration in many savanna species. The impacts of fire on limiting seedling regeneration will require savanna managers to consider fire-free intervals of several years for effective recruitment of dominant woody species.

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