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Individual study: The effect of autumn versus spring burns on survival and seedling recruitment of four Banksia species at Mt Adams, Western Australia

Published source details

Cowling R.M. & Lamont B.B. (1987) Post-fire recruitment of four co-occurring Banksia species. Journal of Applied Ecology, 24, 645-658

Summary

In certain regions of Australia, in order to minimize wildfire hazards in semi-arid shrublands the authorities may adopt a policy of cool season (spring or autumn) prescribed burns, typically undertaken every 4-7 years. There is much evidence from Australia and analogous shrublands in South Africa, that fire season and frequency have a profound effect on population sizes of certain plant species, including Banksia and Protea species characteristic of such habitats. The factors responsible for mortality and survival after autumn and spring burns were studied for four Banksia species: Banksia attenuata, B.leptophylla, B.menziesii and B.prionotes in a 15-year-old stand of fire-prone scrub-heath in southwestern Australia.

Study area: The study was undertaken near Mt Adams (29º25'S, 114º 58'E) in southwest Western Australia. The site was located in and beside a 50-60 m wide strip between two fire breaks. Vegetation comprised a 15-year-old Banksia-dominated scrub-heath on deep, infertile sands.

Burns: As part of its fire control programme, the Western Australian Bushfires Board burnt portions of the strip on 5 May, 1983 (autumn) and 21 September, 1983 (spring). Fires in both seasons were intense, achieving 100% Banksia leaf scorch.

Banksia monitoring: Studies were undertaken in the autumn-burn site (1.9 ha), spring-burn site (0.8 ha) and in the adjacent unburnt area recording: seed release; post-fire, pre-dispersal seed predation; post-dispersal seed predation; germination; seedling establishment and mortality; and adult mortality.

Seed release: After 140 days post-burn (regardless of burn season) more than 95% of seeds of all species were released, except for B.leptophylla after the spring burn, which retained 17% of seeds in cones through the summer. The rate of seed release from burnt cones was significantly slower for all species after the spring than the autumn burn.

Seed predation: Post-dispersal seed predation after the autumn burn was negligible but was very high, ranging from 90% (B.prionotes) to 98% (B.menziesii), after the spring burn.

Germination: Seed of all species germinated optimally at 15-20ºC. After the autumn burn, field germination inside enclosures ranged from 9% (B.menziesii) to 69% (B.leptophylla). More seeds of all species germinated in the unburnt than in the autumn burn site. No seed sown after the spring burn had germinated by the end of the summer and their viability was severely reduced.

Seedling establishment: From 54 to 97% of seedlings which established after the autumn burn died, largely due to predation, by the end of the summer. Predation was probably unnaturally high, due to the small size (c. 2 ha) of the resource-rich, autumn-burnt patch relative to the vast tracts of adjacent unburnt scrub-heath. Fewer seedlings died inside enclosures after the autumn burn (32-52%) than in the unburnt site (71-80%), indicating the importance of drought-induced mortality.

Seedling recruitment: By the end of the first winter, the number of seedlings recruited per parent of all species was over twice as high after the autumn than after the spring burn. More than 2 years after both burns, seedling replacement levels were higher than pre-burn adult densities for all species except B.menziesii. There was no recruitment in the unburnt area over the same period.

Conclusions: Results from this study suggest that in order to maintain the populations of these four Banksia species, tracts of scrub-heath should be burnt in autumn, prior to the onset of winter rain.


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