Resprouting and mortality of juvenile eucalypts in an Australian savanna: impacts of fire season and annual sorghum
Patricia A. Werner A B C and Donald C. Franklin BA The Fenner School of Environment and Society, The Australian National University, Canberra, ACT 0200, Australia.
B School for Environmental Research, Charles Darwin University, Darwin, Northern Territory 0909, Australia.
C Corresponding author. Email: wernerpa@ufl.edu
Australian Journal of Botany 58(8) 619-628 https://doi.org/10.1071/BT10139
Submitted: 28 May 2010 Accepted: 10 September 2010 Published: 9 December 2010
Abstract
In northern Australian savannas, canopy tree species often have juvenile tree banks that are composed mainly of small individuals of indeterminate age that have resprouted repeatedly after fire. Little is known about their demography. We report the initial responses (mortality, topkill and resprouting type) of 3133 marked juvenile eucalypts to set fires of different seasons (early dry season, late dry season, wet season, unburnt) in a 32 400 m2 field experiment. Fire treatments were repeated in plots dominated by a native annual grass (sorghum) that becomes senescent before the early dry season and provides the main fuel of savanna fires, and in others with little or no sorghum, but instead other native grasses and forbs that remain green well into the dry season. Most juvenile eucalypts <150 cm high were topkilled but resprouted from underground tissues regardless of fire season or understorey (86–100% vs <5% in unburnt plots). Few saplings 200–500 cm high died or were topkilled, but impacts of fire were harsher in sorghum than in non-sorghum vegetation. The response of eucalypts 150–199 cm high was intermediate, suggesting a ‘tactical’ transition from suppressed persistence to growth toward maturity. Counter-intuitively, genet death of juvenile trees was >22% in the low-intensity early dry season fire in plots with little or no annual sorghum, compared with <2% in all other fire/understorey combinations. We suggest results are related to fire behaviour, seasonal carbohydrate storage dynamics and competition with ground-layer plants.
Acknowledgements
The field work was initiated under a grant from the Australian National Parks and Wildlife Service (A. J. Press, Project Manager for Project SM22 to I. R. Noble subcontracted to P. A. Werner) and the auspices of the CSIRO who were managers of the land. We are indebted to CSIRO technical staff I. D. Cowie, K. L. Tidemann, J. S. Cusack (dec.) and J. McCutcheon (dec.) for assistance in setting up the plots and data recording, to CSIRO fire managers P. Brady and M. Gill and their crews for conducting the prescribed burns with great skill, to J. S. Cusack, I. D. Cowie and C. R. Dunlop (NT Herbarium) for species identification and to J. W. Shirley and two anonymous reviewers for constructive reviews of earlier drafts of the paper.
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