University of Tasmania
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A history and interpretation of fire frequency in dry eucalypt forests and woodlands of Eastern Tasmania

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posted on 2023-05-26, 06:27 authored by von Platen, JK
There is scant information available on the occurrence of fire in the Eastern Tiers of Tasmania. Whole, very large and fire scarred eucalypt cross sections were used as source material for the reconstruction of a fire history for this region. Accurate dating of fire scars in Eucalyptus has been problematic due to the unreliable nature of the annuality of growth rings. Eucalypts have a proclivity for growth anomalies such as false and or missing rings. The annuality of eucalypt tree rings was assessed using dendrochronological techniques on a sub-set each of young trees <140 yrs (n = 15) and old trees > 150 yrs (n = 27). A software program was developed to assist with data capture and analysis. Detect Rings‚ÄövÑvp identified ring boundaries and measured ring widths from high resolution photographs. However, the seven sampled eucalypt species (E. amygdalina, E. obliqua, E. dalrympleana, E. tenuiramis, E. delegatensis, E. pulchella, E. globulus), were not amenable to withinor between-tree cross-dating. Multiple radial ring counts from 104 large trees (photographs: n = 27, in situ: n = 77) were aggregated and tested for reliability with a mean error margin of ¬¨¬±7 rings being calculated where tree age was estimated at > 200 years. Additional sources of error were progressively eliminated. The integrity of fire scar capture from thirteen sites, each with variable sample numbers, was addressed by the development and application of a sample size adjustment procedure analogous to the bootstrap. This process indicated that 9-10 sample trees per site were sufficient to detect a high proportion of fire events large enough to generate injurious fire scars. There was no effect on fire scar distribution resulting from tree age, species composition, landscape position, bark thickness, diameter over bark, slope or elevation. The age of the oldest sample tree was estimated to be ~570 years. The sample size adjustment procedure was used to derive the mean decadal fire years for each tree at each site. Temporal and spatial patterns were then discerned. Temporal patterns were related to variation in annual rainfall. Approximately 29% of fire years which occurred across three or more sites were related to years of low rainfall indicating a relationship between low rainfall and widespread fires. A composite fire scar chronology was developed 1740 ‚Äö- 2004 from which distinctly different periods of fire years were defined. Fire years were recorded as mean fire years per decade, per period, thus: 0.7 in the Aboriginal era 1740 ‚Äö- 1820, 0.4 in the Transitional era 1820 ‚Äö- 1850, 1 in the 2nd European era 1850 ‚Äö- 1910, 1.5 in the 3rd European era 1910 ‚Äö- 1990, and 0.7 in the Current or 4th European era 1990 ‚Äö- 2004. Between-decade fire scar variability was highest in the Aboriginal era. The incidence of fire scars massively increased across most sites from the 1850s and continued at high levels until the late 1980s, although a reduced number of fire scars were recorded in the first decade of the 20th C. Occurrence of fire scars in the most recent period, 1990 ‚Äö- 2004, was shown to approximate that of the earliest period 1740 ‚Äö- 1820. These distinctly different temporal periods were interpreted as being caused by cultural activity. Intensive use of the forests for timber-getting co-incided with the Victorian gold-rush of the early 1850s and is the most likely explanation for the sharp increase in fire years at this time. Land use analysis further defined differences in fire years between public and private land with many more fire years being recorded on private land in the first half of the 3rd European period. A tradition of burning for fresh pick in sheep 'run' country, and cultural familiarity with fire, are reflected in this distribution.


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Copyright 2008 the author

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