Fire scars

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Another type of hollow often observed in eucalypts is the fire scar. Fire scars form at the base of trees (Figure 4.4C) and are also called basal hollows or butt hollows. Hollows at the butt created by fire scars occurred in 2-12% of eucalypts measured in southern NSW (Linden-mayer et al. 2000c). Of all trees in unlogged damp sclerophyll forest in East Gippsland and south-eastern NSW, 16% contained fire scars (Gibbons 1999). In forest dominated by

TREE HOllOWS AND WILDLIFE CONSERVATION IN AUSTRALIA

Figure 4.6 Results of three separate logistic regression models of the proportions of trees with fire scars, main stem hollows and hollows in branches of the crown. For main stem and crown hollows, the proportions are of all living trees with hollows. For fire scars, the proportions are of all trees on unlogged sites. Data are primarily from Brown Barrel, Messmate, Gippsland Peppermint and Mountain Grey Gum in East Gippsland, Victoria. (Source: Gibbons 1999.)

Figure 4.6 Results of three separate logistic regression models of the proportions of trees with fire scars, main stem hollows and hollows in branches of the crown. For main stem and crown hollows, the proportions are of all living trees with hollows. For fire scars, the proportions are of all trees on unlogged sites. Data are primarily from Brown Barrel, Messmate, Gippsland Peppermint and Mountain Grey Gum in East Gippsland, Victoria. (Source: Gibbons 1999.)

Mountain Ash, which is considered sensitive to fire (Attiwill 1994), approximately 10% of stands supported fire-scarred living old trees (McCarthy and Lindenmayer 1998). In River Red Gum forest along the Murray River, basal hollows were found in 5% of all measured trees (Newton-John 1992). This figure may have been higher before European settlement when fire was more prominent in woodlands.

Fire scars form as a result of death to part of the cambium from high temperatures caused by ignition of fuel accumulated at the base of trees (Jacobs 1955, Vines 1968), or from the formation of a vortex on the leeward side of trees exposed to fires (Gill 1974). Small scars may be occluded, but the susceptibility of a tree to additional scarring is enhanced for a considerable period after the initial fire scar has formed (Banks 1994). Repeated burning of scarred tissue either exposes, or leads to the formation of, a hollow in the main stem (Gill 1974). Larger trees generate greater heat on their leeward side when exposed to fire (Gill 1974), and older trees are more likely to have been exposed to repeated fires — explaining why hollow-forming fire scars are more common in older trees (Figure 4.6).

Fissures

Fissures, or cracks in branches or the main stem, also form hollows in eucalypts (Figure 4.4D). These hollows have longitudinal entrances which are typically much longer in one dimension. Fissures (>lcm width) accounted for 0-6% of all hollows in live and dead trees among six species of eucalypts from montane and foothill open forest in southern NSW (Lindenmayer et al. 2000c). In tall, open forest in coastal southern NSW and eastern Victoria, fissures with entrances >2cm width represented 3% of all hollows in live, hollow-bearing trees (Gibbons 1999). Fissures appear to be more common in dead trees, or in dead sections of live trees.

Unlike hollows occurring in branches of the crown or on the main stem, fissures are not associated with branch breakage. They appear to form when the main stem or large branch of a tree cracks longitudinally from movement associated with wind loading (Lindenmayer et al. 1993b), or from the branch or tree bending under its own weight (Mattheck etal. 1995). In studies of Balsam Fir (Abies balsamea), fissures were more likely to occur where there was an existing column of decay in the main stem (Ossenbruggen et al. 1986).

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