19.1.2  How plant tissues succumb to heat

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A temperature of about 60°C will cause the instantaneous death of most hydrated cells while the resinous bonds holding the valves of some woody seed follicles closed melt at about 75°C (Gill 1976; Wardrop 1983). Water boils at about 100°C; at this point water vaporises causing total thermal arrest. Charring of tissues begins at about 300°C and ash forms above about 500°C.

Combustion of leaves is possibly the simplest response to consider in a fire because leaves are usually thin (fractions of millimetres) and hydrated. They are therefore poorly insulated. Leaves subject to hot dry conditions prior to the onset of fire quickly reach 60°C and begin to die (Gill 1995).

Unlike leaves, many tissues are insulated. Typically, the cambium of a tree is insulated from external heating by bark, sometimes fully alive and hydrated, sometimes partly dead and superficially flammable. Other examples are seeds in the woody fruits of plants of genera such as Hakea, Casuarina, Eucalyptus and Callitris and seeds present in the soil. To predict how heat affects temperature of tissues, it is necessary to quantity the relationships between temperature and heat.

The rate of temperature change in plant tissue is determined by thermal diffusivity, κ (m2 s–1). Surface tissues of plants heat up relatively slowly because of low κ. Thermal diffusivity is described by the following equation:

where k is thermal conductivity (kJ s–1 m–1 °C–1), c is specific heat of the tissue ( kJ kg–1 °C–1), and ρ its density (kg m–3).

While k of plant tissues varies widely, κ is often regarded as a constant for large variations in moisture content and density (Gill and Ashton 1968; Vines 1968). Thickness of bark is therefore the main factor determining how well the base of a tree (bole) is protected from fire. Severe thinning of the flammable bark of stringybark trees (Eucalyptus spp.) and some paperbark trees (Melaleuca spp.) by fire can expose underlying tissues to damaging temperatures.

Bark thickness varies widely according to plant species, position on the plant, vigour and history of exposure to previous fires. Bark thickens through differentiation of phloem initiated in the vascular cambium (Section 7.1). Meanwhile, bark becomes thinner due to regular decortication, which is under physiological control (as in most ‘gumbarked’ species of Eucalyptus), or sloughing of dead tissues, which is under mechanical and environmental controls (as in most ‘stringybarked’ Eucalyptus). In general, bark thickness increases with girth in woody plants and decreases with height: stems well above ground have thinner bark than trunks of the same girth at ground level (Gill and Ashton 1968). In some eucalypts, allometric relationships have been established between cross-
sectional areas of bark and stems (Brack et al. 1985).

Non-dicotyledonous plants such as tree ferns and woody monocotyledons have vascular bundles dispersed throughout their fibrous stems. By contrast, cycads have a ring of vascular tissue embedded deep within a stem cortex. Consequently, modes of tissue regeneration and long-distance transport in these plants are dissimilar to dicotyledonous plants, where an annulus of phloem overlies a cylinder of xylem with each cell type distinctly separated by a vascular cambium. The principles of high temperature damage as heat is conducted to successively deeper layers of tissue are the same for all these plant groups but the physiological consequences differ. Death of the vascular cambium after severe fires have removed bark can kill an entire tree through preventing restoration of phloem activity.

 

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