Bark thickness is the thickness of the bark in mm, which is defined here as the part of the stem that is external to the wood or xylem – hence, it includes the vascular cambium. Thick bark insulates meristems and bud primordia from lethally high temperatures associated with fire, although the effectiveness depends on the intensity and duration of a fire, on the diameter of the trunk or branch, on the position of bud primordia within the bark or cambium and on bark quality and moisture. Thick bark may also provide protection of vital tissues against attack by pathogens, herbivores, frost or drought. In general, this trait has special relevance in trees or large shrubs subject to surface-fire regimes. Be aware that the structure and biochemistry of the bark (e.g. suberin in cork, lignin, tannins, other phenols, gums, resins) are often important components of bark defence as well.
What and how to collect?
Healthy, adult plants should be sampled as indicated above (see Section 1.1). Measure bark thickness on a minimum of five adult individuals, preferably (to minimise damage) on the same samples that are used for measurements of SSD (see Section 4.1). Measure this trait on main stem near the base between 10- and 40-cm height, because that is where surface fires occur (but see Special cases or extras in the present Section). If you do not use the same sample as for SSD, cut out a new piece of bark of at least a few centimetres wide and long. Avoid warts, thorns or other protuberances and remove any bark pieces that have mostly flaked off. The bark, as defined here, includes everything external to the wood (i.e. any vascular cambium, secondary phloem, phelloderm or secondary cortex, cork cambium or cork).
How to measure?
For each sample or tree, five random measurements of bark thickness are made with callipers (or special tools used in forestry), if possible to the nearest 0.1 mm. For species with fissured stems, see Special cases or extras in the present Section. In situ measurement with a purpose-designed forestry tool is an acceptable alternative. Take the average per sample. Bark thickness (mm) is the average of all sample means.
Special cases or extras
(1) Bark quality. In addition to bark thickness, several structural or chemical components of bark quality may be of particular interest (see above within the present Protocol). An easy but possibly important one is the presence (1) v. absence (0) of visible (liquid or viscose) gums or resins in the bark.
(2) Bark surface structure (texture) may determine the capture and/or storage of water, nutrients and organic matter. We suggest five broad (subjective) categories, including (1) smooth texture, (2) very slight texture (amplitudes of microrelief within 0.5 mm), (3) intermediate texture (amplitudes 0.5–2 mm), (4) strong texture (amplitudes 2–5 mm) and very coarse texture (amplitudes >0.5 mm). Bark textures may be measured separately for the trunk and smaller branches or twigs, because these may differ greatly and support different epiphyte communities.
(3) Fissured stems. In each sample, take five random measurements of both the maximum (outside the fissure) and minimum (inside the fissure) bark thickness. Then calculate bark thickness as one-half the difference between them.
(4) Alternative height for measurements. Typically, in forestry surveys, bark thickness is measured at breast height (as DBH). Measuring at the base of the tree, as suggested here, has advantages (more related to fire resistance) and problems (often the base of the tree is deformed). An alternative can be to make this measurement at ~50–60 cm (and in any case bark thickness at 50 cm is strongly related to bark thickness at breast height) or directly at DBH.
(5) Decorticating bark. Decorticating bark is usually considered as standing litter, so it is not included in bark-thickness measurements (however, specific objectives may imply its measurement).
(6) Bark investment. The complementary measurement of stem diameter can be useful to compare species for bark investments (by dividing the bark thickness by the stem radius).
References on theory and significance: Jackson et al. (1999); Brando et al. (2012).