Protocol
Author
This article is modified from Perez-Harguindeguy et al (2013). The “New handbook for standardised measurement of plant functional traits worldwide” is a product of and is hosted by Nucleo Diversus (with additional Spanish translation). For more on this and on its context as part of the entire trait handbook visit its primary site Nucleo DiverSus at http://www.nucleodiversus.org/?lang=en
Overview
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.
Units, terms, definitions
- SSD – Stem specific density
Procedure
What and how to collect
Healthy, adult plants should be sampled as indicated above (see Selection of species and replicates for functional trait analysis). 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 (Stem specific density). Measure this trait on the main stem near the base between 10- and 40-cm height, because that is where surface fires occur (but see Notes and troubleshooting tips below). 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 Notes and troubleshooting tips (below). 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.
Notes and troubleshooting tips
(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).
Literature references
Brando PM,Nepstad DC, Balch JK, Bolker B, Christman MC,Coe M,Putz FE (2012) Fire-induced tree mortality in a neotropical forest: the roles of bark traits, tree size, wood density and fire behavior. Global Change Biology 18, 630-641. doi:10.1111/j.1365-2486.2011.02533.x
Jackson RB (1999) The importance of root distributions for hydrology, biogeochemistry and ecosystem function. In -Integrating hydrology, ecosystem dynamics and biogeochemistry in complex landscapes’. Eds JD Tenhunen, P Kabat, pp. 219-240. Wiley: Chichester, UK.