4.1. Stem-specific density

Stem-specific density (SSD, mg mm–3 or kg dm–3) is the oven-dry mass (at 70°C for 72 h, but see Special cases or extras in the present Section) of a section of the main stem of a plant divided by the volume of the same section, when still fresh. This trait is a synonym for ‘stem density’; we distinguish it from ‘wood density’ in that SSD can also be measured on herbaceous species and includes the stem bark (i.e. secondary phloem and cork if any), which in some cases accounts for a significant proportion of the overall stem structure (see Special cases and extras in the present Section). SSD is emerging as a core functional trait because of its importance for the stability, defence, architecture, hydraulics, C gain and growth potential of plants. Stem density partly underlies the growth-survival trade-off; a low stem density (with large vessels) leads to a fast growth, because of cheap volumetric construction costs and a large hydraulic capacity, whereas a high stem density (with small vessels) leads to a high survival, because of biomechanical and hydraulic safety, resistance against pathogens, herbivores or physical damage. In combination with plant size-related traits, it also plays an important global role in the storage of C.

What and how to collect?

Healthy adult plants should be selected according to previous suggestions. Depending on the objectives of the study, either stem density or wood density should be measured (see Special cases or extras below in the present Section). For stem density, remove only the loose bark that appears functionally detached from the stem (see Section 4.3). Stem density can increase, decrease or remain constant from pith to bark, so a representative sample must include a proportional representation of the complete stem. Stem density is higher at the insertion point of branches (because of tapering of vessels), and lower for branches and for outer bark (which includes more air spaces and cork). These density gradients have important implications for the sampling procedure. Collect either a whole stem cross-sectional slice or, for large trees, a triangular sector from the bark tapering into the centre (like a pizza slice) of cross-sectional area, ~1/8 of the total area. For herbaceous species or woody species with thin main stems (diameter <6 cm), cut with a knife or saw a ~10-cm-long section near the base of the stem (between 10- and 40-cm height). If possible, select a regular, branchless section, or else cut off the branches. For woody or thick succulent plants with stem diameters >6 cm, saw out a (pizza) slice from the trunk at ~1.3-m height; the slice should be 2–10 cm thick. Hard-wooded samples can be stored in a sealed plastic bag (preferably cool) until measurement. Soft-wooded or herbaceous samples are more vulnerable to shrinkage, so should be wrapped in moist tissue in plastic bags, and stored in a cool box or refrigerator until measurement.

Measuring

Stem volume can be determined in, at least, two different ways.

(1) Water-displacement method. This procedure allows the volume of irregularly shaped samples, which cannot be properly measured with the dimensional method, to be measured easily. A graded test tube or beaker, large enough to hold the entire sample, is filled with distilled water (but not completely filled, to be sure that when the stem is placed within the water, the liquid will not escape from the test tube), placed on a balance, and tare the balance. The wood sample is then completely submerged under the water with the aid of a small-volume needle or tweezers, being careful to not touch the sides or bottom of the beaker which can cause variations in the weight being registered by the balance. When the wood sample is submerged, the increase on water level leads to an increase in the weight being registered by the balance (i.e. the weight of the displaced water), which equals the wood sample volume in cm3 (because water has a density of 1 g cm–3). The mass registered in the balance (i.e. the volume of the wood sample) is quickly read and registered. Tare again for each measurement, previously replacing water.

(2) Measurement of dimensions (or dimensional method). The volume of a cylindrical sample can be determined simply by measuring its total length (L), and its diameter (D), on one or more places along the sample, using callipers. If the stem is very thin, determine the diameter on a cross-section of it under a microscope, using a calibrated ocular micrometer. Calculate the volume (V) of the cylinder as
 
V = (0.5D)2 × π × L.

In the case of hollow stems (e.g. young trees of Cecropia or some bamboo species), estimate the diameter of the hollow and subtract its cross-sectional area from the stem cross-sectional area before multiplying by L. This method can be applied to samples having different geometrical forms. After volume measurement (by any of the methods described above), the sample is dried in the oven. Besides free water, stems also contain bound water, which is removed only by drying at above 100°C. Samples should be dried in a well ventilated oven at 101–105°C for 24–72 h (small samples), until a constant weight is obtained. Large samples may need more drying time.
 
Additional useful methods from forestry

In forest ecological studies, samples are often taken with an increment borer, in which a wood core is cut from the bark inward, to just beyond the centre of the stem. Because a core does not taper inward towards the centre of the stem, such a sample may not be perfectly representative of the density of the stem as a whole; however, the difference from a density estimate using a sector or an entire stem section is usually probably small. Large-diameter corers (12 mm) are better because they cause less compaction. Samples with this method are usually taken at ~1.3 m above ground (‘breast height’). After the core is extracted, it can be stored in a plastic drinking straw, with the ends of the straw sealed.

In the timber industry, the ‘wood density’ is often measured at 12% moisture content, and density is reported as ‘air-dry weight’ (ADW) (a misnomer, because density is not weight, but weight/volume). SSD as described in the present protocol is called ‘oven dry weight’ (ODW). ADW can be transformed to ODW by using the formula ODW = 0.800 ADW + 0.0134 (R2 = 0.99) (this formula cannot be correct for ‘dry weights’, but only for densities). We suggest that data for ODW, directly measured or derived from ADW, can safely be used as SSD. This value ignores the contribution of the bark of a tree to its stem density; however, because bark usually makes up only a very small fraction of the mass of a large tree trunk, this error is probably unimportant, except as noted below.

Special cases or extras

(1) Oven drying and wood-specific gravity. Because wood is mainly cellulose and lignin, containing substantial bound water and relatively small quantities of compounds of low molecular weight, many wood scientists and foresters oven-dry wood samples at 100–110°C, before determining both weight and volume. They then refer to the relationship between weight and volume as wood specific gravity.

(2) Stems with holes. Very large holes in the stem are considered to be air or water spaces that do not belong to the stem tissue, whereas smaller spaces such as the lumens of xylem vessels, and intercellular spaces, are part of the stem tissue.

(3) Wood density v. stem density. It might be worthwhile to make separate density estimates for wood and bark, because they have very different chemical and cellular compositions, physical properties and biological functions. Many trees and shrubs in savannah-type vegetation (and some Mediterranean and arid species) have, for example, a very thick corky bark (with very low density), and often the volume of the bark may be an important part (even 50%) of the stem volume. In these species, most of the structural support is given by the wood and wood is generally denser than bark. Thus, from the support viewpoint, wood density is the important parameter.

(4) Xylem density. Some authors make a distinction between wood density (oven-dry wood density of the main trunk including sapwood and heartwood) from xylem density or sapwood density (measured on small, ~1.5-cm, terminal branches of trees). Xylem density has been proposed as a proxy for the tree hydraulic architecture, which, in turn, may limit tree performance in terms of transpiration, C exchange and growth.

(5) Plants without a prominent above-ground stem (rosette plants, grasses and sedges). Try to isolate the short, condensed stem, near ground level, to which the leaves are attached, and obtain its density. In some plants, all the leaves are attached to an underground, often horizontal, modified stem called a rhizome (see Section 2.3), whose density can be determined, but which does not have the kind of mechanical leaf-supporting function that an above-ground stem has. Most rosette plants and basal-leaved graminoids produce aerial inflorescences, the density of whose stem can also be determined; however, this stem again usually has either no, or a reduced, function in supporting photosynthetic leaves, compared with that of an extensive-stemmed plant (see Section 2.3). If the plant has no recognisable above-ground leaf-supporting, stem, qualitatively recording it as ‘stemless’ is probably more convenient from the point of view of further analyses, than quantitatively recording its stem density as zero. If a plant branches from ground level (e.g. many shrubs), select the apparent main branch, or a random one if they are all similar.

(6) Dense woods. When coring trees with very dense wood, a rope can be tied around the tree and fastened at the handle of the borer. When the handle is turned around during coring, the rope will wrap itself around the borer, increasing the tension on the rope, which helps push the borer bit into the tree.

(7) Measuring other than main stem. If wood samples cannot be removed from a trunk or main stem, wood from branches 1–2 cm in diameter may be sampled. Main-stem wood density has been found on average to equal 1.411 × branch wood density, although this relationship can vary among species, so using it can sometimes introduce an error.

(8) Components of stem volume (cell-wall material, water, gas). The volumetric fractions of cell-wall material, water and gas (‘air’) in the stem can be calculated as follows. The volume fraction of water is simply the decrease in weight (in g) on drying, divided by the original volume (in cm3) of the sample. The volume fraction that is cell-wall material equals the dry mass:fresh volume divided by the density of dry cell-wall material (1.53 g cm–3 for cell-wall material in dry wood); if an appreciable fraction of the stem volume is bark, using this number may involve an error because the density of bark cell-wall material is not necessarily the same as that for wood. The fraction of the original volume that was gas is simply 1 minus the foregoing two volume fractions.
 
References on theory, significance and large datasets: Putz et al. (1983); Loehle (1988); Reyes et al. (1992); Niklas (1994); Gartner (1995); Santiago et al. (2004); Van Gelder et al. (2006); Poorter et al. (2008); Chave et al. (2009); Patiño et al. (2009) Anten and Schieving (2010).

More on methods: Reyes et al. (1992); Brown (1997); Gartner et al. (2004); Chave et al. (2005;); Swenson and Enquist (2008); Williamson and Wiemann (2010).