4.2. Twig dry-matter content and twig drying time

Twig dry-matter content (TDMC) is the oven-dry mass (mg) of a terminal twig, divided by its water-saturated fresh mass (g), expressed in mg g–1. Twig drying time is expressed in days (until equilibrium moisture). We consider TDMC to be a critical component of plant potential flammability, particularly fire conductivity after ignition (see Section 2.12). Twigs with high dry-matter content are expected to dry out relatively quickly during the dry season in fire-prone regions. TDMC should be positively correlated with specific density or dry-matter content of the main stem across woody species (see Section 4.1), and negatively correlated with potential RGR, although this has, to our knowledge, not been tested explicitly.

What and how to collect?

Collect one to three terminal (highest ramification-order; smallest diameter-class), sun-exposed twigs from a minimum of five plants. Twigs (or twig sections) should preferably be 20–30 cm long. If a plant has no branches or twigs, take the main stem; in that case, the procedure can be combined with that for SSD (see Section 4.1). For very fine, strongly ramifying terminal twigs, a ‘main twig’ with fine side twigs can be collected as one unit.

Storing and processing

Wrap the twigs (including any attached leaves) in moist paper and put them in sealed plastic bags. Store these in a cool box or fridge (never in a freezer!), until further processing in the laboratory. If no cool box is available in the field and temperatures are high, it is better to store the samples in plastic bags without any additional moisture; then follow the above procedure once back in the laboratory.

Measuring

Following the rehydration procedure (see Section 3.3), any leaves are removed and the twigs gently blotted dry with tissue paper, to remove any surface water before measuring water-saturated fresh mass. Each twig sample (consisting of 1–3 twigs) is then first dried in an oven or drying room at 40°C at 40% relative humidity of the outside air, or lower. Every 24 h, each sample is reweighed. Twig drying time is defined as the number of days it takes to reach 95% of the mass reduction of the sample as a result of drying (interpolating between weightings if necessary), where 100% is the final loss of mass when the weight of the sample ceases to decline further at the indicated temperature. Continue until you are certain that a steady dry weight has been reached. TDMC is defined (analogously to LDMC) as dry mass divided by saturated mass. The final dry weight obtained at 40°C is not the true dry mass of the twig, because some bound water will remain within the cell walls of the material (and probably also in its protein) at this temperature (see Section 4.1). However, the rather low drying temperature adopted here, compared with that in Section 4.1, is chosen so as to produce in twigs a dry condition relatively similar to that from air-drying outdoors, but which can be obtained relatively quickly.

Special cases or extras

(i) Herbaceous plants. For herbaceous plants, the equivalent to TDMC is stem dry-matter content, which can be measured in exactly the same way as LDMC but using the main stem of forbs or the leaf sheaths of grasses.
 
References on theory and significance: Bond and Van Wilgen (1996); Lavorel and Garnier (2002); Shipley and Vu (2002).
 
More on methods: Garnier et al. (2001b).