3.7. Physical strength of leaves

Physically stronger leaves are better protected against abiotic (e.g. wind, hail) and biotic (e.g. herbivory, trampling) mechanical damage, contributing to longer leaf lifespans. Physical investment in leaf strength is a good indicator of C investment in structural protection of the photosynthetic tissues. It also tends to have afterlife effects in the form of poor litter quality for decomposition. Because leaves have different strength properties according to the direction in which the force is applied, the physical strength of the leaves can be defined and measured in different ways. The three most common measured properties are force to tear (Ft), work to shear (Ws) and force to punch (Fp). Ft is the force needed to tear a leaf or leaf fragment, divided by its width, expressed in N mm–1. Note that Ft has been previously referred to as ‘leaf tensile strength’. Ws, sometimes called ‘force of fracture’, reflects the mean force needed to cut a leaf or leaf fragment at a constant angle and speed, expressed in N, or its analogue, J m–1. Fp is the force needed to force a punch through a leaf or leaf fragment (expressed in N mm–1). Both Ws and Fp are strongly influenced by Lth (see Section 3.4).

What and how to collect?

For the selection and collecting procedure, see Section 3.1; for recommended sample size, see Appendix 1.

Storing and processing

Follow the procedure described within Section 3.1 and store leaves in a cool box or fridge. Rehydration is not indispensable, although it may be desirable for accurate measurement of Lth. Measure as soon as possible after collecting, certainly within 72 h for species with soft leaves. Tougher leaves tend to keep their strength for several days. If this is not possible (e.g. if samples have to be sent to distant locations), an alternative is to air-dry the samples, by putting them between sheets of absorbing paper in a plant press, immediately after collecting. In this case, rehydration is needed before measuring. Toughness of fresh and rehydrated leaves is well correlated for sclerophyllous leaves and grass leaves, in the cases of both Ft and Ws.

Measuring

For fresh samples, proceed to measurement straight away. For air-dried samples, first rehydrate by wrapping in moist paper and put in a sealed plastic bag in the fridge for 24 h (gentle spraying may be better for some xerophytic, rotting-sensitive species; see Section 3.1). Here, we describe three methods that have produced good results and for which purpose-built equipment is available. If you have the choice, we recommend measuring the property that is most closely related to the process of interest. In the case of herbivory by vertebrate grazers, Ft is likely to be the most meaningful property. However, if the focus is on chewing insects or trampling by mammals, work to shear tests would be the best approach (Table 2).

    (1) Tearing (tensile) tests

Force to tear (Ft) can be easily and inexpensively measured with a simple apparatus that includes a 0–3-kg-range dynamometer (Fig. 3a). To proceed, cut a leaf fragment from the central section of the leaf but away from the midrib (central vein), unless the latter is not obvious (e.g. some Poaceae or Liliaceae), or the leaf is too small for doing so without using a magnifying lens. The length of the fragment follows the longitudinal axis (direction of main veins). The width of the leaf or leaf fragment depends on the tensile strength and tends to vary between 1 mm (extremely tough species) and 10 mm (very tender species). Whenever possible, we recommend using fragments with a length : width ratio between 5 and 10, to make sure that force is applied along its main axis. Measure the exact width of the leaf sample before placing it in the apparatus. Place it perpendicular to the edges of the clamps. Then fix both ends of the sample with the clamps. Try to do this gently without damaging the tissues. A thin piece of rubber added to the edges of the clamps could help. Very small leaves, very tough and slippery leaves, and slightly succulent leaves may be clamped tightly without much tissue damage, by using strong double-sided tape. Then pull slowly, with increasing force, until the leaf tears. Watch the dynamometer to read the force at the moment of tearing. For unit conversion, remember that 1 kg = 9.81 N. Divide the total force by the width of the leaf fragment to obtain Ft. Express the result in N mm–1. There are some more sophisticated instruments to measure Ft, such as the 5542 (Instron, Canton, MA, USA) or (with adaptations) Mecmesin Ultra Test Tensiometer (Mecmesin, Slinfold, UK).

Leaves too tender to provide an actual measurement with the apparatus are assigned an arbitrary tensile strength of zero. For leaves too tough to be torn, first try a narrower sample (down to 1 mm if necessary and possible). If still too tough, then tensile strength equals the maximum possible value in apparatus (assuming sample width of 1 mm). In the case of highly succulent leaves (or modified stems), which would be squashed if clamped into the apparatus, carry out the measurements on epidermis fragments.

    (2) Shearing (cutting) tests

At least five instruments have been used to measure work to shear (Ws). They all measure how much work is required to cut a leaf, either with a single blade against an anvil, or with a pair of blades (‘instrumented scissors’). A portable and widely used device for measuring the average force needed to fracture a leaf at a constant shearing angle (20°) and speed is detailed in Fig. 3b. A non-portable apparatus for this measurement is showed in Fig. 3c. In both cases, the apparatus consists of a mechanical portion, a power source and a computer in which the output file is recorded. To proceed, be sure to complete the calibration procedure according to the apparatus you are using. Considering that this methodology is highly sensitive, the operator must avoid any possible source of external noise (e.g. vibrations, wind), and must frequently clean the blade(s) with an alcohol-soaked wipe. Measure the exact thickness of the leaf sample (lamina or midrib) with a calliper, before placing it in the apparatus. Then, place the sample in the anvil and fix it with the clamp. Leaves are cut at right angles to the midrib, at the widest point along the lamina (or halfway between the base and the tip, if this is difficult to determine). In some studies, the midrib may be removed so that only lamina tissue is tested (or lamina and midrib are tested separately). Alternatively (and less precisely), one can remove the portion of the data that represents the midrib being cut, and analyse these data separately. The procedure to calculate final values can also differ between apparati (for calculation process, refer to the specific user’s manuals).

A calibrated copy of the apparatus described in Fig. 3c is available for use at CNRS in Montpellier, France (contact Eric Garnier; email: garnier@cefe.cnrs-mop.fr).

    (3) Punch tests

Force to punch (Fp) is the resistance of the actual leaf tissues (particularly the epidermis) to rupture, excluding toughness provided by midribs and main veins. Different penetrometers (Fig. 3d) have been used in the past (there is no standard design), all of which have some kind of fine-needle or a flat-ended punch (diameter ~0.5–5.5 mm) attached to a spring-loaded balance or a counterweight (being a container gradually filled with water and weighed after penetration). The punch goes through a die with a hole in its centre. A clearance of 0.05–0.1 mm between the punch and the edge of the hole in the die is recommended to avoid error in measurements as a result of friction between the punch and the die. A sharp edge in the flat end of the punch will also avoid overestimating toughness values resulting from compression and tension rather than shearing. To standardise the force per unit fraction length, one has to divide the force by the circumference of the punch. The data are therefore expressed in N mm–1. Consistency across the leaf tends to be reasonable, as long as big veins are avoided. Three measurements per leaf are probably sufficient. This test does not work well for many grasses and other monocots. This method has been more widely used in the tropics. Some recent studies have added the punch and die to more sophisticated apparati designed for measuring properties of materials (such as 5542, Instron).

Special cases or extras

(1) Leafless plants. See Section 3.1.

(2) Leaf-tissue toughness or leaf-specific toughness. This interesting additional parameter of leaf strength can be obtained by dividing Ft, Ws or Fp by the (average) thickness of the leaf sample. Measurement of Lth (in mm) for this purpose is most practically carried out with a calliper, immediately after measuring the leaf-sample width and before placing it in the measuring apparatus.

References on theory, significance and large datasets: Coley (1988); Choong et al. (1992); Grubb (1992); Turner (1994); Wright and Vincent (1996); Cornelissen et al. (1999); Lucas et al. (2000); Díaz et al. (2004); Read and Stokes (2006); Kitajima and Poorter (2010); Onoda et al. (2011).

More on methods: Hendry and Grime (1993); Darvell et al. (1996); Aranwela et al. (1999); Wright and Cannon (2001).