2.6. Spinescence

Spinescence refers to the degree to which a plant is defended by spines, thorns and/or prickles. Spines are sharp, modified leaves, leaf parts or stipules; they also occur sometimes on fruits. Thorns are sharp, modified twigs or branches. Prickles are modified epidermis or cork (e.g. rose-stem prickles). Because spinescence is clearly involved in anti-herbivore defence, especially against vertebrate herbivores, the following two separate issues are critical in considering spinescence: (1) the effectiveness of physical defences in preventing or mitigating damage from herbivores; and (2) the cost to the plant in producing these defences. Different types, sizes, angles and densities of spines, thorns and prickles may act against different herbivores. Although in many cases, characterisations of plant spinescence by measuring spines is sufficient, some researchers may decide that experiments with actual herbivores, which examine the effectiveness of anti-herbivore defences, are necessary, e.g. by offering whole shoots (with and without spines) to different animals and recording how much biomass is consumed per unit time (see Special cases or extras in Section 3.16).

Spines, thorns and prickles can be an induced response to herbivory, meaning that some plants invest in these defences only when they have already been browsed by herbivores. Other types of damage, including pruning and fire, can also induce increased levels of spinescence. In addition, spinescence traits can change drastically with the age of the plant or plant part, depending on its susceptibility to herbivory. For this reason, spinescence sometimes cannot be considered an innate plant trait, but rather a trait that reflects the actual herbivore pressure and investment in defence by plants. In other words, although there are species that always have spines, and species that never have them, the spinescence of an individual plant is not necessarily representative of the potential range of spinescence in the whole species (e.g. some members of Acacia and Prosopis show a striking range of spine lengths within the same species, depending on individuals, age and pruning history). Spines, thorns and prickles can sometimes play additional roles in reducing heat or drought stress, especially when they densely cover organs.

How to measure?

Spines, thorns and prickles – summarised below as ‘spines’ – can either be measured as a quantitative trait or reduced to a qualitative, categorical trait. Data on spinescence are preferably measured from specimens in the field, and can also be gathered from herbarium specimens or descriptions in the literature. Spine length is measured from the base of the spine to its tip. If a spine branches, as many do, its length would be to the tip of the longest branch. Spine width, measured at the base of the spine, is often more useful for assessing effectiveness against herbivores and more generalisable across types of spines. The number of branches, if any, should also be recorded because branches can increase significantly the dangerousness of spines to herbivores. Ratio of spine length to leaf length can also be a useful character because it gives an idea of how protected the lamina is by the spine closest to it.

Spine strength or toughness. Spines are ‘soft’ if, when mature, they can be bent easily by pressing sideways with a finger, and ‘tough’ if they cannot be thus bent. Spine density is the number of spines per unit length of twig or branch, or area of leaf.

Biomass allocation to spines is also an important parameter for some research questions. Its estimation is more work-intensive than those above, but still relatively simple. Cut a standard length of stem or branch, cut off all spines, oven-dry and weigh leaves, shoot and spines separately and estimate fractional allocation as the ratio of spine dry weight to shoot dry weight.

These quantitative trait measurements can be converted into a categorical estimate of spinescence by using the classification proposed in Box 3.

Finally, to simply record the presence or absence of spines is sufficient in some cases. Bear in mind that the size, structure and behaviour of herbivores vary enormously, so the degree of protection provided by spine mass, size and distribution can be determined only with reference to a particular kind of herbivore. When selecting the most meaningful measurement/s of spinescence, always consider what herbivores are relevant.

References on theory, significance and large datasets: Milton (1991); Grubb (1992); Cooper and Ginnett (1998); Pisani and Distel (1998); Olff et al. (1999); Hanley and Lamont (2002); Rebollo et al. (2002); Gowda and Palo (2003); Gowda and Raffaele (2004); Agrawal and Fishbein (2006).