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
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 Notes and troubleshooting tips in Leaf palatibility as indicated by preference by model herbivores).
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.
Units, terms, definitions
- 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).
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 allocationto 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 1 (below).
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.
|1. No spines.|
|2. Low or very local density of soft spines <5mm long; plant may sting or prickle when hit carelessly, but not impart strong pain.|
|3. High density of soft spines, intermediate density of spines of intermediate hardness, or low density of hard, sharp spines >5mm long; plant causes actual pain when hit carelessly.|
|4. Intermediate or high density of hard, sharp spines >5mm long; plant causes strong pain when hit carelessly.|
|5. Intermediate or high density of hard, sharp spines >20mm long; plant may cause significant wounds when hit carelessly.|
|6. Intermediate or high density of hard, sharp spines >100mm long; plant is dangerous to careless large mammals, including humans.|
References on theory, significance and large datasets:
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Cooper SM, Ginnett TF (1998) Spines protect plants against browsing by small climbing mammals. Oecologia 113, 219-221. doi:10.1007/s004420050371
Gowda JH, Palo RT (2003) Age-related changes in defensive traits of Acacia tortilis Hayne. African Journal of Ecology 41, 218-223. doi:10.1046/j.1365-2028.2003.00434.x
Gowda J, Raffaele E (2004) Spine production is induced by fire: a natural experiment with three Berberis species. Acta Oecologica 26, 239-245. doi:10.1016/j.actao.2004.08.001
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Pisani JM, Distel RA (1998) Inter- and intraspecific variations in production of spines and phenols in Prosopis caldenia and Prosopis flexuosa. Journal of Chemical Ecology 24, 23-36. doi:10.1023/A:1022380627261
Rebollo S, Milchunas DG, Noy-Meir I, Chapman PL (2002) The role of a spiny plant refuge in structuring grazed shortgrass steppe plant communities. Oikos 98, 53-64. doi: 10.1034/j.1600-0706.2002.980106.x