Spines take resources to build that could be used elsewhere. Plants don’t want to waste effort, so spines are usually associated with something the plant finds important, such as reproductive organs or protecting young shoots. So why would trees build spines on their trunks? Théodore Lefebvre and colleagues at Xishuangbanna Tropical Botanical Garden and the Sorbonne examined how the spines were shaped to see if they could determine what the spines were for.
The botanists analysed the traits of 31 tropical woody species with spines on their trunks, using the living collections of Xishuangbanna Tropical Botanical Garden located in Menglun, Yunnan, China.
“First, we analysed whether species with trunk spines have common morphological syndromes that could inform about their likely function (timing, location, association with organ, maintenance and densities),” write Lefebvre and colleagues in their article.
“Second, we mapped spine placement on the trunks and used computer simulation to evaluate whether the identified syndromes differed in their potential to defend against either debarking or climbing. Finally, we compared the nutritiousness (nitrogen concentration, total phenols and inner bark thickness) of leaf and bark, and the attractiveness of flowers and fruits from 31 spiny species and 25 non-spiny species, to test whether spiny trunk species were defending more nutritious organs than non-spiny species, and whether there were differences in nutritiousness among the identified syndromes.”
The team identified four morphological syndromes of woody plants with spines on their trunks. Two of the syndromes were already known. Crown syndrome is where the tree grows spines near leaves to protect leaves and reproductive organs from ground-dwelling mammals. Another syndrome was the liana syndrome, with curved spines for climbing.
The new syndromes were when a tree grew spines on the trunk after dropping its leaves. Lefebvre and colleagues name these thorny and prickly syndromes.
For the thorny trunk syndrome, trees develop a medium density of long spines displayed in clusters on the trunk at its base (2–3 m). In contrast, prickly trunk species typically develop a high density of medium conical cork spines covering the whole trunk surface at its base and sometimes up to the top of trees.
“Simulations, in accordance with morphology, suggested that trunk spines of the thorny syndrome were likely defending against medium to large ground-dwelling bark-feeding animals, whereas the trunk spines of the prickly syndrome were likely defending against small to large ground-dwelling bark-feeding and climbing animals,” write Lefebvre and colleagues.
“While simulations are not a formal demonstration of their effect nor an ecological demonstration of their function, they can provide indications about their potential function, the likely mammal size that might be affected by the defence, which characteristics are important for the defence, and how these differ according to the spine syndrome.”
READ THE ARTICLE
Lefebvre, T., Charles-Dominique, T. and Tomlinson, K.W. (2022) “Trunk spines of trees: a physical defence against bark removal and climbing by mammals?,” Annals of Botany, 129(5), pp. 541–554. https://doi.org/10.1093/aob/mcac025