For young woody plants, being eaten by large mammals is a major threat and source of sapling mortality. Some species produce spines as a form of defence. How early these spines emerge may be a strong predictor of sapling survival. Until now, however, little attention has been paid to how spine emergence varies across species.
In a recent article published in Annals of Botany, Mohammed Armani and colleagues have used a diverse pool of 45 spiny plant species from different habitats to examine how sapling size, spine type, and the availability of light and water resources affect the timing and size of spines. The authors examined three different types of spine: true spines, which are derived from leaf tissue; prickles, which arise from the epidermis or cortex; and thorns, which come from branch tissue. The plants, which belonged to 17 different plant families, were all grown together under greenhouse conditions. They were harvested at 5 and 15 weeks after planting and checked for spine emergence and the proportion of their mass they had put into the spines.
The authors found that the biggest influences on when the plants’ defences appeared and how big they got were the type of spine being produced and the resources available to the seedling. True spines emerged the earliest, followed by prickles, and thorns last. This is thought to be because true spines, though costly, can be produced along with the earliest leaves. Prickles have a lower production cost, but must undergo longer developmental stages than leaf spines. Thorns, finally, are expected to be the slowest to produce because they must wait until the plant is old enough for lateral branching to occur. Overall, spines tended to emerge earlier in locations with lower precipitation or a more open, sunny habitat. While the timing of the spines was not dependent on the size of the saplings, the amount of biomass allocated to them was, though this was contingent on both light levels and the type of spine being produced.
This is the first study that examines the pattern of spine growth and resource investment across a diverse range of plant species. In light of their results, the authors write that “understanding effectiveness of the different spine types against large herbivores is an important next step in developing predictive frameworks on how herbivory and environmental resources – and potential changes in these – will shape spiny woody communities in the future.” Importantly, the authors also concluded that, given their striking differences, lumping all spine types together may limit our ability to predict their ecological performance.