Tolerance to severe browsing of three shrub species on Mediterranean islands

© 2019 Buxus balearica Lam., Ephedra fragilis Desf., and Juniperus oxycedrus L. are three Pre-Mediterranean shrubs that have become scarce in the Balearic Islands probably due to a climatic change and the anthropogenic activities during the second half of the Holocene. In addition, an endemic artiodactyl, Myotragus balearicus, exerted browsing pressure on these species for thousands of years, and after it disappeared domestic goats were introduced by humans. Adaptations that enable these plants to resist grazing are structural or chemical; however, there is no information about whether their defenses are only constitutive or also inducible. Currently, the mountain ranges of Majorca Island are extensively occupied by feral goats, and a possible increase in the stocking rates could affect the survival of these plant species. The aim of this study was to evaluate and compare the effects of simulated severe browsing on the re-growth capacity and chemical composition of these species in order to determine their tolerance to herbivory. We sampled and compared structural and chemical variables of saplings submitted to two pruning intensities for each species. The results showed that all three species could maintain their growth rate and biomass ratios under simulated herbivory. Sprout production was also stimulated in all them. E. fragilis and B. balearica increased phenolic content after pruning, and E. fragilis also increased the condensed tannins. Pruning treatments did not trigger induction of phenolic content in J. oxycedrus, which could be considered as a constitutive defense. It is noteworthy that alkaloids in E. fragilis decreased with the treatments, which may contribute to preventing a possible addiction to these compounds, and thus reduce browsing damage. The responses induced by pruning show that these species are very tolerant to a level of herbivory, which could indicate a good adaptation to browsing pressure, exerted since the Pleistocene by Myotragus balearicus, and followed by goats during the last millennia. This is especially relevant in Mediterranean islands, such as the Balearic Islands, where practices for conserving relict plant species are based on feral herbivore exclusion.