To be browsed or not to be browsed: differences in nutritional characteristics of blackthorn Prunus spinosa subject to the long-term pressure of herbivores

The impact of ungulates on temperate forest vegetation has been investigated for a long time. Numerous studies on food selection have identified the palatable plant species preferred by large European herbivores. However, intra-specific food selection and the question why particular plants of a given species are ignored during foraging have been neglected in the literature. In central Europe, Blackthorns Prunus spinosa growing in abandoned pastures are an important component of the red deer’s Cervus elaphus diet. In areas densely populated by deer, annual shoot browsing produces dwarf shrubby forms of blackthorns. However, some blackthorns are not browsed by ungulates and tend to adopt a tree-like form. The existence of distinct, browsing-dependent growth forms of blackthorns raises the question of inter-individual differences in the nutritional composition of plants. Based on factor analysis, we discovered differences in nutritional composition between browsed and unbrowsed blackthorns that might explain the individual plant-related drivers of red deer food preferences. The leaves of browsed blackthorns contained higher concentrations of C, N, P and Cu but lower levels of Ca and Mg than unbrowsed ones. Moreover, browsed blackthorns had a higher water content and higher concentrations of insoluble proteins, chlorophylls and carotenoids. We highlight the fact that the nutritional characteristics of an individual plant may explain the observed food selection pattern, leading to the unhindered growth of a fraction of the blackthorn population, in spite of severe pressure on the part of ungulate herbivores. The results of this study underline the important role of herbivores in the dynamics of plant communities, in which ungulates may mediate the persistence of certain individuals of a given species.