Effects of deer herbivory on chemical defenses of Fagus grandifolia

Abstract

Plants exhibit a plethora of chemical responses to herbivory. Compared to studies on plant responses to insect herbivory, there are far fewer relating to mammalian herbivory. However, one well-known plant defense against mammalian browsing is the presence of tannins. Condensed tannins, as opposed to hydrolyzable tannins, are more well-studied. These polyphenols bind with protein to create an astringent taste that is repellent to animals. In general, phenolics are a diverse group of compounds involved with plant defense (Rehmen et al. 2012). Salicylic acid also plays a role in plant defense, particularly against pathogens (Ohse et al. 2016). In an experiment by Ohse et al. in 2016, clipping of Fagus sylvatica (European beech) leaves and addition of Capreolus capreolus (roe deer) saliva caused an increase of leaf salicylic acid content. Condensed tannin content did not change in leaves or buds after application of the clipping and saliva treatment on those structures. This raises the question of whether tannin levels, as well as other defensive phenol levels, are inherent within the plant or change inresponse to herbivory. Our study examined the potential long-term change in chemical composition of Fagus grandifolia as a result of exposure to herbivory by Odocoileus virginianus (white-tailed deer). We chose the deer and tree species present in the Northern Lower Peninsula of Michigan most similar to those used in the Ohse et al. study. In addition, there is no published research about the effects of deer herbivory on Fagus grandifolia leaves and buds. We hypothesized that chemical composition of leaves and buds between the deer exclosure plot and surrounding area would differ, with a significant increase in salicylic acid production in leaves and in general defensive compounds. We examined the relationship between years since browsing and the amount of chemical compounds present. Trees that experienced more recent browsing were hypothesized to have higher levels of defensive compounds.