Effects of tree species richness and composition on moose winter browsing and selectivity

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Mammalian herbivores play important role in forest regeneration. Selective browsing by moose and deer on saplings of preferred tree species may have considerable impact on tree species survival and species composition of naturally regenerated stands. On the other hand, in many countries across Europe game production is an important ecosystem service and ability of forests to support sufficient densities of game such as deer, moose and wild boar is of economic importance as well. Therefore, it is important to know how mammalian herbivores select the patches of forest where they feed and how, by manipulating forest diversity and tree species composition, we can ensure that forests have sufficient game production potential without suffering excessive browsing damage from mammalian herbivores. Ability to regulate and reduce damage by pests is one of the services provided by forest ecosystems, and it is commonly believed that diverse ecosystems are able to regulate pest damage better than species-poor ecosystems and communities. However, majority of mammalian herbivores are generalists and have a fairly broad diet. Therefore, instead of reducing herbivore damage, diverse tree stands may actually attract more mammalian herbivores by providing a kind of ‘smorgasbord’. The FunDivEurope project allows examining the effects of forest diversity and species composition on browsing and habitat use by mammalian herbivores across various types of forests from boreal to Mediterranean. In mature forests most of the damage by deer and moose is caused to the understorey shrub layer. The interesting question is hence how do diversity and species composition of overstorey influence diversity and species composition of understorey and palatability of understorey vegetation to mammal herbivores.  Mammalian herbivores are a management concern within production forests, where, in even-aged regeneration areas, trees often suffer damage and mortality as a result of heavy browsing pressure. It is therefore important to understand how forest tree diversity may influence browsing selectivity and intensity by large mammal herbivores, and consequently how this might inform forest management. To test the hypotheses surrounding the relationship between tree diversity and browsing we assessed browsing damage to young trees in the Satakunta forest diversity experiment, Finland.


Results from the Satakunta experiment indicate the greatest browsing by moose (the most prevalent large mammal herbivore at the site) occurs in more species-rich forest plots. Additionally we find selectivity for tree species within a plot decreases with increased tree species richness, meaning tree species are targeted more equally when growing in a mixture. We also demonstrate that the presence of preferred tree species pine (Pinus sylvestris) and birch (Betula pendula) in a plot leads to greater browsing damage on other less-preferred species growing in association in the same plot (associational susceptibility). The amount of browsing on preferred tree species did not differ significantly between different treatments, and we found no evidence that the least-preferred species, spruce, deters browsing on higher-preference species.

We conclude that species-rich plots provide nutritional benefits for generalist mammalian herbivores such as moose, and a strategy of opportunistic foraging and diet mixing allows complementary intake of nutrients as well as avoiding high concentrations of digestion-inhibiting plant secondary metabolites. The lowered selectivity in species-rich plots means that mixtures may not act as a defense against excessive damage by mammal herbivores, particularly for economically important species such and pine and birch.

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