The value of evolutionary history: Using phylogenetic diversity to identify the long-term effects of deer browsing on Northern Wisconsin forest understory plant communities

Background/Question/Methods

The effects of excessive white-tailed deer (Odocoileus virginianus) browsing on the composition of forest understory plant communities are well-documented. Overabundant white-tailed deer populations remove nutrient-rich forest herbs and tree saplings more quickly than can be regenerated, which leads to plant communities that are dominated by graminoids, ferns, and club mosses. However, these studies usually focus on quantifying plant species diversity through traditional diversity indices. Traditional indices, while useful, fail to provide insight into the evolutionary history of species within a community. In our study, we use a site-specific phylogenetic tree and two abundance-weighted phylogenetic diversity indices: mean pairwise distance (MPD) and mean nearest taxon distance (MNTD) to compare phylogenetic distance among 6 deer exclosure/control pairs to a randomized null model. Furthermore, we develop an herbivore susceptibility index that compares species presence inside versus outside exclosures to identify species particularly susceptible to deer browsing and reliant on exclosures for persistence. 

Results/Conclusions

Consistent with the literature, our herbivore susceptibility index shows control areas predominantly composed of club mosses, graminoids, and ferns. As expected, browse susceptible species such as Veronica officinalis, Tsuga canadensis, Abies balsamea, and Trientalis borealis were predominantly found inside exclosures (>80% of total abundance for each species). However, some species did show unexpected results. Acer saccharum (a browse neutral species) was rarely found outside exclosures (<2%), while two other browse-resistant species (Poa pratensis and Ostrya virginiana) were only found inside exclosures.

Both phylogenetic distance metrics showed control areas to be more phylogenetically clustered (less diverse) when compared to a randomized null model. In most cases, our phylogenetic distance in exclosed areas did not vary significantly from the null model. As predicted, in every control/exclosure pair, exclosures were more phylogenetically even (more diverse) than control areas. These findings highlight the importance of expanding diversity indices to include phylogenetic relationships, and provide new insights into the relationship between deer over-browsing and phylogenetic patterns.