Phylogenetic and ecological ordination of a forest understory community following deer exclusion

Background/Question/Methods

Throughout eastern North America, white-tailed deer herbivory is a key disturbance that can be expressed in terms of periodicity, severity, extent, and intensity. Exclosure studies manipulate this disturbance, allowing researchers to determine the extent to which deer alter plant community composition. Here, we take advantage of a 20-year study of deer exclusion in a hemlock-northern hardwood stand in N Wisconsin. We sampled species composition annually over 8 seasons in four adjacent control-exclosure plots. We used permanent line-intercept transects to estimate percent cover.

We use NMDS ordination to examine similarity in species composition between browsed and unbrowsed plots through time. We perform both a standard NMDS based on Bray-Curtis distances, and a phylogenetic ordination based on Unifrac distances among all pairwise combination of sites, with species weighted by percent cover.

Results/Conclusions

Deer exclusion generates two distinct communities: a browsed community dominated by sedges and grasses, and an unbrowsed community dominated by broadleaved herbs and woody species. Neither ordination revealed a tendency towards convergence through time. Browsed and unbrowsed communities segregate in NMDS space, but segregation is more apparent when Bray-Curtis distance is used (ANOSIM R = 0.95; P < 0.001).  Segregation is less visually apparent where Unifrac distance is used (ANOSIM R = 0.28; P < 0.001). Together, they provide a complementary view of how browsing affects community structure and assembly processes. Bray-Curtis and Unifrac distance matrices are only moderately rank correlated (Spearman rho = 0.33; P < 0.001). The combination of ecological and phylogenetic data extend our understanding of how herbivory in particular, and disturbance more generally, combine to influence community structure.