White-tailed deer (Odocoileus virginianus) have major impacts on vegetation diversity, and decrease overall density and average heights of species upon which they browse in forest understories. Measurements of browse impacts on plant species diversity and abundance typically utilize percent cover estimates from plots laid out at ground level and viewed from above; however such measurements collapse information into two dimensions and can fail to capture impacts in the vertical dimension.
Vertical vegetation diversity has been shown to be important in structuring communities of a variety of taxa such as birds, web spiders, and tropical butterflies. Thus understanding the relationship of deer density to vegetation diversity and density in the vertical dimension may provide critical information about the structure and composition of the forest understory community that could help explain indirect effects on other taxa.
We measured vegetation diversity and density at 20 cm height intervals from ground level up to 2 m at 44 established deer exclosures in New Jersey and Maryland, and at 109 locations across 9 forests in the greater Washington DC area over a gradient of deer densities ranging from 0 to 57 deer/km2.
At deer exclosures, vegetation density was significantly different inside and outside the exclosures at every height class in paired t-tests. The absence of deer browse resulted in the greatest vegetation density at the lowest height classes while at the higher heights the difference was less pronounced. The relationship between mean difference in vegetation density and height was negatively linear (r2=0.8).
Species richness was significantly greater at every height class inside deer exclosures. The greatest difference in species richness inside and outside the exclosures occurred in the lowest height classes. The average difference decreased non-linearly as height class increased (r2=0.92).
Analysis of vegetation density and species richness across deer densities indicated that the relationship between these variables is not the same among the height classes. Deer impacts on vegetation density and diversity were most apparent at heights below 1 m, and our data suggests that decreases in vegetation diversity and density in these height classes may occur even at very low deer densities. Our results imply that the range of heights at which vegetation diversity and density are measured may be important in capturing the impact of deer density on the understory.