Deer Exclosure Monitoring
We have recently installed deer exclosures at multiple locations within Fairfax County, Virginia. The fenced exclosures measure 8-feet wide by 20-feet long and 8-feet tall, which will effectively deny any deer from being able to feed within the exclosure.
At each exclosure we have rebar staked a replicate sized rectangle 10-feet away, as a control to determine the effects of deer browse on natural vegetation community structure. These sites will be monitored for several years to evaluate the trend of possible impact by deer herbivory (feeding pressure).
The monitoring methodology that we are using is called the Browse Impact Protocol, as developed by Bryan Gorsira of the National Park Service. A PVC frame with a grid pattern is used to measure vegetation on the forest floor (point-intercept method). Sixteen points where the layers of string form a cross allow you to align your eye to look straight down and determine if you see leaf litter, forb, grass, fern, moss or mineral soil, which is then recorded. This is done at three corners inside of the rectangle, less the entrance of the exclosure where most of the trampling occurs.
Vertical plant cover is then measured using a vegetation profile board, divided into three 1.6-foot sections with 25 squares in each section. The number of squares not obstructed by vegetation for each section is then recorded by the observer at the opposite end of the plot, with the observer sitting or crouching at the height of the deer.
For the purpose of establishing a baseline, we also identified every plant species within each exclosure and control by stem count. These combined standardized measurements allow us to then go back year-after-year during the growing season to evaluate how deer may affect forest composition.
In recent studies elsewhere within the greater Washington DC metropolitan Piedmont region, excessive deer browse is reducing growth and survival rates of tree seedlings and saplings, and the prevention of adult recruitment into tree populations, thereby causing irreversible shifts for a stable-state forest community by altering species composition.
Studies have documented that annual seedling survival rates are significantly lower in the control than in the exclosures. It can be determined that deer are having a significant impact on the vegetation structure and composition of woody seedlings. Herbivory by deer may severely impact forb cover by suppressing forb densities to levels much lower than would be expected in the absence of deer. After five or more years of on-going studies, forb cover in exclosures may be at least 30-percent greater than in controls.
The appropriate carrying capacity of deer is approximately 10 to 15 deer per square mile. At these densities, most forest floor cover is able to regenerate and allow for vegetative diversity. Many urbanized areas in metropolitan Maryland and Virginia however, have deer densities exceeding 200 deer per square mile. When walking through most any woodlot and if you pay attention, you will notice that deer are degrading the forest as we observe less diversity of wildflowers, an increase of non-native nuisance vegetation and the loss of shrub and tree species necessary for future regeneration.
Deer exclosure studies help to provide both qualitative and quantitative science in helping land managers better defend wildlife management strategies that may include deer harvesting.
Thank you Jessica for your help today.