Deer Impacts on Ann Arbor’s Natural Areas

The City contracted with botanist Dr. Jacqueline Courteau to study the impact of the deer on Ann Arbor’s natural areas. Dr. Courteau studied deer impacts on three different plants – oak seedlings, trillium, and wildflowers. All three studies had fenced and unfenced areas for comparison purposes.

All three studies show that more plants are surviving with reduced deer browse, but neither the oak nor the trillium browse levels are at sustainable levels. That is, the deer are not allowing the regeneration of the oaks forests nor the trillium under the oaks.

Ecological carrying capacity focuses on the interaction between herbivores and the plants that they eat. It is defined as “the maximum density of animals that can be sustained in the absence of harvesting without inducing trends in vegetation” (Krebs 1978). At ecological carrying capacity, the rate of browsing is roughly equal to the rate of food-plant regrowth. Our deer are exceeding the ecological carrying capacity of the City’s natural areas.

From the 2020 Deer Impacts on Oak Seedlings summary by Dr. Corteau:

  • Overall, deer browsed 46% of all monitored red oak seedlings in 2019–20, the lowest level overall since monitoring began in 2015–16.
  • Most sites where deer have been managed have shown steady declines in deer browse levels.
  • Despite declines, deer browse levels remained higher at all sites than the 15% threshold beyond which oak regeneration is likely to fail.

The oak seedlings study has been going on for 4 years. Deer browse damage on the oak seedlings is clearly trending downward with the notable exception of Furstenberg Park, which is greater than 3/8 of a mile to a deer management site.

From the 2019 Deer Impacts on Trillium summary by Dr. Courteau:

  • In 2019, trillium abundance continued to be significantly lower in unfenced plots where deer can browse and trample.
  • Deer browse rates on trillium in 4 of 5 sites are above the 5–15% level that studies have shown will allow trillium populations to persist.
  • Although deer management in 2016–2019 has stabilized or reduced deer populations, unfenced deer accessible plots continue to show strong deer impacts on trillium flowering, while fenced populations show recovery.
Fenced vs unfenced trillium flowering. 30% is the sustainable flowering level..

Trillium Damage in the U-M Nichols Arboretum
(slide to compare the overlayed pictures)

  • The first picture shows a group of about 18 trillium and a couple of solomon seals. Only one trillium is going to bloom. It is hard to see what is missing.
  • The same picture but with all 27 of the browsed trillium stems marked. Deer browsed 60% of the plants.

From the 2019 Deer Impacts on Wildflowers summary by Dr. Corteau:

  • Deer browsed from 60–85% of unfenced aster and goldenrod plants.
  • Deer presence was associated with significantly lower flowering rates and numbers.
Number of wildflower blooms, fenced vs unfenced

Dr. Corteau’s methodology for measuring vegetative deer damage is based on work done by Dr. Bernd Blossey of Cornell University. In this video, Case Study: The Future of the Forest: White-tailed Deer and their Impact, Dr. Blossey explains how his research team has documented the impact of deer on forests. Dr. Blossey’s group also produced a video animation explaining how red oak seedlings can be used to quantify deer browse pressure on woodlands.  Dr. Courteau’s most recent report the City of Ann Arbor is Deer Impacts on Vegetation in Ann Arbor Natural Areas – 2018-2019. The report summaries (also linked above) are:
2020 Deer Impacts on Oak Seedlings summary
2019 Deer Impacts on Trillium summary
2019 Deer Impacts on Wildflowers summary

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