Assessing the Impact of Deer Browse on Forest Regeneration
Forests in Pound Ridge and surrounding areas are under siege by one of the gentlest of animals, the white-tailed deer. Deer are keystone herbivores and eat from five to seven pounds of vegetation per day. In summer, deer dine richly on the buffet of tender plants provided by home landscaping. In winter, our subsidized deer herds turn to the forest to browse buds, tender saplings, and even tree bark. At ideal densities of 10-15 per square mile, deer browse checks the growth of plant life without hindering forest regeneration. At higher densities, they can strip the understory bare. With deer density in Pound Ridge as high as 65 per square mile, we face the loss of wildflower species first, and then the pollinators that depended upon them, and then the birds that can’t find shelter or food, and finally, the forests themselves as trees are not replaced.
Is the situation really so dire? Scientists and natural historians have documented all of these effects, including our friends at Mianus River Gorge who found that tree sapling diversity in a local protected forest decreased from 15 species to 4 over the last thirty years, due to overbrowse by deer. Local land managers seek solutions, but first we must know our problem. I will describe here a simple method for assessing the level of deer browse in virtually any patch of forest in Pound Ridge. Rather than counting deer or deer density, we are going to measure the effect of deer browse on each plot and use that as a baseline that guides future management. This methodology was developed by Thomas Rawinski of the United States Forest Service, who shared his knowledge and skill with us in a training session this summer.
We begin by choosing a site for our study plot(s). The circular plot covers 100 square meters and must include at least ten measurable saplings of one of the indicator species. Measurable means that the sapling is still below the average browse line and therefore vulnerable. Indicator species include any browsed species with beech and sassafras preferred followed by the shrubs; maple-leaf viburnum, sweet pepperbush, alternate-leaf dogwood, red elderberry, choke-cherry, and even the invasive burning bush. I do not mean to imply that deer prefer these plants – rather, they avoid them – so that browse levels reliably indicate the severity of deer pressure on forest growth.
Once the indicator plant has been selected and the plot margins delineated, we set out to find the ten tallest measurable saplings of our chosen species. Then we average those ten measurements to find the definitive “tallest” for that plot. In itself, this doesn’t tell us much. Over time though, we can discern trends and hope to associate those with management actions. We can also make comparisons among plots to measure relative levels of browse across landscapes. For example, Sugar maples at Armstrong were browsed down to 8.9 inches, while those at Halle measured 8.1 inches. Deer pressure may be more severe at Halle than it is at Armstrong.
We can avoid the costly and time-consuming task of measuring deer density by using this simple approach to assess deer impact on our preserves. We will measure again next year and compare our findings both within PRLC holdings and with a larger community of land managers and report back to you then.
For more information, please see http://www.na.fs.fed.us/pubs/2014/NA-IN-02-14_WhitetailedDeerNEForestsWEB.pdf