NPS photo/Dave Bieri Changes to our forestsThe deciduous forests of eastern North America have gone through dramatic changes over the last 100 years. From the large-scale industrial logging of old growth forests at the beginning of the 20th century, to invasive pests like the emerald ash borer or hemlock woolly adelgid killing millions of trees currently, these are all long-lasting changes to the forested landscape that we can observe in present day. But how will these forests look in the future? 
Thomas Saladyga What is causing the decline in oaks?Multiple studies throughout deciduous forests of eastern North America have shown that the decline in oak species is likely multi-factored. A lack of frequent, low-intensity fires and excessive browsing by deer are thought to be the main causes, with the loss of species like passenger pigeons and the American chestnut in the early 20th century and increasing precipitation in recent decades also playing roles in oak decline. Historically, low-intensity fires throughout Appalachian oak forests would happen every 5-15 years and wouldn’t have been severe enough to kill most of the large, overstory trees. Evidence for this frequent fire history is derived from tree ring records throughout Appalachia that date back to the early 1700s and from fossilized charcoal fragments thousands of years old found in soil and wetland sediments. These fires would have mainly been ignited by Native Americans for clearing land or for driving game while hunting. In the New River Gorge, tree ring studies have shown that fires since the 19th and 20th century were small and connected to regional industrialization, such as the establishment of railroads and mining. These low intensity fires would clear the understory of the seedlings of fire intolerant species such as red maple and American beech, which are adapted for shady conditions and thrive in areas void of fire. Oak species have higher light requirements and the open conditions created by frequent fires makes for ideal habitat. Oaks are also more resistant to fire damage, with their thick bark adding protection from the flames and their large underground root structure allows for small seedlings killed by fire to resprout more rapidly. Wildfires across the eastern United States have been less frequent in the decades following the 1930s after an aggressive campaign to put out all wildfires began. This coupled with deindustrialization and the establishment of various public lands has led to a near cessation of fires within the New River Gorge.
The eastern United States has also seen a wide fluctuation in white-tailed deer populations over the past century. While almost extinct 100 years ago due to overhunting, hunting laws and the extirpation of natural predators like wolves and mountain lions have allowed deer to rebound to populations as high if not higher than what they were prior to European settlement despite the region having less forest land in present times. Deer prefer to browse oak seedlings compared to other tree species and this pressure along with a lack of fire and a wetter overall climate creates compounding factors that lead to shade-tolerant species like maples replacing oaks in the understory. These compounding processes are called mesophication, (mesic being Latin for moist) where the transition to a denser, shady maple forest creates the conditions for the forest to be less flammable and more moist in the future, further decreasing oak regrowth in a snowballing effect. The decline in oaks will have many substantial impacts on the entire ecosystem. For instance, oak forests are home to more species of birds, moths, butterflies, and insects than maple forests and a transition from an oak understory to one dominated by maple is associated with a one-third reduction in herbaceous plant diversity.
NPS photo Studying the effects of fire and deer exclusion in the New River GorgeNew River Gorge National Park and Preserve was chosen as a research site to monitor the long-term changes experimental controlled burns and the exclusion of deer via 2.5-meter-high fences have on oak seedling regeneration. Sites were established in oak forests at Grandview and Backus Mountain with each site having a combination of 4 possible treatment classifications: 1) burned 2) deer fence 3) burned and deer fence and 4) untreated control. Fires during the controlled burns were small surface fires and didn’t cause any overstory trees to die. In 2015 initial data from established plots was collected and controlled burns were conducted in spring of 2017 at Grandview and spring of 2018 at Backus Mountain with post-fire vegetation data collected again in 2019. Half of the canopy dominant trees at both sites were comprised of oak species with maples only being a small component. However, maples made up 2/3rds of total tree seedling species at Backus Mountain and over 90% at Grandview. Grandview was established as a state park in 1939 where hunting was banned. Because of this, the total amount of seedlings, including oaks species, were much lower than Backus Mountain, which has a long history of hunting and deer populations being smaller. The excessive deer browse at Grandview also led to an abundance of herbaceous plants that deer avoid and an absence of those they preferentially browse. Consequently, Backus Mountain initially had more understory cover and diversity of plants. 
NPS photo ResultsResults from this long-term study show that the initial conditions of the forest understory before controlled burns and deer fences are implemented play a major part in successfully recruiting oak seedlings. Controlled burns at both Grandview and Backus Mountain sites reduced the total amount of seedling abundance, but Backus Mountain was the only site to effectively reduce maple seedlings and increase oaks. Fire at Grandview severely reduced oak seedlings to levels even lower than what they were prior to the controlled burn. The low oaks seedling populations at Grandview are stressed after decades of excessive deer browse and do not have enough energy reserves in their root systems to effectively resprout after a fire compared to those at Backus Mountain. Despite controlled burns reducing the understory cover, fire was shown to not reduce the diversity of understory species. 
NPS photo The future of oak forestsThis study highlights how controlled burns and deer exclusion can lead to increased oak seedlings and herbaceous diversity in the understory, however, knowledge of the history and degree of deer browse should be noted before controlled burns are implemented, as seen from the further decline in oaks after controlled burns implemented in the already over-browsed forests at Grandview. Fences alone could not successfully regenerate oaks due to the low light levels in the closed canopy forest. Future treatments to promote oak could include methods that will lead to some overstory mortality and increased light reaching the forest floor, such as more intense fires created through controlled burns or the creation of canopy gaps by selectively cutting certain overstory trees to allow more light for the oak seedlings. Deer fences will be necessary to protect oak seedlings from browsing to ensure they will reach large enough sizes to not be killed by fire in subsequent controlled burns. This study will continue to be monitored by National Park Service land managers and scientists and the information provided by this research will help aid in the preservation of all oak forests throughout the deciduous forests of eastern North America. Further ReadingBrose, P.H., Dey, D.C., Philips, R.J., Waldrop, T.A. 2013. A meta-analysis of the fire-oak hypothesis: does prescribed burning promote oak reproduction in eastern North America? Forest Science 59(3): 322-334. |
Last updated: August 13, 2021