Summary: Oak-dominated forests in the E.S. George Reserve and elsewhere in the eastern U.S. are being replaced by maples and other fire-sensitive trees. Oak forests may be a legacy of Native American fire practices that predate European settlement.
Oak trees in the Big Woods Plot of the E. S. George Reserve
Two years ago a U-M team* established a Forest-GEO tree inventory plot in the E.S. George Reserve (see 2014 blog post; or listen to this NPR interview). Students and faculty mapped and measured all woody stems ≥1 cm diameter in 23 hectares of oak-hickory forest (the plot is the size of 46 football fields!). “Big Woods” plot includes > 45,000 stems from 41 tree and shrub species.
Oak-hickory is perhaps the most extensive deciduous forest type of eastern North America. In the Big Woods Plot the most common large trees are black oak (Quercus velutina) and white oak (Q. alba) with subdominant shagbark hickory (Carya ovata) and pignut hickory (C. glabra). These are drought-tolerant (xerophytic) species that do well on sandy glacial debris. Tree core studies indicate that the large oak cohort established ca. 150 years ago (J. Vandermeer, pers. com.).
The tree inventory data tells us that oaks and hickories are on the way out; the young generation is overwhelmingly comprised of red maple (Acer rubrum) and black cherry (Prunus serotina) (see figure below). Unlike oak and hickory, black cherry and red maple are mesophytic, meaning they require moderate levels of soil moisture year round. Their seedlings are also more shade tolerant than oak or hickory and they cast more shade as adults.
Size distribution of trees in the Big Woods Plot (from Allen 2012 defense). Oaks are the largest trees (measured in circumference) with few juveniles represented. Black cherry and red maple dominate the subcanopy sizes. Witch-hazel is a multi-stemmed shrub.
Mesophytic species outcompete and will eventually replace oaks as the dominant canopy trees. This process of mesophication is happening throughout eastern North America (1). Red maple, which was relegated to swamps when Europeans arrived (hence its old name swamp maple) now ranks as the most abundant tree species in the eastern U.S. (2)
In this photo of a slope adjacent to a kettle bog, all the larger trees are oaks, while all the saplings and leaves in the foreground are red maple.
Why is mesophication only happening now? In part because of “Smokey the Bear” fire policies implemented In the early 20th century (3). It turns out that oak-hickory forests, and many other landscape ecosystems (4), require periodic fires to subdue their fire sensitive mesophytic competitors. Oaks have thick bark that protects them from moderate fire and they can resprout after severe fires; maples and cherries have neither trait.
We know that the first European settlers in Michigan found a pyrogenic landscape. This is based on the notes of land surveyors in the 1830s. The Michigan Natural Features Inventory (MNFI) has used these survey data to reconstruct detailed circa 1800 vegetation maps.
1800s vegetation map of southern Livingston County with E.S. George Reserve (ESGR) shown inside the dark rectangle. In pre-settlement times, the ESGR contained fire-dependent plant communities, including oak barrens (light orange) and oak-hickory forest (brown) alongside hardwood swamps (red) and wet prairie (grape color).
Where did the fires come from? Lightning is a natural source of fire, but it was probably too infrequent to maintain such an extensive pyrogenic landscape. We think the fires were mostly anthropogenic. Native Americans used fire to alter landscapes for a variety of reasons, including clearing the understory for woodland travel, modifying habitat for game animals such as turkey and deer, enhancing soil nutrients in fields, eliminating pests, or favoring food sources, among which acorns were of major importance.
There may have been a climate-change component as well: during the Little Ice Age, lasting roughly from 1300-1850, North American landscapes were drier and more prone to drought and fire than they are today (3) .
The early settlers didn’t seem to realize how extensively the “pristine” landscape they encountered was the product of Native American land-use. Perhaps this is because the numbers of Native Americans they encountered were at a historical low. The diseases transmitted by European colonists would have reached the midwest by 1800, well before the land surveys were performed (3, 5).
The transition from oak-hickory forests to mesophytic (possibly beech-maple) forest types will herald many ecological changes including, potentially, less ability of our forests to withstand drought condition (3) and major shifts in the oak-associated fauna (including a decline in the huge numbers of wild turkey in the reserve). We will be watching these dynamics take place in the Big Woods forest plot. The next census will be done in 2019.
(1) Nowacki, G. J. and Abrams, M. D. (2008) The Demise of Fire and “Mesophication” of Forests in the Eastern United States. BioScience 58: 123-138.
(2) Abrams, M.D. (1998). The red maple paradox. BioScience 42, 346–353.
(3) Abrams, M.D. andG. J.Nowacki (online advance access) An interdisciplinary approach to better assess global change impacts and drought vulnerability on forest dynamics. Tree Physiology doi:10.1093/treephys/tpw005
(4) Cohen, J. G., M. A. Kost, B. S. Slaughter, D. A. Albert (2014) A Field Guide to the Natural Communities of Michigan. Michigan State University Press. 362 pp.
(5) Mann, C. C. (2006) 1491. Random House LLC. 560 pp.
*The Big Woods plot was established in 2003 by John Vandermeer and Ivette Perfecto and their students. The 2014 census first applied the Smithsonian protocols which included a much larger number of stems. This effort was led by Dave Allen, a U-M EEB alumnus and assistant professor at Middlebury College. We’d like to acknowledge the many students who worked on this project: Omodele Ajagbe, Bob Barretto, Hillary Butterworth, Richard Byler, Vera Chan, Ben Crotte, David Hudson, Lindsay Ford, Katie Gallagher, Jasmine Gramling, Kate Heflick, Rodica Kocur, Carley Kratz, Rachael Lacey, Isaac Levine, Kathleen Parks, Andrew Phillips, Jayna Sames, Margot Sands, John Schroeder, Leah Spaulding, Jordan Trejo, Justin Waraniak, Padhma Venkitapathy, Connor Velzy, and Ash Zemenick.