Did you know that the U.S. has a national tree? I actually didn’t before I started researching for this post, and unlike Canada, we don’t make it super-obvious by putting it on our national flag! Back in 2004, following a public poll by The Arbor Day Foundation, Congress passed legislation establishing “the mighty oak” (all 60+ native species belonging to the genus Quercus) as our official national tree. Here in Illinois, where the WIGL Collaborative is headquartered, oaks hold extra-special status. The white oak, Quercus alba, is our official state tree, and October has been officially designated as oak awareness month, when conservation-minded folks celebrate and educate on the importance of oaks and oak-dominated ecosystems. Many of us lovingly call this month OAKtober.
OAKtober shouldn’t just be for Illinois! All of the Great Lakes States have at least a handful of native oak species. White oak (Q. alba), northern red oak (Q. rubra), burr oak (Q. macrocarpa) and northern pin oak (Q. ellipsoidalis) are distributed throughout the Basin. Northeastern Illinois, Northwestern Indiana, Michigan’s Southern Lower Peninsula and Southeastern Wisconsin are known for oak savannas, unique and increasingly threatened fire-adapted ecosystems. If you’d like to learn more about oak savannas for OAKtober, oaksavannas.org is a fantastic information-rich website all about these unique places. Because we’re an invasive species collaborative, we’re going to honor OAKtober 2021 by taking a look at oaks through an invasive species lens.
When it’s functioning properly, the lifecycle of an oak-dominated forest or woodland looks like this: mature oaks dominate the forest canopy, and are relatively widely spaced to allow room for their roots and branches. This spacing allows light to reach the forest floor – more light than occurs under most other canopy tree species. During mast years, the mature oaks drop hundreds of thousands of acorns, some percentage of which eventually germinate into seedlings and small saplings. These saplings seemingly just “hang out” for several years, with most of their growth occurring in the roots. The saplings wait for a major disturbance in the forest canopy causing the death of one or multiple mature oaks and opening a canopy gap. The saplings that were most successful at establishing root systems use their stored energy to grow rapidly, filling these gaps and eventually replacing their parents in the canopy. (Dey 2014)
When a forest understory is invaded with shade-tolerant plants, the oak reproductive cycle is interrupted because the new generation of saplings doesn’t receive enough light to endure the often lengthy wait for a canopy opening. Forest understories choked with buckthorn, honeysuckle, burning bush, barberry, or sapling Norway maples simply don’t have enough room or light to sustain oak saplings. Native shade-tolerant trees like sugar maple, red maple and tulip poplar can also shade and crowd out young oaks. Over-abundant white-tailed deer also play a part by preferentially browsing oaks. A recent synthesis of data from Eastern U.S. forests revealed that only 4% of oak-dominated sites were showing adequate regeneration to replace canopy trees (National Park Service 2011).
Knowing that oaks are fire adapted, forest managers have often implemented prescribed fire programs to reduce competition. However, as our prescribed fire page describes, fire alone isn’t terribly effective at controlling most mature invasive shrubs or vines, though it will usually take care of seedlings. Another potential issue is that at least in certain soils, frequent low-intensity fire seems to increase soil nitrogen levels significantly, which favors the growth of invasives and other non-oak species (Taylor and Midgley 2018). This isn’t to say that prescribed fire shouldn’t play a role in oak recovery, but that additional methods will be necessary to remove competition from mature woody invasives and that fire intensity and frequency may need to be adjusted to maximize benefits to oaks specifically.
Is there ever a situation where the oak itself might be the invasive species in the Great Lakes Region? It could happen. The Biota of North America Program reported five non-native species and one non-native/native hybrid occurring outside of cultivation on the continent (Kartesz 2015). Of those, sawtooth oak, Quercus acutissima, probably has the greatest potential to become invasive in this region. As the common name suggests, sawtooth oak leaves are serrated like a saw blade rather than lobed like the quintessential oak leaf. It has some characteristics that give it a competitive edge over native oaks, including rapid growth and rapid maturity, allowing it to drop its first acorns in its first decade of life. By contrast, white oaks typically don’t develop significant masts of acorns until they’re at least 50 years old! And, at least in southern climates, sawtooth oak produces masses of acorns every year instead of undergoing mast year booms like native oaks. Sawtooth oak is primarily spread over long distances by human planting, both as a landscape tree and as game animal forage. It is minimally hardy in the Great Lakes region, so we haven’t really seen it establish outside of cultivation here yet, but it is definitely a species to watch for range expansion under climate change. The sale and cultivation of sawtooth oak has been pre-emptively prohibited in Wisconsin.
How can you help with oak recovery? Almost all managers of urban forest systems are looking for help from volunteer stewards. It’s a great way to get exercise and do some good while remaining socially distant. If you’re thinking that the season for volunteering outdoors is over, not so! A lot of woody invasive species management tasks can be done during winter. If you own private oak-dominated forest land, consider developing and enacting an oak recovery plan. If you have an open, sunny spot on a residential or commercial property suitable for a large tree, plant a native oak! If you like the unlobed leaf of sawtooth oak, Chinquapin (Q. muehlenbergii) or shingle oak (Q. imbricaria) may be a good choice. Scarlet oak (Q. cocinea) has excellent fall color. Northern pin oak (Q. ellipsoidalis) and bur oak (Q. macrocarpa) do better than other species in alkaline/limestone-based soils. Swamp white oak (Q. bicolor) may be easiest to find in nurseries. No matter which species you choose, consider your oak sapling an investment for future generations!
Dey, D.C. 2014. Sustaining oak forests in Eastern North America: Regeneration and recruitment, the pillars of sustainability. Forest Science. 60(5): 926–942.
Kartesz, J.T. 2015. Quercus in: The North American Plant Atlas. The Biota of North America Program (BONAP). Chapel Hill, N.C.
National Park Service. 2011. The future of our oak forests: Can fire and fences sustain oak forests for the future?
Taylor, Q.A. and Midgley, M.G. 2018. Prescription side effects: Long-term, highfrequency controlled burning enhances nitrogen availability in an Illinois oak-dominated forest. Forest Ecology and Management. 411: 82-89.