Many of you in the wildlife world are likely familiar with the term savanna (or savannah – both are correct, but why use an extra letter if you do not have to). Most of us think that it is a grassland with a tree here and there, but that’s not always correct. In the Southeast, the long-leaf pine savanna is iconic and closely associated with plantation style quail hunting (not to mention red-cockaded woodpeckers, gopher tortoises and many more at risk species).
Wikipedia defines savanna as “a mixed woodland-grassland ecosystem characterized by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses.” I would argue their point of being “primarily grasses”, as savannas can be some of the most forb diverse plant communities on earth…but who’s counting?
In the rain rich southeast, savannas are not as easily maintained as they are in more arid regions. And in all but the rarest of cases, they do not exist long without fire. In Virginia, we have ample experience turning loblolly pine stands into savanna like ecosystems. To become a fully functioning savanna may take decades. This involves heavy thinning, or tree density reduction, fire applied over multiple years and, in some cases, judicious use of herbicides. There are more and more examples on our public and private lands, though these ecosystems are still rare relative to others.
An ecosystem much less commonly discussed is oak savanna. In 2006 we were given the opportunity to develop examples of oak savanna on DWR lands in the central piedmont. A site was selected on Amelia Wildlife Management Area that was ideal in some ways. It was close to the entrance of the WMA and easily accessible. It was surrounded by roads and trails making fire line establishment simple. And it was a typical mature oak stand with a few pines, gums and non-native invasives mixed in. The project has been ongoing now for 16 years and we have learned some valuable lessons along the way. Note that our goal on this site was not timber income. Also note – we are not opposed to clear-cutting hardwoods when it is necessary to create early-successional vegetation and establish a young forest. Oak savanna is an alternative to clear-cutting, as would be timber stand improvement cuts, shelter wood cuts and others – all based on habitat goals. We wanted to develop an open oak stand that would provide great herbaceous early-successional habitat in its understory. Secondarily, we wanted to learn as we went, and develop lessons for future endeavors. Further, we wanted to determine if using understory herbicides to help control hardwood saplings could reduce the dependence on fire. Many landowners are hesitant to burn, or will not burn frequently enough to establish oak or pine savanna. When using herbicides, care must be used to make sure rare or important plants in the understory are not impacted. Using herbicides in a forest understory to help create savanna should be considered on a case by case basis after careful analysis of the plant community.
The harvest in 2006 reduced basal area to 30 – 40 sq. ft. per acre which in retrospect perhaps opened the canopy too much too quickly. Some over story tree mortality occurred through time due to stress from sunlight and weather exposure. Surveys of understory stem density done in 2008 found 201 small sapling stems per acre consisting of Tree-of-Heaven, black gum, cherry, sweetgum, hickory, red, black and white oak, yellow poplar, winged elm, dogwood, pin oak, cedar, and non-native lespedeza. A common non-soil active broad-leaf hardwood control herbicide was applied by ground based arc spray rig in 2009 to kill hardwood saplings, but allow native grasses like broom sedge bluestem to flourish. A control burn was conducted in March 2011. Analysis during April 2013 found only 13 sapling stems per acre. The treatments were initially effective and the understory became more herbaceous. Another prescribed fire was conducted in March, 2014. By 2015, woody encroachment from seed and residual stumpage had taken over again. The stand was sprayed again from the ground with 4 quarts per acre of a common non-soil active forestry herbicide in summer 2016. The stand was prescribe burned again during March of 2017. This brought a temporary setback of hardwood saplings, but by 2019, hardwood saplings and blackberry dominated the stand. A burn was planned during fall 2019, but extremely dry weather conditions prevented it. Fall burning is now considered more effective at controlling hardwood under growth if it is done before leaf drop and before the saplings and seedlings are dormant. COVID-19 prevented any burning in 2020, and by 2021, the saplings had grown beyond control by fire. The site was forestry mulched in March of 2021 – essentially set back to “ground zero” at that point. This is the second growing season post mulching and oak seedlings and saplings again dominate the understory. We plan to try fall burning in the coming months if conditions allow it, but if the saplings begin to dominate again, we will allow this site to become what it wants to be…an oak forest! It has excellent oak regeneration in the understory now.
It does not appear that a reduced fire return interval of over 5 years combined with herbicide application between burns is effective at creating oak savanna on a site that was meant to grow nice white and red oaks. Our decision of which oak stand to work with was as much a matter of convenience as anything. In hindsight lessons learned are: Look closely at the site index (the ability of a site to grow trees). In our case it is close to 80 for white oak – about as good as it gets (consult a forester to help determine your site index). And look at the soils and drainage class – sites with good soils (in our case Cecil and Appling fine sandy loams) that are well drained but not dry, or that are slightly mesic will make it hard to develop oak savanna. In short – don’t try to turn apples into oranges (a head slapping, earth shattering, duh-huh revelation I am sure…LOL). Look for sites with lower quality soils that are xeric (dry) and perhaps already growing indicator species like chestnut oak, scarlet oak, and southern red oak. They may have been hardwood savannas historically and have only grown closed canopies through time due to a lack of fire. Every landowner needs to accept that their goals for habitat may not always be compatible with the land and site conditions they have. And as professionals we should be able to recognize site conditions and offer the best advice possible.