Saving the Old Forest oaks through research

When Overton Park Conservancy’s Director of Operations, Eric Bridges, began working on his PhD in Forestry at Mississippi State University, he wanted to answer two questions with huge implications for the Old Forest State Natural Area: why are the forest’s signature oak species experiencing such decline, and is there anything we can do about it?

Now, the newly-minted Dr. Bridges has some answers to those questions, a trove of data about the forest, and plans for future research and restoration that could give oaks a fighting chance. 

Laying the Groundwork

Creating a plan for forest management requires us to understand how the ecosystem is currently functioning, and how it’s changed over time. This meant Eric needed to develop a strong research foundation, a process he began more than 10 years ago by engaging students at Rhodes College as Urban Forestry Fellows. Since then, he has worked with interns from Christian Brothers University, the University of Memphis, and several local high schools to gather data in the forest.

A key piece of Eric’s research was replicating a 1987 inventory of the Old Forest by Dr. James Guldin. At the time, Dr. Guldin noted that while oaks were the dominant species in the forest, the amount of young oak trees was worryingly low. This meant that as our mature oaks reached the end of their natural lifespans, there were fewer and fewer young trees that would grow up to replace them.

Eric and Rhodes College Urban Forestry Fellows in 2014

In 2019, Eric and his students replicated Dr. Guldin’s research and found that the downward trajectory of the oaks was getting worse. The number of seedling oaks had declined by two-thirds since 1987. “The overstory trees are heading for the exit,” Eric says, “and the seedlings can’t get in the door.”

But what was causing this decline? Were the seeds failing to germinate because of a lack of light on the forest floor, the moisture content of the soil, the heat of our summers, or the lack of cold in the winters? Or was the problem with their dispersal—the ability to get some distance from their parent tree and begin growing? The answer (or at least part of it) has four legs and a bushy tail.     

Maybe They’re Too Delicious?

In his next research phase, Eric created 12 experimental plots located throughout the forest, where he planted acorns in order to study how invasive species, leaf litter, and seed predators like squirrels affect the ability of seeds to germinate and establish themselves. The amount of leaf litter, and whether invasive species were present, did not significantly impact whether the seeds sprouted. But there was one key factor: whether or not there was a cage over the acorns.

In the plots where there were no cages, “the squirrels just destroyed the acorn crop,” Eric says. “There would be seven or eight squirrels in one plot just going to town.” Using trail cameras pointed at the plots, he was able to watch them in action, sometimes excising the end of an acorn for a small snack that ultimately prevented it from germinating. The uncaged acorns hung on longer in areas where there was a gap in the forest’s canopy (likely because squirrels were more vulnerable to hawks in these locations), but the end result was the same. The next generation of oak trees were winding up in someone’s belly.

Before we stoke anti-squirrel sentiment, Eric is quick to point out that this was one experiment and the results can’t be accepted as universal. In mast years, oaks might produce so many acorns that squirrels couldn’t possibly eat them all. And squirrels—along with blue jays—are actually crucial for the dispersal of acorns. Because the tannins in acorns can upset their stomachs if they eat too much at once, they often move and cache them to eat later. Today’s forgotten acorn becomes tomorrow’s mighty oak tree. Our squirrel population in Overton Park may just be too dense, and the forest too small, to spread them out properly (and keep them hidden from other predators).

“We Have to Plant”

There was good news in this experiment, too: the acorns that were covered by cages germinated at a rate of over 85%. “They were germinating just fine if they got the chance,” Eric says. If a key problem for the oaks is that their seeds are being eaten, that means we could help them along by intervening at that part of their life cycle. “That means we have to plant.” Getting seedlings into the ground past the point at which they’re appealing to predators gives them a better shot at getting established.

Creating a long-term planting plan will be the next phase of Eric’s research work for the Conservancy, starting this summer with two student interns. But the experiments have already begun. Thanks to the Tennessee Division of Forestry, we’ve begun a pilot project with 100 oak seedlings in four different plots. By comparing how the seedlings fare in an open vs. closed area of the forest, and with competing vegetation either pared back or left alone, Eric and his students can see how to plant to give new oak trees the best chance at survival. 

This research will expand into a larger seedling survival study, which will likely involve choosing three oak species and planting a few thousand seedlings in the forest. In some locations, Eric and his team will manipulate the surroundings, because it’s likely that competition from other plants (especially non-native invasive species) is hindering the oaks’ access to sunlight. This intervention could look like anything from spraying invasive honeysuckle plants once a year to trimming back some surrounding vegetation six times a year. We’re looking for the sweet spot of how often to check in with these trees in their early years, given inevitable time and resource constraints.

This summer, Eric and his students will monitor the 100 seedlings planted this winter, to see if these plots provide a good model for the multi-year process of expanding out to thousands of seedlings. They hope to design the long-term study based on this data, with a goal of acquiring equipment, obtaining funding for seedlings, and preparing the sites this fall for planting over the winter. It all points to a busy time and a lot of opportunities for volunteers to get involved!       

Eric demonstrates proper planting technique to Compass School students, who helped us plant 100 oak seedlings that will form the basis of our next research project.

The Work is Just Beginning

Although Eric has finished his degree, he sees the potential for so much more in the Old Forest. “When we started working with students all those years ago,” he says, “we had a vision that was so far away. The nature of ecological change means you need multi-year studies, but we first had to build the capacity with students to conduct those studies.” 

Now Eric—and the Conservancy—are dreaming big about what those studies could do for the forest. We are seeking support to create a Department of Conservation, Research, and Education that would continue the research that will allow us to create a more resilient forest. Urban old-growth forests are so rare that Overton Park’s is one of only three that remains in the entire United States. To care for such a priceless resource is an incredible gift, and dedicating more resources to preserve it for future generations is a huge part of the Conservancy’s reason for being.

Helping the oaks that give the forest its unique character will rely heavily on both financial supporters and volunteers, like the invasive-removal crew Bill Bullock has been leading since 2017. But that’s one of the things that excites Eric the most. “The dream is a project led by a group like Bill’s: they’re planting oak trees in a way that was informed by the work and research they’ve been a part of.” It’s an investment that could pay off not just now, but for centuries to come.

If you feel moved to support this work, please get in touch! And if you’d like to read Eric’s dissertation, you can do that here.

Bill Bullock leads volunteer events all winter in the Old Forest. His crews have removed countless invasive plants like English ivy, cherry laurel, and Japanese honeysuckle.