Eliminating one species with another--graduate research on using Verticillium fungus to stop the tree-of-heaven

It is said that Ailanthus altissima was given the name "Tree of Heaven” because of how quickly and how tall it grows, appearing as if it’s throwing its arms high up into the heavens. But as an invasive species that causes a plethora of problems, a more fitting analogy would be that its roots stretch far, far down, all the way to hell.

Nobody is more aware of this deceptive tree’s habits than Rachel Brooks. A second-year Ph.D. student in the Department of Plant Pathology, Physiology, and Weed Science at Virginia Tech, Rachel is researching new viable control options for the tree species.

Fortunately, research on this subject started back in 2009 in Ohio, Pennsylvania, and Virginia, supplying Rachel and her team with ample background knowledge on the problem. Unfortunately, the tree’s uninterrupted growth has created expensive projects dedicated to the repairment of roads and sidewalks, along with re-cutting the unstoppable trees, adding time and fiscal pressure to the outcome of the research.

The tree-of-heaven is less like an actual tree and more like a 30- to 40-foot weed that refuses to stop growing. Rachel refers to it as “kudzu in tree form.” Native to China, the tree-of-heaven was introduced into North America in the late 1700s and has since spread to occupy thousands of acres of land. The invasive tree and its persistent growth has most famously been summarized in the introduction of the popular Betty Smith novel, A Tree Grows in Brooklyn:

It grows in boarded up lots and out of neglected rubbish heaps. It grows up out of cellar gratings. It is the only tree that grows out of cement. It grows lushly...survives without sun, water, and seemingly earth. It would be considered beautiful except that there are too many of it.

Their rapid growth has caused a multitude of problems, such as the blocking of roadways, the disruption of crop growth, and an overall decrease in biodiversity of the landscapes where it’s found.

As an invasive species, Ailanthus altissima releases biochemicals that prevent competing plant life from growing. And while it’s busy disrupting the growth of plants around it, the tree-of-heaven spreads simultaneously through its seeds and its roots, which re-sprout rapidly even after they’ve been cut.

Rachel described a project done in Shenandoah National Park involving the treatment of just a few trees along a section of road. The cost was about $25,000, and after seeing the treated section in person, Rachel could tell the treatment will have to be redone and then redone again. If it’s not, all of the scenic views will turn into views consisting only of the tree-of-heaven.

As methods of cutting and managing the rapid growth of the tree-of-heaven continue to prove costly and, for the most part, ineffective, researchers have had to think outside the box to look for new solutions. This is where Rachel and her research comes in.

Rachel is studying two different species of naturally occurring fungi, Verticillium nonalfafae and Verticillium dahliae. These species of fungi are extremely important because in addition to the fact that they grow naturally and don’t have to be harvested, they are responsible for what’s known as “verticillium wilt,” a fungal disease that can invade plants through their roots when the fungus is in the surrounding soil.

Once the verticillium makes its way into the plant, it blocks the vascular bundles, which perform the function of moving water and nutrients up and down the tree. This verticillium wilt affects hundreds of trees, plants, and shrubs, including Ailanthus altissima.

Girl holding a machete up to a tree
Rachel Brooks wields her machete. Photo credit: Andy Dechaine.

Prior to researching this invasive tree and alternative methods of controlling it, Rachel gained rich field experience through several organizations around the country. After graduating from the University of Vermont with a bachelor’s in environmental science, Rachel spent the next five years moving throughout the United States in different research projects and organizations. She did field work for the University of Minnesota studying effects of increased temperatures on prairie systems. And in between summers spent living onsite, Rachel also worked for a non-profit land conservancy in her home state of New Hampshire, helping write legal documents. Eventually, she found herself in Washington state conducting outreach programs and fisheries research for the U.S. Fish and Wildlife Service.

Although she had gained a variety of professional experiences, Rachel still felt unsure as to where she wanted to end up--and unsure of what she wanted to be studying in the future. That’s when she heard that Dr. Scott Salom of Virginia Tech needed help collecting insect samples. After getting her foot in the door sending samples to Virginia from Washington, Rachel made the decision to apply to grad school and officially become a part of Salom’s research team.

Her journey getting to Virginia Tech was not defined by a single moment of clarity or pure instinct. Rather, it was defined by the culmination of all that she had experienced up to that point. After spending so much time in the field as a firsthand witness to the changes our environment is undergoing, Rachel knew it was time for her to dedicate her knowledge and available resources to addressing the mounting problems we see with invasive species.

“The fact that the composition of our forests is changing during my lifespan played a huge role. We’re losing our beech trees, we're losing our hemlock trees, we're losing our ash trees,” she lamented.

So far, the verticillium fungi have proven to be extremely effective in combating the tree-of-heaven, specifically targeting the invasive tree without disrupting other plant species growth in the surrounding ecosystem, which is another huge plus of this research. 

tree canop from below
Tree canopy declining from below view
These tree canopy photos show a cluster of Ailanthus altissima, tree-of-heaven, before and two months after Verticillium was introduced to the environment. Photo credit: Rachel Brooks.

This biological control option could have incredibly valuable future implications in investigating effective ways to prevent the propagation of invasive tree and plant species. Using another naturally occurring species to combat the invasive tree opens the possibility of scaling back on traditional control methods, such as traditional herbicides or mechanically cutting them down.

The ultimate goal in Rachel’s research is to create a purchasable product to stop the growth of the invasive tree. The biggest challenge now is running the tests and measurements that are required to register the product with the U.S. Environmental Protection Agency. The regulation process is costly, and if even one of the safety tests required for registration is failed, the whole project could be scrapped. While it may seem like a daunting challenge, the public support is what pushes her:

“The support from the public for this project is probably the most encouraging and exciting part that wasn’t necessarily expected,” Rachel said.

General land managers who’ve been firsthand witnesses to the invasive tree have been especially encouraging, Rachel observed.  

Despite the unexpected wealth of public support, Rachel says there are still some people who misunderstand what the researchers are trying to do. Those outside the research who don’t understand the scope of the problem often get confused and sometimes even upset when they hear someone is working to “kill the tree-of-heaven.”  

Although public perception may not always be on their side, Rachel and her team’s biggest threat may not be realized just yet. While she hasn’t encountered any major setbacks, Rachel is always mindful that they could happen at any time. For example, once the trees wilt completely and stop growing, there is room for another species to grow. This “weed-shaped hole,” as it’s sometimes referred to, could simply create an opportunity for another invasive species to take hold.

The extensive nature of the project opens up a number of questions about the future of the environments they’re working to clear the tree of heaven from. As of now, the momentum of the research is pushing Rachel and her team forward, ready to investigate whatever they need to reach their goal.

“Besides the fact that everything’s working so far, the breakthrough won’t be until we’re handed the registration form [from the U.S. Environmental Protection Agency],” Rachel said.

And until that day comes, Rachel will continue to entrench herself in the world she works so hard to protect. Because if she isn’t in the lab growing pure cultures of fungus from her field samples, or bushwhacking in the forest just to find samples to begin with, she’s outside, just staying connected to what’s important.

“I like to spend time outside. If [I’m] ever stressed or stuck, spending time outside solves all of my problems,” she said.

And maybe this is what we should really take from Rachel. Maybe this insight is as valuable as her research and her extensive field knowledge. If we all spent more time reconnecting with nature and away from the stresses of our day-to-day lives, who knows what we might find? We might come across a foreign species. We might find a beautiful view we haven’t seen before. We might find ourselves. Heck, we might even find a control option to a costly and pesky invasive tree species.

Article written by Connor Buza in ​ENGL 4824​: Science Writing in Spring 2018 ​as part of a collaborative project between Fralin, the Department of English, the Center for Communicating Science, and Technology-enhanced Learning and Online ​Strategies (TLOS). Learn more.