The Invasive Amur Corktree

During a trip to my property in Adams County last winter, I stumbled upon a tree that was approximately four inches in diameter. The bark resembled that of Ailanthus (Tree-of-Heaven), but not exactly. Given the time of year, leaves were not present to help with an accurate identification, but the branching characteristics of the tree appeared to be opposite.  I knew Ailanthus had an alternate arrangement, and the bark did not resemble any of the typical species we think of that have an opposite arrangement. I made a note of where the tree was found and vowed to come back next summer to take another look when the leaves were still on the tree.

I had forgotten about the tree until this past fall. When I went back to make another attempt to identify it, I had a friend with me.  When we looked at the tree together, we were both stumped. It was something neither one of us could recall having ever seen before, and it was not something we'd learned in Dendrology class 30 years ago (or we had forgotten we learned it). 

We were able to pull a lower branch off the tree to take a closer look. It clearly had an opposite leaf arrangement (meaning the leaves were attached at the stem directly opposite of each other) and it had a bright yellow inner bark. So, I took some pictures and sent them to other forestry professionals I know, and a reply came back that it was a cork tree, an invasive species that is being found more often than in the past.

Identification and Ecology

As I began to research the cork tree, I discovered there are several different species that have been found in Pennsylvania, but the one I found is Amur corktree (Phellodendron amurense).  It is native to Eastern Asia and was first brought to the United States in 1850. Literature that I found states that the tree grows about forty feet tall and prefers full sunlight, but can grow in the understory.  The tree I found was in the middle of a 200-acre forest and was not in full sunlight, but was on the edge of a small opening.  The tree has compound, opposite leaves having 5-11 egg-to-lance-shaped leaflets with smooth edges on each leaf.  The leaves are 10-15 inches long, and the leaflets are 2½-4½ inches long and half as wide. The inner bark has a bright yellow color to it (which has been used as a dye in China).  As the tree matures, it develops a thick, corky bark that may also have a spongy feel to it. 

The tree is considered dioecious (meaning the male and female flowers are on separate trees). The fruit is round, about ½ inch in diameter, green in color, and turns black when they mature in the fall. Birds that eat the fruit, which remains on the trees long into winter, are one of the primary ways that this tree is being spread.  The seed can remain viable for several years in the seedbank and sprout to create new trees when there is a disturbance to release them.

Impacts

Impacts of the Amur cork tree include dense shade where stands develop, affecting the growth of native understory plants and hindering regeneration of the desirable overstory trees.  Amur corktree has also been found to be allelopathic, which means it puts out chemicals in the soil that will hinder growth of other species.  This reduces competition and increases its chances of survival. 

Control

There does not seem to be much published research about control techniques for the cork tree. It will resprout if simply cut down, and it has also been documented to sprout from the roots, so a long-term management plan should be developed to include monitoring and follow-up treatments, as necessary.

Young seedlings that are found can be pulled out by hand. The best time to do this is when the ground is very moist to make it easier to get the roots. Older trees will need to be girdled or treated with an herbicide to achieve control. If the tree is girdled, be sure to get a complete girdle around the tree or it may not be effective.  Cut stump treatments and hack and squirt using triclopyr or glyphosate products can offer better control. Trees with smaller diameters can be controlled using a basal bark treatment using triclopyr ester products.  Regardless of the treatment method used to control Amur corktree, plan to follow up with long-term monitoring and treatment to be sure the tree is completely controlled in the area. 

While corktree is not a widespread problem in Pennsylvania yet, it is here, and it has spread into forested areas.  As with all invasive plants, early detection and action is critical to stopping the spread and will save a lot of time and money spent on controlling well-established populations.