As the deciduous trees slowly lose their leaves, they start to reveal long-held secrets. At first the most obvious secrets are the nests of birds that you may have spent the summer looking for. Skillfully placed in areas that are thick with summer foliage, the nests become obvious once the veil is dropped, and it can be amazing to discover a nest that you may have passed dozens of times while you were completely unaware that it was there. Despite how frustrated you may feel, you really have to admire the birds that built them.

Then there are other “secrets” that might just fall into the unobtrusive category. This is usually the case with certain plants that were growing in plain sight; making no attempts to hide, but being lost in the background, nonetheless. These plants are often revealed either by the colors of their leaves, or by the sudden prominence of their fruits. Today’s focus is on one of these plants.

Bittersweet is a vine that can climb to the top of any tree that it manages to latch onto. In some cases a tree may be draped with a collection of these vines, which produce something that resembles a living web. The vines are aggressive, fast-growing, and can be potentially detrimental to the trees that they cling to because they compete for sunlight with their hosts. In the winter, extensive webs of bittersweet vines can accumulate large quantities of ice and potentially damage the trees by pulling them down under the added weight.

Another significant problem can occur when the vines are present for several years. Wrapping themselves around the trunks of trees, the vines grow thicker and stronger every year. If this goes on long enough, then the vines can actually start to strangle the trees that they are climbing, potentially killing the tree by cutting off the flow of water and sugars through the xylem and phloem tubes that serve the same basic purpose of human arteries and veins.

But, as always seems to be the case, there are exceptions. When it comes to bittersweet, there are actually two different species that we may be dealing with. The American Bittersweet (Celastrus scandens) is a native plant that has been prized as an ornamental species that produces gorgeous red berries with golden capsules that open in the autumn (see inset). These berries have been used in holiday wreaths, and they are also grown on fences and trellises to provide an attractive look to homes and gardens.

An added bonus is the fact that the berries of the American Bittersweet are rich in fats and they provide high-quality food for birds during the fall and winter. Birds swallow the berries whole, digest the fruits, and then poop out the seeds in their droppings. The plants rely on this arrangement for having their offspring spread to new places. The freshly deposited seeds need only survive the attention of small rodents during the winter months before they germinate, grow, and then produce fruits of their own. Thus, birds end up accidentally planting more valuable food plants.

The “problem” occurs when we learn that there is a second species of bittersweet that also spreads across the landscape, the Asiatic Bittersweet (C. orbiculatus). This species is highly invasive and detrimental to the landscape. First collected in Japan and then introduced to North America as an attractive landscaping plant in 1879, the Asiatic Bittersweet “escaped cultivation” almost immediately. This happened because of those attractive berries that birds are so happy to consume.

Unfortunately, the berries of the Asiatic Bittersweet do not provide the same high-quality food as the native American Bittersweet. These berries look very similar, but they lack the high levels of valuable fats that are found in the native variety. As a result, the berries are often described as “starvation” food. Birds will eat them, but they don’t get much energy from them. They are often used as a food of last resort; better to eat something rather than nothing at all.

Further exacerbating the problem is the fact that the two species will readily hybridize. This tends to spread the negative characteristics of the invasive species, which threatens the existence of the native American Bittersweet. Sadly, the genie is out of the bottle, and it may now be impossible to stop the spread of the invader.

So, how do you tell the difference between the two species? Once again, it basically comes down to those attractive berries. The American Bittersweet has dark red berries that are covered by reddish outer capsules. The berries only occur at the ends of each branch of a vine. In contrast, the berries of the Asiatic Bittersweet are covered by golden-yellow capsules, and they can be found along the entire length of each vine. The invasive species produce more berries, so the birds tend to eat (and spread) more of them. It is a diabolical problem.

For now, why don’t you spend a little time being vigilant as you drive to and from work. The bittersweet plants (regardless of variety) are going to be at their most obvious at this time of year. I have a long drive to work and I am starting to notice them all over the place. In most cases, the combination of red berries and golden capsules make for an attractive splash of color that you will notice as you drive along. You will also begin to understand the huge challenge that might be involved in attempting to remove the invasive Asiatic Bittersweet. A good lesson on the dangers of bringing new species to new places.

Bill Danielson has been a professional writer and nature photographer for 29 years. He has
worked for the National Park Service, the U.S. Forest Service, the Nature Conservancy and the Massachusetts State Parks and he currently teaches high school biology and physics. For more information visit www.speakingofnature.com, Speaking of Nature on Facebook, or the Speaking of Nature Podcast.