An invasive species of plant called “the scourge of the South” has a new strike against it. Recent research shows that the impact of kudzu is more troublesome than had been previously thought. When it takes over ecosystems, this invader causes soil to surrender its carbon, releasing it as greenhouse gas.
Kudzu is one of the most impressive invasive species in the world. Introduced to the US as a handful of plants in 1876, this invader now occupies over 3m hectares (over 11,500 sq. mi.) of land in the US, largely in the southeast of the country. It is estimated to be “consuming” land in the USA at a rate of 50,000 hectares (123,552 acres) per year.
If anything could be said to grow like a weed, it is kudzu, which can extend by up to a meter every three days. The plant moves across terrain like a wave, smothering everything in its wake—trees, utility poles, and even buildings.
It is a member of the legume family of plants—like beans—and it grows in a vine-like manner, laying down roots whenever it comes into contact with the ground. Originally introduced as an ornamental plant and then used for livestock feed and erosion control, it has overrun entire ecosystems, destroying native long-needled pine forests, woodlots, and grasslands alike.
Kudzu has indirect effects as well. Most notably, it carries a foul-smelling, invasive insect called the kudzu bug. Unfortunately, the bugs' taste extends beyond its namesake plant and includes other legumes, such as beans grown for human consumption, giving kudzu an impact on agricultural productivity.
Kudzu’s total cost to the US economy is estimated to be in excess of $500 million annually, and that cost may be set to increase. Rising temperatures and lengthened growing seasons in the northernmost front of the kudzu’s range are creating a welcoming environment for the vine. Where it was once restricted to southeastern states, Kudzu has made it as far north as New Jersey and Ohio.
New research suggests that kudzu’s negative impact may extend beyond these problems. Its invasion may also be contributing greenhouse gases to the atmosphere by altering soil composition.
What lies beneath
Soil holds a phenomenal amount of carbon. In fact, there is more carbon stored in soil than in the atmosphere and all living terrestrial plants combined. Soil carbon is composed of dead matter and waste from plants and animals, as well as a vast population of microbes. (The plant matter is mainly dead leaves, but it also includes dead roots, as well as stems, branches, and tree trunks that have fallen to the ground.) Collectively, this material is known as soil organic matter.
The carbon in the soil organic matter largely stays in the soil, which acts as an enormous reservoir. Over time, microbes release this carbon as greenhouse gases—carbon dioxide and methane—at a rate that’s determined by its susceptibility to microbial digestion.
The problem with kudzu is that it changes the rate at which the soil organic matter is digested.
In a paper published in the journal New Phytologist, plant ecologist Nishanth Tharayil and graduate student Mioko Tamura of Clemson University show that kudzu invasion results in an increase of carbon released from the soil organic matter. Tharayil and Tamura investigated the impact of a kudzu invasion in native pine forests. They found that although kudzu increased the amount of leaf material contributed to the soil, soil carbon actually decreased by nearly a third in those forests.
Tharayil and Tamura attribute the release of carbon from kudzu-invaded forests to the fact that kudzu adds material to the soil that is easily digested, especially relative to that produced by pine. Simply put, kudzu leaves and stems are easy for microbes to eat; pine needles and stems are not. This means that carbon has a chance to get locked into the soil when it’s in the form of waste from pines; it gets released when it’s kudzu.
This has a three-fold effect. First, over time, the hard-to-degrade pine matter decreases in abundance. Second, the kudzu material “primes” the soil microbes, making them more effective at degrading the plant material in the soil, including that previously contributed by pines. Finally, the kudzu matter is more rapidly degraded. The net result of these three effects is that plant material doesn’t persist like it did in the pine forests.
The south will rise again?
Tharayil has estimated that kudzu invasion might cause the release of 4.8 million tons of carbon per year. This is the equivalent of the amount of carbon stored in roughly 5 million hectares (12.3 million acres) of forest, or the amount released by burning 2.3 million tons of coal. It’s also the annual carbon footprint for a city of 1 million.
The release of this amount of carbon into the atmosphere could itself contribute to global warming. This could create a feedback effect, as elevated temperature would enable kudzu to extend its range to more northern latitudes.
Not all news from Tharayil and Tamura is bad. The researchers also looked at the impact of the invasion of another noxious weed, knotweed. They found that knotweed drove a net increase in carbon locked away in the soil. This is not to say that allowing knotweed to run rampant is the solution to kudzu’s carbon-releasing menace. Instead, the findings point to the fact that plant composition in different ecosystems could be managed to optimize soil’s carbon retention.
In the meantime, though, we're going to have to find a way to restrain the plant that ate the South—for a large number of reasons.
This story originally appeared at The Conversation.