Kudzu vs. Honeysuckle: Helping the "little guy" win

Competition, it’s something that many of us have experienced in one way or another in our lives. Whether it was for a little league tournament, video games, or a family game night, competition is something that everyone has experienced in one way or another in their lives. But competition is not a solely human experience. This phenomenon is something we see everywhere in the natural world. Lions fighting hyenas for food, gorillas competing for territory, or plants battling it out for water and light. These are just a tiny few in a seemingly endless list of competitive interactions we see in nature.

However, by this point, I think it’s clear that I will not be discussing lions or gorillas in this blog, but rather the intricate world of competition amongst plants. One characteristic of invasive species of any flora or fauna is its competitive advantage over native inhabitants. So, in this blog I will dive into the competitive advantages of the widely spread invasives, kudzu and honeysuckle, as well as the consequences of this competition on the ecosystem as a whole.

Competitive advantages

Both honeysuckle and kudzu are known to be highly successful invasives in the US, as is evident from their widespread range. This invasiveness can be caused by numerous factors, however one of the key differences between native species and invasives is the ability to out-compete the other. Honeysuckle, for instance, has been shown to bud earlier, hold onto its leaves longer in the fall, and hold-up better during freeze-snaps in springtime than its potential native competitors (McEwan et al, 2009). The three factors basically mean that honeysuckle has a longer period of time with leaves present than its neighbors and therefore can out-compete them to higher amounts of sunlight.

Similarly to honeysuckle, the battle for light is won by kudzu over native plants. Kudzu, we all know, is a vine that can cover pretty much anything in its path, once it gets a foothold (as is seen in Figure 1). With this ability to spread and cover entire landscapes, any native plant that would have previously grown there is no longer able to succeed. Additionally, kudzu has been shown to be able to withstand biomass removal (and even come back stronger afterwards), and kudzu seedlings can survive several weeks with no light (Frye et al, 2012).

Figure 1: Photo of kudzu entirely covering a house. (credit: Jonathon Van Buren)

Impacts of this competition on biodiversity

As you can imagine, this ability to over-grow anything below it, has dire consequences for the overall health and biodiversity of the effected ecosystem. For example, a 2008 study showed that a more mature (and therefore larger and more established) honeysuckle plant resulted in a significant decrease in the both density and species richness of herbaceous plants, seedlings and saplings growing under and around these invasive shrubs (Hartman & McCarthy, 2008). This pattern can be seen in Figure 2, however the bottom-most graph in that figure depicts a difference between honeysuckle and kudzu.

As was discussed earlier, kudzu grows over anything in its path, including full grown trees. We see that honeysuckle does not have this direct impact on overstory, or treetops in Figure 2. So, while kudzu indiscriminately takes over, on a surface level, honeysuckle does not impact the more mature trees.

Figure 2: From Hartman & McCarthy, 2008. Without diving into the details, we can see from this figure that the understory density (seen in the top three graphs) significantly decreased as the honeysuckle bushes matured and grew. This is because the amount of light reaching the understory and the overall accessibility of the understory is prevented by the thick clustering of honeysuckle branches above it. We also see that the overstory (tree tops) is not impacted by the honeysuckle, because this invasive is a shrub and does not reach heights to effect trees.

You may be asking, why does this biodiversity matter? Honeysuckle is a beautiful, sweet-smelling shrub and kudzu does create quite a lush, green blanket. Well, this biodiversity goes beyond the direct impact to native plants. There are other organisms that rely on natural, native ecosystems to survive. Once this balance is thrown off, animals that prefer native plants to survive on will be displaced. What once may have been an intricate collage of plants and animals, will become a monotonous landscape of invasives.

So, what now?

I know that these posts can seem a bit daunting and negative, but I always want to leave things off on a hopeful note! Because these plants have been studied for decades now, we have a pretty good understanding of how to try to control them. So not all hope is lost!

For honeysuckle, we know that they grow in both forested and open areas. However, they prefer higher light areas. This being said, avoiding the thinning of the forest canopy and forest fragmentation will lessen the likelihood of honeysuckle taking over that area (Luken et al, 1997). Clipping honeysuckle has been shown to have an impact on growth of the shrub. In forests, this technique may be all it takes to control honeysuckle. However, in more open areas, where honeysuckle prefers, clipping plus the use of herbicide have been shown to be a decent control (Luken & Mattimiro, 1991).

Kudzu is a bit trickier. It’s thought that the only plants that can truly compete with this vine are other invasives (such as Chinese privet and our beloved honeysuckle)(Blaustein, 2001). However, it has also been shown that planting certain grasses in the fall can help control kudzu once it is weakened (Everest et al, 1999). Biocontrol (aka bringing in things to eat the vine) has also proven to be a useful mechanism for controlling the spread of kudzu (Frye et al, 2013).

All of these control methods require persistence. Honeysuckle has been shown to come back with a vengeance if cut and then abandoned (Luken & Mattimiro, 1991). So, if clipping is the solution, it is suggested that this happens at least once a year, but more often if possible. Kudzu requires more than 2 or 3 years of biocontrol (Frye et al, 2012) but this is known to be a very effective method, if done correctly. All of these controls disrupt the competitive advantage these invasives have and give the native plants a fighting chance to survive.

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So, while competition is a natural part of the world, whether that be our family game night becoming heated, or a gorilla defending its territory, it can be a destructive force is left unchecked. The competitive advantage of invasive plants can be compared to an older and younger sibling wrestling, the little one doesn’t stand a chance. But we can at least try our best to give the little guy (in this case our native plants) any advantage we can.

References:

Blaustein, R. J. (2001). Kudzu’s invasion into Southern United States life and culture. The Great Reshuffling: Human dimensions of invasive species, 55-62.

Everest, J. W., Miller, J. H., Ball, D. M., & Patterson, M. (1999). Kudzu in Alabama history, uses, and control. Alabama A&M and Auburn Universities, Alabama Cooperative Extension System ANR-65.

Frye, M., Hough-Goldstein, J., & Kidd, K. (2012). Response of Kudzu (Pueraria montana var. lobata) Seedlings and Naturalized Plants to Simulated Herbivory. Invasive Plant Science and Management, 5(4), 417–426. https://doi.org/10.1614/IPSM-D-12-00001.1

Frye, M., & Hough-Goldstein, J. (2013). Plant Architecture and Growth Response of Kudzu (Fabales: Fabaceae) to Simulated Insect Herbivory. Environmental Entomology, 42(5), 936–941. https://doi.org/10.1603/EN12270

Hartman, K., & McCarthy, B. (2008). Changes in forest structure and species composition following invasion by a non-indigenous shrub, Amur honeysuckle (Lonicera maackii). The Journal of the Torrey Botanical Society, 135(2), 245–259. https://doi.org/10.3159/07-RA-036.1

Luken, J., & Mattimiro, D. (1991). Habitat-Specific Resilience of the Invasive Shrub Amur Honeysuckle (Lonicera Maackii) During Repeated Clipping. Ecological Applications, 1(1), 104–109. https://doi.org/10.2307/1941852

Luken, J., Kuddes, L., Tholemeier, T., & Haller, D. (1997). Comparative Responses of Lonicera maackii (Amur Honeysuckle) and Lindera benzoin (Spicebush) to Increased Light. The American Midland Naturalist, 138(2), 331-343. doi:10.2307/2426826

McEwan, R., Birchfield, M., Schoergendorfer, A., & Arthur, M. (2009). Leaf phenology and freeze tolerance of the invasive shrub Amur honeysuckle and potential native competitors. The Journal of the Torrey Botanical Society, 136(2), 212–220. https://doi.org/10.3159/08-RA-109.1