Historically, studies that aimed to identify how invasive plant species interact with their new environments tend to focus on aboveground interactions. The majority of these studies have an emphasis on patterns of herbivory, and other aboveground influences, rather than what’s going on beneath the forest floor. There are several reasons for this bias, but for one, these underground interactions are exceedingly more complex to study. Even inserting probes into the belowground mesocosms essentially changes its structure and function. Lucky for us, we live in a time of technological advancement when cheaper, more reliable techniques have progressed. In a new paper in Ecology & Evolution, scientists Krystal Nunes and Peter Kotanen use some of these new techniques to study how one widespread invasive plant interacts with its aboveground and belowground environment.
Cirsium arvense, commonly known as the creeping thistle is native throughout Asia and Europe. It’s known for producing abundant amounts of nectar, which diverts generalist pollinators away from North American flowers, effectively reducing its competitor’s reproductive success. The North American invader has such a wide distribution mainly for this reason. Additionally, as species escape from their native range, they too escape from their natural predators. Together for millennia, herbivores that co-evolved with Cirsium arvense can recognize the plant as food, and easily digest the plant material. Though, upon its arrival on a new continent, there seems to be a learning curve with its potential herbivores. The time it takes for those hungry animals to recognize C. arvense as food was more than enough time for it to spread throughout temperate North America, especially with its copious, enticing, nectar production.
To gain an in-depth ecological perspective of this plant, Nunes and Kotanen collected seeds from three different populations of C. arvense across a 509-km latitudinal gradient in Canada. They germinated seeds and grew the plants for 4 weeks, eventually transplanting all representing cultivars to each of the three sites. In a fully factorial design, plants where assigned with one of the following treatments: aboveground enclosure, belowground enclosure, dual enclosure or lack of any enclosure. With this design, these researchers could begin to assess the relative importance of aboveground, vs. belowground interactions. Knowing that this invasive plant pairs with arbuscular mycorrhizal fungi, they hypothesized that belowground interactions to have equal to, or greater importance on plant fitness than aboveground interactions.
Quite clearly, their results indicate that plants grown within enclosures grew more robustly. Unsurprisingly, plants grown with the combined, dual enclosures had the highest biomass across all three sites, while plants exposed to both above and belowground biota grew most stunted. Intriguingly, but like the researchers predicted, plant performance was better in individuals with belowground enclosures compared to plants with aboveground enclosures. Interpreting the interaction of seed location and plant performance was a bit more challenging.
Unexpectedly, seeds sourced from the mid latitude location grew most poor at every latitude. This was surprising to me because I had predicted that directional selection would impose itself on more northern plants, as colder temperatures would limit photosynthetic output, thus resulting in generally more stunted plants. Though, that wasn’t the case. Why would mid latitude plants grow the least, in every location, even the location they were sourced from? After investigating the text and figures in this publication, I do have another testable hypothesis that could explain the relationship between plant growth and seed source.
In addition to testing plant performance, these scientists quantified and identified the potential belowground herbivores by closely examining soil samples at each location. The community of mesofaunal soil organisms was most rich in the intermediate latitude, and the number of individuals counted at that same location was also generally high. I think this is a key component largely left out in the study. I hypothesize that over time, populations of C. arvense growing at mid-latitude where the highest species diversity of potential belowground herbivores coincides has changed the evolutionary trajectory of those plants. Individuals naturalized in the mid-latitude region deal with the most diverse mesofaunal communities. Thus, individuals that produce the most anti-herbivory compounds have a better change of reproducing. These plant exudates are exceedingly “expensive” for plants to synthesize, but in these mid-latitude regions, really do enhance plant fitness in the long run. The reason for the stunted growth in plants sourced from the middle latitude; they are allocating more resources to secondary compounds as opposed to plant tissues.
This study is a great example of a transplant experiment, and like all good science, we can definitely build on this. Ultimately, they found that belowground interactions had a greater influence on fitness than aboveground interactions. This mechanism still remains unclear, but it is a great start. They stated within the study that they wanted to focus on the plant-herbivore interaction, which made the paper quite a concise read, but I would love if they took this a step forward in an additional study. If they repeated this experiment, and added a fungicidal treatment they could gage importance of the invaders mycorrhizae. Furthermore, they should isolate the and quantify the secondary compounds in each plant genotype. I bet that the more secondary compounds produced, the less energy will be allocated to plant growth. This would link latitudinal mesofaunal diversity with plant strategy, and highlight the dynamic evolutionary nature of an adaptive invasive weed that competently competes with other species living from the forest floor.