Chicken or egg?

Guest post by Jan Clavel

By now we know quite well that non-native plant invaders are fond of human disturbances. We have seen many times, across many different ecosystems, that these foreign plants use the more welcoming disturbed sites as their entry points towards new ecosystems. Roadsides and trails in particular make for great pathways for these non-native plants, which use them to spread out to new ranges that would have otherwise remained unattainable.

While this pattern clearly exist, we are still in the process of trying to understand exactly what it is about these disturbed sites that makes them so welcoming for non-native plants. Is it due to reduced competition? Abiotic changes to pH and nutrient availability? Or maybe increased seed availability thanks to transport by cars and blissfully unaware hikers? As is often the case in ecology the answer is most likely: “a bit of all of the above”.

Cars are known to transport seeds of non-native species along mountain roads, like here in northern Norway.

However, there is another factor that could prove to play a large role in facilitating non-native success and that has remained largely unstudied. That is, the relationship between plants and their fungal mutualists: mycorrhizas. This crucial symbiosis between plants and fungi, in which plants exchange sugars for precious nutrients, is a difficult one to study as it takes place wholly belowground and at microscopic level. However with ongoing improvements to our methods, it is becoming clear that these little fungi are instrumental in shaping the processes and composition of ecosystems all across the world. And as such, it is not unreasonable to think that they could shape how non-native plants benefit from human disturbance. 

To test for that hypothesis we went back to our trusty roads in the beautiful northern Scandes. The great thing about these roads is that not only do we already know that they favor non-native species thanks to our previous surveys (see chapter 2 here), there is also a clear distinction between the type of mycorrhizas that naturally associate with the local plants (called ericoid and ecto-mycorrhizas) and the type that non-native plants like to associate with (arbuscular mycorrhizas or AM). This clear distinction in mycorrhizal types is really helpful as we can simply track the changes in AM and have an idea of how impacted our non-native plants are going to be.

Part of the team digging for roots of non-native species along Norwegian mountain roads

After many a day of root collecting, soil sampling and mosquito swatting, we sent a few hundred of samples taken all over our mountains roads to our home base in Antwerp, where we went looking for mycorrhizas. We did so both visually, under the microscopes, to know the quantity of mycorrhizas there was in each root, as well as through DNA analysis, to know which ‘species’ of mycorrhizas were present . 

Figure 1: we found many more roots with arbuscular mycorrhizae in the roadsides than in the natural vegetation (left), as well as a much high diversity, with 23 OTUs (‘species’) unique for roadside roots.

By far the clearest result that came from all that work is that there is a very clear effect of the road disturbance on AM. There are way more of these arbuscular mycorrhizas in roadside roots than in the natural vegetation and they are also much more diverse! More AM in the roadsides means more non-native plants in the roadside, thus disturbance impacts mycorrhizas which in turn facilitate invasion success. Question answered, right? Well, if you have been around ecology for a while, you will know that things are never that easy. After all, the higher presence of AM could be a consequence of non-native plant success instead of a cause, or maybe these two things are wholly unrelated and they are both the consequence of some third factor, changes in pH for example. That’s what’s both fun and sometimes frustrating about ecology, it always keeps you guessing. But what we have found for sure is that this big difference in mycorrhizas is truly there. And that opens many exciting future research opportunities that are only asking to be seized! That chicken-and-egg problem of who’s driving what? That needs to wait till next summer, when we’ll get back to see how things have changed over time.

So we can be clear here, roadsides are full of mycorrhizae that can benefit our non-native plant species. Yet, there is something rather funny with this observation: while non-native plants have not yet reached the highest elevations of our mountain roads, that is not for a lack of adequate mycorrhizas. Indeed, contrary to our expectations we found AM to be present all over our elevation gradient, even well above the highest elevation reached by non-native plants. Moreover, the difference in mycorrhizas between roadside and natural vegetation was also constant all over that gradient. That tells us that adequate mycorrhizas are already available for non-native plants climbing up the roads, which is one less potential barrier to further invasion. So that is another chicken-and-egg problem for which we do have the answer already: while we don’t know who came first in the system, non-native plants or their mycorrhizae, the mycorrhizae where clearly first to reach the top. That non-native plants haven’t colonized highest elevations yet, is thus not from a lack of mycorrhizae. At least one potential explanation we can erase from the drawing board.

Fieldwork with a view, above a Norwegian fjord close to the city of Narvik

More information:

Clavel, J., Lembrechts, J., Alexander, J., Haider, S., Lenoir, J., Milbau, A., … & Verbruggen, E. The role of arbuscular mycorrhizal fungi in non‐native plant invasion along mountain roads. New Phytologist.

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