Mathematical model explains how plant communities evolve when uninhabited lands are conquered

Some plants are able to appropriate uninhabited spaces such as sand dunes and volcanic substrates. These first colonizers have specific characteristics that allow them to develop in such hostile environments. Other plants do not have these characteristics but will soon follow these pioneers.

Ricardo Martínez-García, from the Center for Understanding Advanced Systems (CASUS) at Helmholtz-Zentrum Dresden-Rossendorf, and collaborators from Spain and Brazil studied the type of interaction between different species on these newly conquered lands, using a mathematical model of plant root physiology.

Their model links the type of interaction between species to the general availability of a scarce soil resource.

Plants interact in different ways and very often we see one individual supporting another of a different species. Experts use the term facilitation for this. With symbiotic facilitation, both plants support each other. With commensalistic facilitation, the nourished plant neither positively nor negatively affects its benefactor.

The third type of facilitation is called antagonistic facilitation. In this case, the nurtured partner benefits at the expense of the benefactor. The latter, for example, leaves the partner with a self-produced resource, when in fact he could use it himself. The benefactor seems to “accept” this situation: there is neither a defensive reaction against the removal of resources, nor a complete cessation of production.

“There is currently a debate about the existence of antagonistic facilitation. Our study provides a clear result: this type of interaction between plant species could occur in nature,” says Dr. Ricardo Martínez-García, a young researcher at CASUS and corresponding author of the study published in New phytologist.

“Experimentally proving antagonistic facilitation requires a great deal of effort. To begin with, it must be ruled out that this type of interaction is neither symbiotic nor commensalistic facilitation. Furthermore, it must be shown that it is not a classic competition where the two plants harm each other in the fight for resources.”

Plants as miners

Martínez-García, together with Ciro Cabal (King Juan Carlos University, Madrid, Spain) and Gabriel A. Maciel (South American Institute for Fundamental Research, São Paulo, Brazil), the study’s two lead authors, focused their modeling efforts on an example from nature where antagonistic facilitation had long been suspected: pioneer plants starting to grow on uninhabited soil and other plants soon following in their footsteps.

These pioneer plants can modify their environment to increase the availability of certain scarce soil resources, such as nitrogen and phosphorus. Their abilities certainly help them thrive, and they don’t seem to be bothered by opportunistic plants that help themselves to the buffet.

Ultimately, the pioneer plant always benefits from its special characteristics. From an experimental point of view, the pioneer plant example seems to be a manageable system. However, practitioners have not yet been able to determine with certainty the type of interaction, for this example.

The results of the model presented here now constitute a strong argument in favor of the existence of antagonistic facilitation in these pioneer areas. Of course, it goes without saying that this type of interaction probably exists not only here, but also elsewhere in nature.

“Our model also shows that plant interactions are an emergent property of resource availability,” Cabal adds.

“It turned out that in environments where resource availability is low or intermediate, antagonistic facilitation is the best strategy. This too was suggested some time ago, but so far it has not been supported by either experimental data or theoretical models.”

The research team was thus able not only to provide reliable results on the general existence of this type of interaction, but also to demonstrate that antagonistic facilitation is the optimal interaction between two plant communities under certain environmental conditions.

Over time, the soil changes and more and more plant species flourish, which changes the interaction between species. Although pioneer species continue to increase resource availability, this no longer affects other plants due to the overall resource situation. The antagonistic facilitation phase is over and all plants are in competition with each other.

In the future, the extraction capacity of pioneers will even become an obstacle in this competition. Pioneers will be at a disadvantage. In the end, other plant species will win the competition and pioneer plants will no longer be present on the site.

How Root Modeling Helps Explain Ecological Patterns

Modeling is an important tool in ecology because it allows us to test hypotheses and explore ideas that are difficult to study in the field or laboratory. In these cases, computer simulations can help us understand ecological dynamics and patterns and even guide the design of field and laboratory experiments.

In 2020 Science Cabal, Martínez-García, and others presented a mathematical model that predicts the density and spatial distribution of interacting plant roots. A comparison with greenhouse experiments showed a large overlap with the model’s prediction.

For the New phytologist study, the 2020 root model was extended and refined to represent the interaction of pioneer plants with their environment as well as with other plants.

The model takes into account in particular the dynamics of a soil resource in demand (supply, decomposition, availability for plants, mining character of pioneer plants), the size and shape of the root system of plants and the costs of growth and maintenance of the roots, exploitation of the resource as well as its transport within the plants.