Some plants are able to take over uninhabited areas such as sand dunes, volcanic substrates and rocky areas. The first settlers have specific traits that allow them to grow in such hostile environments. Other plants lack such properties, but will soon follow these pioneers. Ricardo Martínez-García from the Center for Advanced Systems Understanding (CASUS), an institute of the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), and collaborators from Spain and Brazil investigated the types of interactions between different species in these newly conquered areas with the help of a mathematical model based on existing knowledge of root physiology. Their new model links the type of species interaction with the general availability of a scarce soil resource. It also reveals the best strategy for the pioneer who can exploit a resource that is not freely available.
Plants interact with each other in many different ways and very often we see one individual supporting another individual of a different species. The term experts use for this is facilitation. With symbiotic facilitation, both plants support each other. With commensalistic facilitation, the cared for plant has no positive or negative impact on its benefactor. The third type of facilitation is called antagonistic facilitation. Here the cared for partner benefits at the expense of the benefactor. For example, the latter leaves a self-produced resource to the partner, even though he could actually use it himself. The benefactor seems to ‘accept’ this situation: neither some kind of defensive reaction against the removal of resources nor a complete cessation of production can be observed.
“There is an ongoing debate about whether antagonistic facilitation actually exists. Our study provides a clear result: this kind of interaction between plant species could occur in nature,” says Dr. Ricardo Martínez-García, CASUS Young Investigator and corresponding author of the study. which will be published in the second October issue of the magazine New phytologist. “To experimentally prove antagonistic facilitation requires a major effort. To begin with, it must be ruled out that this type of interaction is neither symbiotic nor commensalistic facilitation. Moreover, it must be shown that it is not a classical competition in which both plants harm each other in the struggle for resources.”
Plants as miners
Both Martínez-García and Ciro Cabal (King Juan Carlos University, Madrid, Spain) and Gabriel A. Maciel (South American Institute for Basic Research, São Paulo, Brazil), the two lead authors of the study, focused their modeling efforts on an example from nature where the existence of antagonistic facilitation has long been suspected: pioneering plants began to grow on uninhabited land and other plants soon followed in their wake. Such pioneer plants can adapt their environment in such a way that the availability of certain scarce soil resources, such as nitrogen and phosphorus, is increased. Their abilities certainly help them thrive and they don’t seem bothered by opportunistic plants appearing to help themselves at the buffet. The bottom line is that the pioneer still benefits from its special qualities. From an experimental point of view, the pioneer plant example appears to be a manageable system. Nevertheless, 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 provide a strong argument for the existence of antagonistic facilitation in these pioneer areas. It goes without saying that this type of interaction probably occurs 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 with low and medium resource availability, antagonistic facilitation is the best strategy. This too was suggested some time ago, but has not been supported by experimental data or theoretical models until now.” Consequently, the research team could not only provide reliable results for the general existence of this type of interaction. In fact, antagonistic facilitation is even the optimal interaction between two plant communities under certain environmental conditions.
As the soil changes over time and more and more plant species bloom, the interactions between the species change. Although the pioneer species continue to increase resource availability, this no longer affects the other plants due to the generally good resource situation. The phase of antagonistic facilitation is over and all plants compete with each other. Further in the future, the mining capacity of the pioneer will even become a burden in this competitive battle. The pioneers are at a disadvantage. Ultimately, other plant species prevail in the competition and there are no longer any pioneer plants to be found on the site.
How root modeling helps explain ecological patterns
Modeling is an important tool in ecology because it allows hypotheses to be tested and ideas that are difficult to investigate in field or laboratory experiments. In these cases, computational simulations can help understand ecological dynamics and patterns and even guide the design of field and laboratory experiments. In a 2020 Science article Cabal, Martínez-García and others presented a mathematical model that predicts the density and spatial distribution of roots of interacting plants. A comparison with greenhouse experiments showed a large overlap with the model’s prediction.
For the New phytologist In the study, the 2020 root model has been expanded and refined to reflect the interaction of pioneer plants with their environment and with other plants. The model takes into account, among other things, the dynamics of a high-demand soil resource (input, decay, availability to the plants, mining properties of the pioneer plants), the size and shape of the plants’ root systems and the costs of growing. and maintaining roots, mining the resource and transporting it within the plants.
