Why do rival plants coexist? The secret is in the soil beneath the oaks

How can plants that compete for the same resources grow in the same area without one driving the other to extinction? Ecologists have been trying to answer this question for decades, and a surprising new explanation has now emerged: the soil surrounding oak trees acts as a silent mediator that restrains the dominant species and gives an advantage to weaker ones, allowing both to coexist.

A team of researchers led by Ezequiel Antorán and Joaquín Calatayud from the Global Change Research Institute at Rey Juan Carlos University (IICG- URJC) has published a study in the journal Ecology Letters — one of the most prestigious journals in the field — revealing how Pyrenean oaks (Quercus pyrenaica) alter the chemical and microbial composition of the surrounding soil.  This creates conditions different from those found elsewhere in the landscape. These conditions reduce the germination of gum rockrose (Cistus ladanifer), the dominant and more aggressive species, while promoting the growth of laurel-leaf rockrose (Cistus laurifolius), the weaker species. The result: both species coexist.

A mediator in the shadows

The team designed experiments in which seeds from both rockrose species were grown in soils collected near oak trees and compared with soils unaffected by oaks. The results were clear: seeds of the dominant rockrose germinated less successfully in oak soil, while seedlings of the weaker rockrose grew better in it. These effects are driven both by chemical compounds that oaks accumulate in the soil — through their roots and decomposing leaves — and by the specific microorganisms living in that soil.

“It is as if the oak redistributes resources from below,” explains Ezequiel Antorán. “Without its presence, gum rockrose ends up dominating and laurel-leaf rockrose disappears. But with the oak acting in between, there is room for both,” adds the IICG-URJC researcher.

Theory and nature agree

To test whether these effects were sufficient to sustain coexistence over the long term, the team developed computer simulations based on the experimental data. Ezequiel Antorán worked on these simulations while visiting IceLab at Umeå University. 

Linking ecological theory with mathematical models is one of the core strengths of IceLab. The computer simulations developed during Antorán’s research stay reproduced with remarkable accuracy the distribution patterns observed in nature.

The models reproduced the distribution patterns observed in nature with remarkable accuracy: the weaker rockrose species clustered near the oaks, while the dominant species thrived farther away. The simulations also showed that both populations remained stable over a period of 100 years.

"What makes this study special is that we not only explain the 'why' through experiments and field observations, but, thanks to the simulations I carried out while I was a visitor at IceLab in Umeå, we can also see the true importance of the mechanisms isolated in those experiments. Their effects hold up mathematically over time and are reflected in the field. That is quite difficult to achieve in ecology," says Ezequiel Antorán.

Why this matters beyond the forest

The findings have implications that extend far beyond the Central Mountain Range in Spain, where the study was carried out. Understanding how indirect interactions between species maintain biodiversity is crucial for ecosystem management, habitat restoration, and predicting how plant communities will respond to climate change or species loss. If we remove ecosystem mediators — trees such as oaks — we may unknowingly disrupt the balances that allow many other species to coexist.

The study involved researchers from the University of Valladolid, Umeå University (Sweden), the Doñana Biological Station (CSIC), and the Autonomous University of Madrid. Funding was provided by the Spanish Ministry of Science, Innovation and Universities and the Community of Madrid.

Connection to IceLab at Umeå University  

The discovery was made possible through close international collaboration. A key part of the research took shape at Umeå University, where researcher Ezequiel Antorán was a guest researcher in 2025 and 2026. 

"I took advantage of the strongly interdisciplinary environment at IceLab in Umeå to carry out the computer simulations and data analysis together with Rubén Bernardo," explains Ezequiel Antorán. "The scientific ecosystem at IceLab, which is highly focused on connecting mathematics with theoretical ecology, provided the perfect environment for digitally testing what we observed in the field," he adds.