A long-term field study has shown that soil fertility plays a decisive role in determining the distribution of tree species associated with different mycorrhizal types.

The findings were published in the Journal of Ecology.

Most forest plants form symbiotic relationships with mycorrhizal fungi, among which arbuscular mycorrhizae (AM) and ectomycorrhizae (EcM) are the most common. AM fungi typically penetrate root cells and facilitate nutrient uptake, while EcM fungi form a sheath around roots and are more efficient at accessing nutrients in nutrient-poor environments. As a result, AM-associated tree species are generally more abundant in fertile soils, whereas EcM-associated species dominate in less fertile conditions. Although this distribution pattern has been widely documented, the processes that generate it have remained under debate.

To address this question, the study led by Drs. ZHU Meihui and WANG Xugao from the Institute of Applied Ecology, Chinese Academy of Sciences, analyzed more than 15 years of data from a 25-hectare forest dynamics plot in the Changbai Mountain temperate forest.

The researchers found that as soil fertility increased, EcM-associated tree species exhibited higher mortality rates and lower growth rates, while AM-associated species showed the opposite pattern, with improved growth and reduced mortality. These results support the environmental filtering hypothesis, which proposes that soil nutrient conditions directly regulate species performance and thereby shape community composition.

In contrast, the alternative hypothesis that trees actively modify soil conditions to reinforce their own dominance was not supported in terms of shaping spatial distribution. The researchers reported that EcM-dominated communities were associated with increased total phosphorus in soils and reduced losses of available nitrogen and phosphorus, these changes did not translate into a shift in tree dominance consistent with that hypothesis. Instead, plant-driven soil modification appeared to occur more slowly and did not override the primary role of existing soil conditions. Further analysis highlighted the importance of leaf litter nutrient balance (the relative proportions of elements such as carbon, nitrogen, and phosphorus) in regulating plant–soil feedback processes.

The study concludes that environmental filtering is the dominant mechanism structuring the distribution of mycorrhizal tree types along soil fertility gradients. The results also provide empirical support for the Mycorrhizal-Associated Nutrient Economy framework.

Figure 1. Conceptual framework illustrating two hypotheses for the distribution of AM- and EcM-associated tree species along a soil fertility gradient (Image by ZHU Meihui).