In temperate forests, canopy trees shape understory herb diversity through the fungal partnerships they make

Beneath the forest floor, fungal partnerships help to determine which plants thrive above. The underground fungal networks of canopy trees shape the diversity of the herbaceous plants growing at ground level, a new study finds. Forests with a greater variety of these partnerships support more diverse understory plant communities. 

Herbaceous plants make up the forest understory and play an essential role in nutrient cycling and leaf litter production. However, some forests support a richer mix of these plants than others, and why that is remains poorly understood. The success of herbaceous plants within forest communities depends on several factors, including fungi that help plants absorb and exchange nutrients from the soil.

Soil fungi form partnerships with plant roots, called mycorrhizal symbiosis, wherein they help plants absorb nitrogen in exchange for organic carbon compounds. These plant-fungus interactions can also influence the interactions and competition amongst neighboring plant species, including those that form the forest canopy.

Jie Li of the Beijing Forestry University in China and colleagues examined how the environment and mycorrhizal partnerships influence the verdancy and diversity of the understory in a temperate region.

In the summer of 2017, researchers marked 437 circular field plots and tagged a total of 34,850 trees across 72 species. They also sampled and surveyed the herbaceous vegetation that made up the understory of the temperate forest, totaling 389 species of herbaceous plants and ferns. Using a statistical framework, the team analyzed all of this data to calculate the diversity of plants, herbaceous and otherwise, within the field plots.

They also classified the fungal associations of these plants and trees on the basis of known databases and previous research. In the most common variation, mycorrhizal fungi penetrate deep inside plant root cells, promoting nutrient exchange. A less common variation involves the growth of mycorrhizal fungi around the exterior of roots and between root cells, never directly penetrating.

Within the field plots, trees tended to have the second type of mycorrhizal fungi, called ectomycorrhizal (EcM), or a mix, while the first type, called arbuscular mycorrhizal (AM) fungi, was prevalent among the herb species.

Across forests, partnerships with fungi shape the understory. Herbaceous plants are known to depend on such partnerships to different levels: some plants are unable to survive without mycorrhizal fungi, others can choose to call on them in adverse survival situations and others do not need them at all.

Across the field plots, herbaceous plants that need mycorrhizal fungi for survival were predominant, followed by those who could choose to use fungi when required, with nonmycorrhizal plants being sparse. Forests where fungal partnerships were a mix of these different forms were more diverse, with a richer variety of herbaceous plants making up the understory. Forests where trees primarily partnered with ectomycorrhizal fungi had less diverse understories.

In order to understand the environmental factors influencing understory diversity, Li’s team took into account the levels of nitrogen and phosphorus in soil along with precipitation levels and temperatures.

Climate patterns, primarily rainfall, were found to control herb diversity by favoring certain types and dependencies on fungal partnerships. In the same way, fungal partnerships influenced the presence and availability of soil nitrogen, influencing which trees did well, indirectly driving herb diversity in the forest understory.

“Canopy trees and understory herbs were predominantly associated with EcM and AM fungi, respectively, and their contrasting mycorrhization patterns differentially regulated herb diversity,” write the researchers. “This pattern suggests an intriguing trade-off whereby different mycorrhizal types jointly shape forest plant diversity.”

Denser forests were also found to be more diverse forests. Dense forests made up on trees that had different heights encouraged a greater variety of herbaceous plants to flourish. “Further research is needed to examine the effects of various mycorrhizal statuses and types on species coexistence across different environmental conditions,” add the researchers.

Reference: Jie Li et al., Mycorrhizal symbiosis and environmental conditions shape understory herb diversity in a large temperate forest region, Functional Ecology (2026). DOI: https://doi.org/10.1111/1365-2435.70323

Featured Image Credit: Mike Faulkner via Pexels