A forest's diversity may begin somewhere you cannot see, as hidden fungal partnerships beneath the soil help shape the plants above

A forest ecosystem is characterised by a diversity of tree species, with some forming a canopy, whereas ferns, herbs, and other plant species fill the space below.

However, according to a recently conducted study, there may be more than meets the eye when it comes to this diversity and its causes.

It has been shown that the diversity of relationships that the trees forming a forest canopy have with fungi affects the variety of plant species that grow below the canopy layer.


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Thus, the study reveals yet another important factor affecting the plant ecosystem of a forest. In addition to rainfall, temperature, soil nutrient availability, and competition between different plant species, interactions between trees and mycorrhizal fungi may play an important role.

Mycorrhizal fungi are associated with plant roots.

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These fungi help plants to receive nutrients from the soil in exchange for carbon compounds that they provide from themselves. Different species of plants form different relationships with the fungi, and it has been shown that this may affect the whole plant community in a forest.

The study was performed by a team of researchers headed by Dr Jie Li from Beijing Forestry University.

Thousands of trees reveal a hidden pattern

The researchers conducted their field study during the summer of 2017. They established 437 circular plots in a temperate forest and tagged 34,850 trees of 72 species, along with assessing the vegetation present near the surface of the ground.

A total of 389 herbaceous plants and ferns were recorded.

The researchers merged field observations with data on mycorrhizal symbioses of the plants and trees. The symbioses of fungi and plants may have varied types depending on the relationship between fungi and plant roots.

While ectomycorrhizal fungi, or EcM, are found in association with roots and root tissues without penetrating into the roots, arbuscular mycorrhizal fungi or AM, infect roots and form structures to facilitate the exchange of nutrients.

The type of symbiosis played a significant role in the comparison of trees and plants growing below them.

Canopy trees had mostly EcM and diverse symbiotic fungi types, whereas herbaceous plants were mostly AM symbionts.

The study discovered a correlation between the type of mycorrhizal partnerships and plant diversity of the forest understory.

However, it doesn’t mean that fungi work independently. Rather, it is part of the interaction network where trees, fungi, soil properties, and understory plants affect each other.

The fungi beneath trees may change the plant community

But not all plants need fungi in the same manner.

While some herbaceous plants have very strong relations with fungi in the form of mycorrhiza in order to thrive, others require them only when living conditions become adverse. There are some plants which do not require any relations with fungi at all.

According to the results obtained in the study, it became clear that the variety of plant-fungi relations leads to greater diversity of forest floor flora. For instance, forests in which the majority of trees have ectomycorrhizal fungal relations tend to contain less diverse flora in their understory.

There could be several explanations for the phenomenon. One of them relates to the influence of fungi on the nutrient composition of soils. Fungal associations have been proven to have an impact on soil nitrogen.

Thus, there is an interesting link between the upper parts of the forest and plants growing beneath it.

Not only are the trees able to determine what type of fungi will be present in the soil near their roots, but also the relations between fungi and trees might influence the environment where other plants such as herbs and ferns live.

Rainfall also changes the underground equation

The scientists also considered nitrogen and phosphorus in soil, temperature, and precipitation to see what determines the understory diversity. Precipitation was found to be one of the factors that contribute to it by favouring plants that depend on mycorrhizae differently.

Mycorrhizae were also associated with the presence of soil nitrogen, which in turn affects tree growth and the plants below them.

It is found that more diverse forests, which consist of trees of various heights, have more understory plant species because the varied canopy structure provides more conditions for their coexistence.

It is found that forest diversity is determined by the complex interplay of climate, soil nutrients, canopy structure, competition, and mycorrhizae.

Scientists consider the connection between canopy trees, mycorrhizae, and understory plants to be some kind of trade-off of different mycorrhizae.

The forest floor tells only part of the story

These results reveal the extent to which forest ecology is hidden underground.

Even though trees, shrubs and herbs compete for light and space, fungi associated with the roots of plants may regulate nutrient fluxes and affect the composition of forest species.

The paper does not claim that fungi alone regulate forest biodiversity.

Rather, mycorrhizal associations combine with environmental factors (rainfall, temperature and soil conditions) and plant competition to affect plant growth under the canopy.

Such relationships might become particularly relevant under climate change since environmental alterations will probably affect fungal community composition and availability of plant nutrients.

Knowledge about underground connections might facilitate better forest conservation and restoration because protection of a particular ecosystem will not depend solely on the presence of certain trees, but also on organisms that grow around the roots of those trees.

According to the authors, additional studies are required to understand how specific mycorrhizal associations affect plant coexistence under environmental variations.

In any case, the presented research provides one more explanation for the high diversity of forest understory in areas with visually identical trees: some part of the answer lies hidden in the soil.

FAQ

1. What are mycorrhizal fungi?

Mycorrhizal fungi establish associations with roots of plants and can help plants get nutrients from the soil in exchange for carbon compounds produced by plants.

2. How can fungi affect forest diversity?

Fungal associations can affect nutrient availability and the ability of plants to grow and compete. It was determined that forest diversity is higher in forests where there is a greater number of fungal relationships.

3. What is the difference between AM and EcM fungi?

AM fungi penetrate into root cells of plants, forming structures responsible for nutrient exchange between the fungi and plants. EcM fungi develop outside root cells, surrounding plant roots.

4. What other factors affect understory plant diversity?

Rainfall, temperature, nitrogen and phosphorus content of soil were studied, along with forest density and structure.