5 Ways Mycorrhizae Influence Forest Productivity

• Ectomycorrhiza is commonly found in forests, especially in temperate and boreal regions. Arbuscular mycorrhizae, a type of endomycorrhiza, is more common in the tropics, grasslands, plantations, and deserts.
• A tree can have more than one type of mycorrhiza, each making a distinct contribution to its hosts.
• Mycorrhiza increases access and absorption of nutrients and water, protects against pests and pathogens, improves seedling establishment, and helps plants cope with stress.
• On large scales, mycorrhizal fungi impact species and structural diversity to influence forest productivity.

Mycorrhizal fungi are ubiquitous and associated with almost 90% of plant species. Most plants require mycorrhiza for their growth. There is a vast diversity of mycorrhizal fungi, which perform various functions for their host plants. The accumulative effect of these benefits is an improvement in forest productivity. Find out more about the roles mycorrhizal fungi play in a forest.

Forest Productivity Importance and Drivers

Forest productivity is the rate at which forest vegetation adds biomass over time due to photosynthesis. This refers to the cumulative increase in forest volume. Some definitions also consider forest productivity the net annual increase of forest volume.

Forest productivity can be expressed as gross primary productivity, the rate of total photosynthesis during the measurement time. Net primary productivity (NPP) is the rate of organic matter storage after use by respiration during the period of measurement.

Biomass accumulation by all plants in the forests, trees, shrubs, and herbs are included in forest productivity. Since biomass contribution by shrubs and herbs is small, NPP usually refers to tree biomass accumulation. Above and below-ground biomass can be counted to consider forest productivity.

Forest productivity is essential due to ecosystem service provisions like biodiversity and wildlife habitat creation and protection, carbon storage, and rainwater harvesting to replenish water aquifers. Forest productivity is crucial for people because they rely on forests for timber and pulp.

The productivity of a forest is influenced by the availability of resources like light, nutrients, and water, as well as the optimal conditions of the site, including factors such as slope, soil type, aspects, weather, and climate. Ecosystem dynamics and the web of life that develops in forests also help sustain a forest’s productivity. Mycorrhizae are integral to forests and help in providing many of these forests’ needs.

Mycorrhizae Found in Forests

Mycorrhizae is a symbiosis between mycorrhizal fungi and plant root tissue. The fungi get energy in the form of carbohydrates from the root tissue, and in return, the extensive network of fungal hyphae access and provide nutrients and water for roots.

The endomycorrhiza, particularly the Arbuscular Mycorrhizae (AM), are common and are associated with 80% of all plant species. They are present in many tropical regions, where mycorrhizae constitute 50% of all fungal associations with plants. Arbuscular mycorrhizal fungi are also found in grasslands and deserts. AM fungi are less prevalent in temperate and boreal forests but can be found associated with poplars, alders, eucalyptus, and olive trees.

Ectomycorrhizas (ECM) are commonly found in forests, especially temperate regions, with some tropical tree species. ECM is formed with fungal partners by 60% of all tree species. Ectomycorrhizas are found in shallow soils to deep roots growing 2-4 meters below the soil surface.

ECM fungus forms an external mantle around root tips, and its hyphae grow between root cells and may occasionally grow into root cells to get plant photosynthates. AM sends its hyphae into root cells.

Artificial applications of ECM are standard practice in tree nurseries for forest restoration sites degraded by mining or timber extraction.

Mycorrhizae help in keeping forests productive in the following ways:

1. Improve access to nutrients and water
2. Help in seedling establishment
3. Protect against pests and root pathogens
4. Cope with environmental stress and climate change
5. Maintain structural and species diversity

A tree can have more than one species or type of mycorrhizal fungi, which occupy different niches and provide various benefits to their host plants. One species may be essential for a particular nutrient’s acquisition and another for protection against stress or pests. High mycorrhiza diversity is crucial for the healthy functioning of woodland plant communities and forest productivity.

1. Improve Nutrient Availability for Trees Through Mycorrhizae

ECM and AM contribute to forest productivity by increasing the supply of water, a crucial element in photosynthesis, and nutrients for biomass formation and accumulation. The fungi help by improving plant access to nutrients and water, and through their role, in cycling phosphorus and nitrogen.

Only phosphorus (P) in inorganic forms is useable by plants, and it would be difficult for plants to absorb P in this form without help from mycorrhiza. With the fungi’s help, trees can get more P, as shown in Table 1.

• AM fungi recruit phosphate-producing bacteria to break down organic forms of P into inorganic forms. AM also produces acids to release P in fertilizers and rocks, thereby increasing P cycling and availability in soil.
• ECM can also mineralize P in rocks and organic P to make it available for tree roots.

Both endo- and ecto- mycorrhizae increase nitrogen cycling for the benefit of their hosts:

• AM cycle nitrogen (N) by acting on organic matter.
• ECM also acts on soil organic matter (SOM) composed of root exudates, dead matter from plants, microbes, animals, etc. They degrade SOM with oxidative and hydrolytic enzymes and nonenzymatic mechanisms to mineralize N. The organic forms are oxidized into small N-containing molecules, leaving carbon substrates behind for soil storage.

ECM also provides potassium, calcium, and magnesium to host plants by increasing the weathering of rocks and minerals through physical fragmentation, acidification, and complexation. Mycorrhizae also acquire sulfur and zinc for their host plants.

By improving availability and access, mycorrhiza increases plant growth and forest productivity. Mycorrhiza can help plants recover from nutrient deficiency, which helps restore degraded lands.

Table 1: “Mycorrhizal dependency of some forestry tree species saplings,” Govindu et al. 2020. (Credits: https://www.intechopen.com/chapters/74173)

2. Establishment of Seedlings

Ectomycorrhiza is essential for the establishment of seedlings. Without ectomycorrhiza in a site or nursery bed, seedlings find it hard to establish, even with artificial fertilizers. This phenomenon has been seen in pine transplantation worldwide. Adding forest soils helped seedlings due to the presence of mycorrhizal fungal propagules. Mycorrhizal contribution to the seedling establishment is often indirect.

In forests, mycorrhizal fungi improve soil structures by aggregating soils. AM hyphae bind soil particles to form aggregates, and glomalin secretions bind the aggregates to give hydrophilic macro aggregates. AM soil aggregation has been studied more than ECM. However, a few studies show that ECM does produce large-sized soil aggregates that are water stable. The improvement in soil structure also helps forest seedlings germinate and establish.

Moreover, in myco-heterotrophs, plants that parasitize fungi, for example, orchids, fungi support the establishment phase by providing carbohydrates, reversing the usual symbiosis in mycorrhiza. These beneficiary plants have tiny seeds with little or no stored carbohydrates, and the early association with compatible mycorrhizal fungi is vital for orchid survival and establishment.

Mycorrhizal benefits of protecting plants from environmental stress also help young seedlings establish.

3. Protection Against Pests and Diseases

Mycorrhizal fungi protect plants against pests and diseases in many ways.

AM are means of biocontrol of diseases. They reduce pathogen numbers by their presence as a competition to pathogens and by parasitizing nematodes. AM also enhances phytohormone, protein, and secondary metabolite concentrations that help plants fight pests and pathogens.

The outer mantle that ECM form around roots physically shields them from pest and disease attacks. ECM selects bacteria that are antagonistic against plant pathogenic bacteria and fungi. These associated bacteria, for example, streptomycetes, produce secondary metabolites and protect plant root tissue against pathogens without affecting ECM growth.

Moreover, mycorrhizal fungi help access nutrients and water and compensate for plant tissue damage and loss of functions by pests and diseases to increase plant resistance.

It has been observed that pine has survived Pine wilt disease caused by nematodes, in soils with a rich ectomycorrhizal population. The ECM helped pines by supplying water and overcoming drought.

4. Coping With Environmental Stress and Climate Change

Figure 1. “Mycorrhizal fungi alleviate abiotic stress affecting tree growth in temperate and boreal forests, by improving plant water and mineral nutrition and helping to adapt to stressful environmental conditions,” Adapted from Usman et al. 2021 (Image credits: https://www.frontiersin.org/articles/10.3389/ffgc.2021.742392/)

AM and ECM also help their hosts tolerate abiotic environmental stresses like drought, salinity, and flooding through various means.

Drought: Drought stress, also triggered by high temperatures, is one of the main reasons for lower forest productivity since it affects photosynthesis, nutrient uptake and transport, enzyme structure, and hormone balancing. As a result, it can cause osmotic, nutritional, and oxidative stress.

AM and ECM mycorrhizal fungi help plants develop drought tolerance by the following methods:

• The fungal hyphal network increases plant access and absorption of water and helps vegetation cope with lower soil water.
• ECM fungi trigger more lateral root formation during drought to increase water absorption. Similarly, mycorrhizal trees also have more root hair growth to enhance the water absorption surface.
• Improving soil structure through aggregation formation enhances the infiltration of precipitation and the soil’s water-holding capacity.
• Mycorrhizal fungi affect gene expression of plant aquaporin, which is needed for water transport within root tissue.
• Mycorrhizal fungi increase phosphate secretions to increase P cycling and make more nutrients available during drought.
• Mycorrhizal symbiosis lower osmotic damage by accumulating proline and soluble sugars.
• Water use efficiency in plants with mycorrhiza occurs due to more stomatal closure and abscisic acid signaling.

Salinity: Due to increased irrigation, more land is suffering from salinity. An accumulation of sodium (Na+) interferes with the uptake of potassium (K+) by roots due to ion similarity. Sodium can also be toxic to cell functioning and enzyme activity. Mycorrhizal fungi exclude sodium and improve water and nutrient absorption, especially K. AM fungi also enhance the activity of various antioxidants in roots to alleviate salt stress. In some trees with mycorrhiza, there is more leaf chlorophyll and less Na in shoots compared to trees without mycorrhiza.

Flooding: More AM fungi are present in water-logged soils compared to ECM. The tolerance of fungi to flooding depends on the tree species and the severity of stress. AM helps trees during flooding through nutrient acquisition and reducing ethylene production.

Temperature: There is little research on the benefits of mycorrhizal association during higher temperatures or heat stress. However, temperature can affect mycorrhizal fungal populations.

Higher temperature, radiation, salinity, flooding, and drought are also conditions associated with climate change. AM and ECM can help alleviate these stresses and maintain forest productivity, as shown in Figure 1.

5. Improve Vegetation and Structural Diversity

A structurally diverse forest is more productive. Structural diversity is more important than species diversity as an indicator of forest productivity. Species diversity’s importance lies in its contribution to structural diversity. AM associations have been shown to influence plant community species diversity and composition. Though the exact mechanisms are unknown, assistance in seedling establishment can be a factor.

In a structurally diverse forest, tree canopies are distributed in different strata. Higher species diversity increases growth forms and tree height, and canopy size. As a result, the tree canopies in the various forest strata can capture more solar light and fix more carbon, leading to increased NPP.

By increasing the species and structural diversity of forests, mycorrhizae also influence large-scale forest dynamics to increase forest productivity.

Practical Applications of Mycorrhizae

Mycorrhizae occur naturally in forests and support plant growth and health to contribute to forest productivity. Therefore, AM and ECM are cultured for artificial inoculation for large-scale forestry and reforestation projects. However, before practical applications, matching fungi to hosts, learning about lifespan in soils, and interactions with other microbes must be established. Minirhizotrons like CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager can be used with preinstalled root tubes to scan and study mycorrhizal fungi and root interactions to answer these research questions for broad applications of the versatile mycorrhizae.

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