Ectomycorrhizal Inoculum: A Key Tool for Rehabilitation of Natural Forests

Abstract

Deforestation is among the greatest challenges the Earth is facing. The annual deforestation rate is more than 3%. To uplift the economic growth of any country, the forest cover should be at least 25%. To overcome this problem, rapid afforestation and reforestation strategies are required. Inoculation of ectomycorrhizal fungi growing efficiently in the biodiversity-rich regions could be a leading approach in this regard. Several ectomycorrhizal fungi have been reported in association with many coniferous and deciduous tree species growing in these regions. The success of this association is mainly based on the mutual exchange of nutrients between the symbionts. These ectomycorrhizal fungi can mitigate the stress conditions and enhance the seedling survival. Inoculation of these fungi with indigenous tree species of the region can greatly improve plant growth and survival. This symbiosis may play a major role in the function, maintenance, and evolution of biodiversity and ecosystem stability and productivity.

1. Introduction

Deforestation is indeed one of the greatest challenges that the Earth is facing, and has several detrimental impacts. The removal of forests results in the destruction of diverse habitats that support a wide range of plant and animal species. Many species, including those unique to specific ecosystems, may face extinction as their habitats disappear. Even if some areas remain untouched, the fragmentation of forests into smaller patches can isolate populations, limit genetic diversity, and make it difficult for species to survive and reproduce.

Deforestation releases stored carbon into the atmosphere, contributing to global warming. It disrupts climate patterns, impacting agriculture, water resources, and settlements. Forests play a crucial role in water cycle regulation, and deforestation leads to altered river flow and increased flooding. The removal of trees weakens soil structure, making areas prone to erosion and landslides, especially in hilly regions. Preserving forests is vital to mitigate these environmental challenges. Many indigenous communities rely on forests for their livelihoods, obtaining food, medicine, and materials for shelter and cultural practices. Deforestation can displace these communities, disrupt their traditional way of life, and lead to the loss of cultural and ecological knowledge. While deforestation may provide short-term economic benefits through activities like logging and conversion of land for agriculture, the long-term consequences can be severe.

Loss of ecosystem services, including clean water, pollination, and climate regulation, can have significant economic impacts. Planting new trees to replace those that have been cut down helps restore ecosystems and mitigate the impacts of deforestation. Reforestation projects are essential for combating climate change and preserving biodiversity. Implementing practices that ensure the responsible use of forest resources, such as selective logging and regeneration plans, can help maintain the ecological integrity of forests while meeting human needs for wood and other products. Supporting and promoting alternative livelihoods for communities dependent on forest resources can reduce pressure on forests. This may involve the development of sustainable agroforestry, ecotourism, or non-extractive economic activities.

An annual deforestation rate exceeds 3%. To uplift the economic growth of any country, the forest cover should be at least 25%. Addressing deforestation requires a holistic approach that considers environmental, social, and economic factors. International collaboration and policies that promote sustainable land use practices are crucial in mitigating the negative impacts of deforestation and promoting a more harmonious relationship between humans and the environment. Rapid afforestation and reforestation strategies are essential and effective remedies to address deforestation and increase forest cover.

Ectomycorrhiza is a type of mutualistic symbiotic association between certain groups of fungi and the roots of higher plants. In this association, the fungi do not penetrate the root cells; instead, they form a mycelium between the cortical cells. This mycelium is known as Hartig’s net. This relationship is beneficial for both partners. The fungus enhances the plant’s nutrient absorption capabilities, especially for minerals like phosphorus and nitrogen, which can be less accessible to the plant in the soil. In return, the plant provides the fungus with carbohydrates produced through photosynthesis. The fungal hyphae form a sheath around the outer surface of the plant root tips, creating a structure known as a “mantle”. The mantle layer offers protection against certain soil-borne pathogens, helping to enhance the plant’s resistance to diseases and helps plants in coping with stress. The hyphae which originate from the mantle layer extend into the soil, forming a dense network around the plant roots to play a role in improving water uptake by the host plant, especially in conditions of water stress [1, 2, 3].

2. Evolution of ectomycorrhiza

Numerous studies propose that diverse ectomycorrhizal associations evolved independently, occurring at least once in the Pinaceae and in various lineages of angiosperms [4, 5]. However, the geographic origins and subsequent spread of ectomycorrhizal associations are still unclear. Since ectomycorrhizas are predominantly found in boreal and temperate forests today [6], examined whether these associations originated in these environments and subsequently spread to tropical regions, or if ectomycorrhizas emerged independently in tropical regions. Until recently, the sole fossil evidence for ectomycorrhizas came from the roots of Eocene Pinaceae on Vancouver Island [7], potentially indicating a northern latitude origin for ectomycorrhizas.

3. Plants forming ectomycorrhizal association

Approximately 8000 species of seed plants, which represent about 3% of all seed plant species, engage in this type of symbiosis [8, 9]. Even though being a minority among seed plants, these ectomycorrhizal (ECM) species are incredibly significant ecologically and economically. This is because they play a dominant role in forest and woodland ecosystems across much of the Earth’s surface. Their presence shapes these ecosystems and influences the diversity and abundance of other organisms living within them [4]. Despite the prevalence of woody perennials, some non-woody plants also form ectomycorrhizal associations. For example, certain sedges (plants resembling grasses with triangular stems) e.g., Kobresia spp., and herbaceous species in the Polygonum genus (which includes various types of flowering plants), can engage in ectomycorrhizal symbiosis. This expands the diversity of ECM plant species beyond woody perennials [10].

It is believed that ectomycorrhizal associations are primarily found in temperate and boreal forests, with their presence elsewhere being sporadic and of lesser ecological significance. It is certainly true that dominant tree species in these regions, such as those from the Fagaceae, Betulaceae, Salicaceae, Myrtacea, Leptospermoideae, and Pinaceae families, typically form ectomycorrhizal associations in their natural habitats. Furthermore, the ectomycorrhizal habit exhibits specific adaptations that enhance nutrient uptake in temperate and boreal forests, as documented by [11].

However, contrary to this belief, extensive portions of land beyond temperate and boreal zones also host vegetation with a significant ectomycorrhizal component. For instance, Arctic and alpine habitats in the northern hemisphere are characterized by dwarf shrub communities dominated by species such as Dryas and Salix, which form ectomycorrhizal associations supporting diverse communities of fungal partners. Similarly, regions with winter-wet ecosystems, such as those found in the Mediterranean region, exhibit a pronounced presence of ectomycorrhizal associations among species like Pinus, Cistus, Arbutus, and Arctostaphylos. This demonstrates that ectomycorrhizal associations are not confined to temperate and boreal forests but are prevalent across various ecosystems, highlighting their broader ecological importance and distribution [12].

However, it is in the tropics that the occurrence and importance of ECM host species have been most consistently underestimated. Consider the Dipterocarpaceae, of which all members form ectomycorrhizas. This diverse family of over 500 spp. is the source of most tropical hardwood timber and is of immense economic and ecological importance.

Sarcolaenaceae, sharing a common ancestor with Dipterocarpaceae, has been found to be ectomycorrhizal by [13], suggesting that the ectomycorrhizal habit in this clade was present more than 88 million years ago. The discovery of Pakaraimea being ECM pushes this origin back to 130 million years ago. Understanding ECM fungi in dipterocarps could offer insights into ECM evolution and tropical forest ecology.

Ectomycorrhizas are prevalent in certain members of Fabaceae primarily in African tropical rainforests and savannah woodlands. Genera such as Tetraberlinia, Microberlinia, and Julbernardia dominate rainforests, while Isoberlinia and Brachystegia cover vast areas in African woodlands. These ecosystems support diverse ectomycorrhizal fungi [14, 15, 16, 17].

Two temperate ectomycorrhizal plant taxa; Fagaceae and Pinaceae, have radiated into natural tropical forests. Tropical oak species like Lithocarpus, Castanopsis, and Trigonobalanus occur in Central America and SE Asia along with ectomycorrhizal dipterocarps.

Endemic ectomycorrhizal tropical pines like Pinus oocarpa, P. patula, and P. caribea are found in Mexico, Central America, the Caribbean, and SE Asia. The ectomycorrhizal fungi of these natural tropical pine forests are not well studied. However, several tropical pines have been extensively used in plantations throughout the tropics outside their natural range, and often in areas that have not previously supported ectomycorrhizal vegetation. Ironically, in these plantations, the mycobionts are either temperate species introduced with inoculum from Europe, or the ubiquitous ectomycorrhizal genus Pisolithus [18]. Eucalyptus spp. are widespread throughout temperate and subtropical forest ecosystems in Australia where they support hypogeous, ectomycorrhizal fungi [19]. Like pines, this genus has been extensively planted outside its natural range in tropical countries, and like pines, its mycobiont community appears less diverse in these situations. Acacia mangium is another Australian species extensively used in tropical plantations. Apparently it too can form ectomycorrhizas, at least with Pisolithus sp. [20]. Table 1 describes the list of ectomycorrhizal host taxa reported to contain at least one species forming this association.

Family	Genus
Aceraceae	Acer
Betulaceae	Alnus, Betula, Carpinus, Corylus, Ostrya, Ostryopsis
Bignoniaceae	Jacaranda
Caprifoliaceae	Sambucus
Casuarinaceae	Casuarina, Allocasuarina
Cistaceae	Helianthemum, Cistus
Compositae	Lactuca
Cyperaceae	Kobresia
Dipterocarpaceae	Anisoptera, Balanocarpus, Cotylelobium, Dipterocarpus, Dryobalanops, Hopea, Monotes, Shorea, Valica
Elaeagnaceae	Shepherdia
Epacridaceae	Astroloma
Ericaceae	Arbutus, Arctostaphylos, Chimaphila, Gaultheria, Kalmia, Ledum, Leucothoe, Rhododendron, Vaccinium
Euphorbiaceae	Poranthera, Uapaca
Fagaceae	Castanea, Castanopsis, Fagus, Lithocarpus, Nothofagus, Pasania, Quercus, Trigonobalus
Gentianaceae	Bartonia
Goodenaceae	Brunonia, Goodenia
Hammamelidaceae	Parrotia
Juglandaceae	Carya, Juglans, Pterocarya
Caesalpinoideae	Afzelia, Aldina, Anthonota, Bauhinia, Brachystegia, Cassia, Eperua, Gilbertiodendron, Julbernardia, Monopetalanthus, Paramacrolobium, Swartzia
Mimosoideae	Acacia
Papilionoideae	Brachysema, Chorizema, Daviesia, Dillwynia, Eutaxia, Gompholobium, Hardenbergia, Jacksonia, Kennedya, Mirbelia, Oxylobium, Platylobium, Pultenaea, Robinia, Vicia, Viminaria
Myricaceae	Comptonia, Myrica
Myrtaceae	Angophora, Callistemon, Campomanesia, Eucalyptus, Leptospermum, Melaleuca, Tristania
Nyctaginaceae	Neea, Torrubia, Pisonia
Oleaceae	Fraxinus
Platanaceae	Platanus
Polygalaceae	Comeosperma
Polygonaceae	Coccoloba, Polygonum
Rhamnaceae	Cryptandra, Pomaderris, Rhamnus, Spyridium, Trymalium
Rosaceae	Chaembatia, Cirocarpus, Crataegus, Dryas, Malus, Prunus, Pyrus, Rosa, Sorbus
Salicaceae	Populus, Salix
Sapindaceae	Allophylus, Nephelium
Sapotaceae	Glycoxylon
Sterculiaceae	Lasiopetalum, Thomasia
Stylidiaceae	Stylidium
Thymeliaceae	Pimelia
Tiliaceae	Tilia
Ulmaceae	Ulmus, Celtis
Vitaceae	Vitis
Cupresseceae	Cupressus, Juniperus
Pinaceae	Abies, Cathaya, Cedrus, Keteleeria, Larix, Picea, Pinus, Pseudolarix, Pseudotsuga, Tsuga
Gnetaceae	Gnetum

Table 1.

Plant hosts reported to form ectomycorrhizal association.

4. Fungi forming ectomycorrhizal association

Approximately 5000–6000 fungal species form ectomyorrhizal association with the roots of trees. In recent years, more intensive mycological investigations in tropical forests and studies on hypogeous fungi associated with Eucalyptus vegetation in Australasia have unveiled numerous undescribed ectomycorrhizal species [19, 21, 22]. The use of molecular markers for direct mycobiont identification from ectomycorrhizas has significantly expanded the known taxa [23]. Furthermore, several fungal groups previously classified as saprotrophic, such as tomentelloid fungi, have been discovered to be ECM [24].

The contributing fungi are not evenly distributed over the kingdom Fungi. Basidiomycota is the major phylum in the kingdom that is involved about 95% in this association while members in Ascomycota play a minor role with approximately 5% ectomycorrhizal symbionts. Members in Mucoromycota only form exceptionally ectomycorrhizal associations [25].

Weiss et al. [23] revealed that the significance of heterobasidiomycetes within the Sebacinaceae as ectomycorrhizal symbionts has been underestimated. The members of this family have been strongly involved in forming this association [26]. As molecular studies on mycorrhizal symbionts increase, the taxonomic range of identified mycobionts is likely to broaden.

Certain ectomycorrhizal fungal genera are restricted to particular vegetation types. Members of Inocybaceae have been found forming association with Pinaceae in regions with a temperate climate [27]. Cortinarius, the most species-rich ectomycorrhizal genus, is prolific both in terms of species richness and sporocarp production in northern boreal regions [28], but is conspicuously absent from tropical regions [29]. By contrast, members of the Russulaceae show significant diversity in temperate and boreal regions and present a huge, largely undescribed, diversity in tropical regions where ECM hosts occur [22, 30, 31].

With the exception of some Tuber spp. [32], the knowledge of the ecology of ascomycete ECM fungi is very limited. Recent work [33, 34] suggests that the occurrence and importance of the Helotiales as ectomycorrhizal fungi may have been underestimated [35]. It demonstrated that an isolate from the Hymenoscyphus ericae aggregate [33] was capable of simultaneously forming ectomycorrhizal association on Pinus sylvestris. In recent years, the ectomycorrhiza of Paragalactinia succosella (=Peziza succosella) and Geopora spp. has also been reported with members of Pinaceae [36, 37, 38, 39]. Table 2 describes the list of ectomycorrhizal fungal taxa reported forming the ectomycorrhizal association.

Division	Family	Genus	Species
Ascomycota	Balsamiaceae	Balsamia	magnata, platyspora, vulgaris
	Elaphomycetaceae	Cenococcum	geophilum
		Elaphomyces	anthracinus, granulatus, muricatus, mutabilis, reticulatus, variegatus
	Geneaceae	Genabea	cerebriformis
		Genea	gardneri, harknessii, intermedia
	Helvellaceae	Hydnotrya	tulasnei
	Pezizaceae	Pachyphloeus	citrinus, ligericus, melanoxanthus
		Paragalactinia	succosella
	Pyronemataceae	Geopora	pinyonensis
	Terfeziaceae	Choiromyces	alveolatus, venosus
	Tuberaceae	Tuber	aestivum, borchii, brumale, californicum, excavatum, melanosporum, puberulum, rapaeodorum, rufum
Basidiomycota	Amanitaceae	Amanita	aspera, fulva, gemmata, inaurata, muscaria, pallidorosea, pantherina, phalloides, rubescens, solitaria, spissa, strobiliformis, subjunquillea, vaginata, verna, virosa
	Astraeaceae	Astraeus	hygrometricus, pteridus
	Boletaceae	Boletus	minatioolivaceus, pulverulentus, regius
		Gyroporus	castaneus, cyanescens
		Phylloporus	Rhodoxanthus
		Pulveroboletus	Ravenlii
		Tylopilus	chromapes, felleus, gracilis, porphyrosporus
		Xerocomus	armeniacus, badius, chrysenteron, rubellus, spadiceus, subtomentosus, truncatus
	Cantharellaceae	Cantharellus	cibarius, infundibuliformis, tubiformis
	Clavariaceae	Ramaria	aurea, botrytis, flava, formosa, mairei, subbotrytis
	Corticiaceae	Byssocorticium	Atrovirens
		Byssoporia	Sublutea
		Piloderma	byssinium, croceum, sulphureum
	Cortinariaceae	Cortinarius	acutus, anomalus, bicolor, biveluseverneus, hemitrichus, leucophanes, mucosus, multiformis, obtusus, phrygianus, saniosus
		Dermocybe	anthracina, cinnamomea, malicoria, palustris, phoenicea
		Hebeloma	crustuliniforme, cylindrosporum, hiemale, longicaudum, mesophaeum, minus, pumilum, sinapizans
		Hymenogaster	bulliardii, calosporus, citrinus, decorus, lilacinus, luteus, olivaceus, populetorum, tener, vulgaris
	Inocybaceae	Inocybe	asterospora, brunnea, cincinnata, curvipes, dulcamara, lacera, petiginosa
		Inosperma	adaequatum, bongardii
		Mallocybe	Terrigena
		Pseudosperma	Rimosum
		Rozites	Caperata
	Hydnaceae	Dentinum	Repandum
		Hydnellum	Velutinum
		Hydnum	imbricatum, rufescens, scabrosum
	Hygrophoraceae	Hygrophorus	capreolarius, camarophyllus, chrysodon, discoideus, hypothejus, karstenii, marzulus, pudorinus
	Hysterangiaceae	Hysterangium	Membranaceum
	Leucogastraceae	Leucogaster	Nudus
	Melanogastraceae	Melanogaster	ambiguus, broomeianus, euryspermus, intermedius, tuberiformis, variegatus
	Paxillaceae	Paxillus	Involutus
	Pisolithaceae	Pisolithus	Tinctorius
	Polyporaceae	Albatrellus	Cristatus
	Russulaceae	Elasmomyces	Mattirolianus
		Lactarius	decipiens, fuliginosus, helvus, necator, piperatus, repraesentaneus, rufus, scrobiculatus, spinosulus, uvidus, vellereus, volemus
		Russula	aeruginea, albonigra, amoena, anthracina, cyanoxantha, densifolia, emetica, foetens, heterophylla, lutea, nigricans, ochroleuca, odorata, olivacea, paludosa, palumbina, parazurea, vesca, virescens, xerampelina
		Zelleromyces	Stephensii
	Sclerodermataceae	Scleroderma	bovista, cepa, citrinum, hypogaeum, laeve, polyrhizum, verrucosum
	Strobilomycetaceae	Boletellus	betula, chrysenteroides
		Gautieria	graveolens, mexicana, otthii
		Strobilomyces	floccopus
	Thelephoraceae	Thelephora	anthocephala, atrocitrina, penicillata, terrestris
	Tricholomataceae	Laccaria	amethystina, bicolor, laccata, montana, proxima
		Tricholoma	caligatum, columbetta, flavobrunneum, flavovirens, myomyces, saponaceum, sulphureum
Mucoromycota	Endogonaceae	Endogone	lactiflua

Table 2.

Fungal partners known to form ectomycorrhizal association.

5. Ectomycorrhiza in forestry

Since ectomycorrhizal fungi are known as integral to the ecosystem, and a huge diversity of these fungi form associations with a number of economically important tree species cultivated for timber, their potential importance in tree nutrition has been acknowledged since the earliest experiments [40]. Plantation forestry is on the rise globally due to the growing demand for timber and pulp, as well as efforts to boost carbon sequestration by expanding forested areas. Although seedlings that are outplanted or naturally regenerate can form ectomycorrhizal associations from any naturally present mycorrhizal inoculum, the evident benefits of nursery stock being mycorrhizal prior to outplanting are apparent.

Many commercial practices, especially those aimed at enhancing hygiene in tree nurseries, are detrimental to the growth of most ectomycorrhizal fungi species, except for a few ruderal ones. As a result, specific techniques have been devised to facilitate the colonization of plants by selected fungi before outplanting. The application of these techniques has facilitated superior performance in tree crops in many parts of the world, particularly those which lack natural local sources of inoculum [41]. Grove and Le Tacon [42] provide a comprehensive outline of the imperatives for developing successful strategies to maximize the benefits of ectomycorrhizas in forestry. Meanwhile, there has been an increasing interest in the potential of harvesting edible fruiting bodies of ECM fungi, which have been utilized as commercial inoculants to enhance both diet and revenue. Inoculation can provide benefits at two key stages of timber production systems: in the nursery phase and post-outplanting in the field.

A significant portion of experimental research has concentrated on the benefits associated with producing well-developed seedlings that, along with their fungal symbionts, can thrive once transplanted into the field. While this focus primarily aims to ensure successful establishment, there are also direct cost savings realized through the accelerated throughput resulting from rapid growth in nurseries.

Furthermore, the utilization of inoculated seedlings has provided valuable insights. It has been observed that the responses to ectomycorrhizal colonization are particularly pronounced under challenging conditions, such as drought, metal contamination, and pathogen exposure. These observations have prompted an in-depth analysis of the functional mechanisms underlying the beneficial effects of ectomycorrhizal fungi, shedding light on their role in ameliorating adverse environmental factors.

Beyond the extensively studied effects on plant nutrition and growth, significant progress has been achieved in comprehending the roles of symbiotic relationships in imparting resistance to various stresses. These stresses, while occurring naturally in ecosystems, are often exacerbated locally due to prior land-use practices or the afforestation process itself. Discrepancies in performance between nursery and field conditions following inoculation may indeed arise from differential impacts of soil and environmental conditions on the specific mycorrhizal partnerships established through inoculation.

Forest productivity faces threats from global change factors such as increased inputs of nitrogen and sulfur directly into soils, leading to simultaneous decreases in pH, which can negatively affect both symbiotic partners. These effects are believed to contribute to forest decline syndromes witnessed in various parts of the world. Additionally, there are indications that the availability of base cations may eventually constrain forest productivity, emphasizing the potential benefits of ectomycorrhizal associations in enhancing nutrient uptake under such circumstances.

Acknowledgments

Sincere thanks to my students; Memoona Azeem, Wasiqa Arshad, and Mehboobullah Khan who helped me in formatting the text.

Conflict of interest

The author declares no conflict of interest.