Monodominance is an ecological condition in which more than 60% of the tree canopy comprises a single species of tree. [1] [2] Monodominant forests are quite common under conditions of extra-tropical climate types. Although monodominance is studied across different regions, most research focuses on the many prominent species in tropical forests. Connel and Lowman, originally called it single-dominance. [3] [1] Conventional explanations of biodiversity in tropical forests in the decades prior to Connel and Lowman's work either ignored monodominance entirely or predicted that it would not exist. [4]
Connel and Lowman hypothesized two contrasting mechanisms by which dominance can be attained. [3] The first is by fast regrowth in unstable habitats with high disturbance rates. The second is through competitive exclusion in stable habitats that have low disturbance rates. [2] [3] Explanations of persistent monodominace include the monodominant species being more resistant than others to seasonal flooding, or that the monodominance is simply a sere. [4] With persistent monodominance, the monodominant species successfully remains so from generation to generation. [4] [2]
Examples of monodominant forests under temperate climate conditions include widespread boreal coniferous forests of the northern hemisphere, temperate Fagus grandifolia (American beech) forests in southern New England, Tsuga canadensis (Eastern hemlock) forests in northeastern United States, Populus tremuloides (quaking aspen) forests in mountainous regions of the western United States, Fagus sylvatica (European beech) forests in central Europe, Fagus crenata (Japanese beech) forests in Japan, or high-altitude monodominant Nothofagus menziesii (silver beech) and Nothofagus solandri (mountain beech) forests in New Zealand.
In tropical lowland forest environments, a minimum of 22 species from eight different families are known to create monodominant forests. [1] Examples of persistent monodominance are seen in Africa, Central and South America, and Asia. [5] Dipterocarpaceae is one example of a plant family that is recognized as persistently dominant in Asia. [5] The ectomycorrhizal tree Dicymbe corymbosa, found in central Guyana, creates wide ranges of monodominant forests containing more than 80% of the canopy tree species. [6]
Dominant plants in the Neotropics and Africa are usually in the family Leguminosae. [5] The species Gilbertiodendron dewevrei , Cynometra alexandri , and Julbernardia seretii are pronounced as exclusive dominants in their individual forests in equatorial Africa. [7] G. dewevrei dominated forests are more widespread on the highlands adjacent to the central basin of the Zaire River. [7] This species in the Ituri forest forms monodominant stands that occupy more than 90% of the canopy trees. [5]
Monodominance often occurs also on oceanic islands in the tropics. [8] Examples are Ochrosia oppositifolia forests on the Marshall Islands, Barringtonia asiatica forests on the Samoan Islands, Pisonia grandis forests on Rose Atoll, Palaquium hornei forests on Fiji Islands, Leucaena leucocephala forests on Nuka Hiva and Ua Pou of the Marquesas Islands and on Vanuatu, and Metrosideros polymorpha forests on the Hawaiian Islands.
Connel and Lowman originally hypothesized ectomycorrhizal association causing the replacement of other species as one of two mechanisms by which a species becomes persistently monodominant; the other is the simple colonization of large gaps. [3] However, subsequent research over the years has shown that there is not a single, simple mechanism by which monodominance occurs. [9] [10] [4] [11] Monodominant species have been recorded forming at various times after forest clearance, though this has not been shown to be a predictor of monodominant species persistence. Reliance upon ectomycorrhizae and poor soils have not been demonstrated. [2] Instead, multiple traits of adult monodominant species hinder the ability of other species to grow, including a dense canopy, a uniform canopy, deep leaf litter, slow nutrient processing, mast fruiting, and poor dispersal.
Several causal mechanisms have been proposed for the formation of monodominant forest in tropical ecosystems, [9] [1] including features of the environment such as low disturbance rates, and intrinsic characteristics of the dominant species: escape from herbivores, high seedling shade-tolerance, and the formation of mycorrhizal networks between individuals of the same species. [12]
The dense canopy of the adult trees prevents light from getting into the understory. In the Ituri Forest of the Democratic Republic of the Congo a monodominant Gilbertiodendron forest understory only receives 0.57% full sunlight while a mixed-forest understory received 1.15% full sunlight. This difference may prohibit many plant species from living in that environment due to the low light conditions and their resulting inability to sufficiently and effectively photosynthesize. Even some species that are more shade tolerant cannot survive the severe low light conditions. [4]
A monodominant forest has generally very deep leaf litter because the leaves are not decomposing as quickly as in other forests. In some monodominant forests the decomposition rates can be two to three times slower than mixed forests. [1] Low ammonium and nitrate could be the result of this slow decomposition which in turn, means less nutrients in the soil for other plant species to use. [4]
Nutrient processing is somewhat different from one forest to another. In the Gilbertiodendron forests there is low availability of nitrogen due to the low levels in the leaves that fall to the ground and the slow decomposition. This could prevent other plant species from colonizing because the soil lacks necessary nutrients. [4] In Parashorea chinensis forests, trees are known to require more fertile soils than in other areas. There is a large amount of manganese though that prevents other plants from taking root. Manganese can poison other trees if the levels are too high and possibly cause leaf chlorosis and necrosis and prevent the nutrient uptake of calcium and magnesium. [13]
Mast fruiting is a mass fruiting event that overwhelms the animals that consume fruit and helps the seeds' survival rate. Well-defended leaves also assist in the prevention of predation. In the Gilbertiodendron forests this mast fruiting does not assist in lesser predation, but in Asia and the Neotropics this does induce fitness benefits [14] and sometimes is actually important to monodominant maintenance. [4]
A monodominant forest has poor dispersal because of the lack of animal dispersers, so many of the seeds are just dropped from the parent tree and fall to the ground where they germinate. This can create a regular and radial path around the parent tree that results in a "tree-by-tree replacement" in a mixed forest. [1] In a monodominant forest the dominant species do not need all of the described traits to overwhelm the area. Though many have a combination, all monodominant forests have at least one of these traits to create the monodominant habitat. [4]
Many of the tropical monodominant trees are associated with ectomycorrhizal (ECM) fungi networks. Mycorrhizal fungi are known to effect plant diversity trends in a variety of ecosystems around the world. [6] Ectomycorrhizal relations with trees can increase nutrient supplies through a more effectual use of larger capacities of soils or through the direct decomposition of leaf litter. This has been suggested to provide a competitive advantage to such tree species. [1]
Examples of ectomycorrhizal trees in tropical rainforests can be found in Asia, Africa, and the Neotropics. There is a strong correlation between the ECM association in tropical trees and the occurrence of monodominance. [6]
Fungi like mycorrihizae appear not to harm the leaves and even display a symbiotic relationship. [15] ECM fungi are derived from saprotrophs and retain some ability to decompose organic material. Because tropical soils are often nutrient-poor, ECM trees are predicted to have a competitive advantage over neighboring trees because of their ability to attain more nutrients. With time this could lead to dominance in a tropical rainforest.
A study of Dicymbe corymbosa individuals show that (in terms of total basal area) the adult trees dominate resources and space. Additionally, they form coppices, also known as epicormic shoots, which allow their perseverance over time. Hence, if one stem of the tree dies, it is replaced by another living stem in the canopy. This creates same-species regrowth at stem level. All of this requires high levels of carbohydrates and nutrients that are accumulated from the ECM association. There is evidence that masting tree species rely on ECM associations to accumulate these requisite nutrients for reproduction during inter-mast years. Associations between resource levels stowed in plant tissue, timing of masting, and ECM patterns propose that ECM fungi are essential in the procurement of nutrients required for large masting trees. [6]
Seeds of monodominant trees typically have higher rates of germination and seedling survival when planted in monodominant forests rather than mixed forests. Monodominant seedlings planted in mixed forests have significantly lower levels of ECM colonization of roots. The lower percent of ECM colonization can cause the low survival rates of these seedlings in mixed forest. [6] Another mechanism that can be important for seedling and growth survival is a connection to a common ECM network. By connecting their small root systems to ECM networks that emanate from larger adults, more benefits can be received. [6]
Slower decomposition rates in monodominant forests have been hypothesized to be a result of competition between saprotrophic bacteria and fungi. ECM fungi may be suppressing saprotrophs in the monodominant forest to slow decomposition and return organically bound nutrients back to the tree. This is also called the "Gadgil" hypothesis. [6]
All of the traits that contribute to creating a monodominant forest over time hinder the growth of other plant species and force them to move to a more mixed forest. Even though this is inconvenient for the plant species that were there, there has not been any evidence that suggests that this is a negative effect of monodominance. [4] Monodominant forests are also found to have significantly less nitrogen in their soil than mixed forests. In these monodominant forests there are a lot of dominant tree species from the legume family that have nitrogen fixation. Nitrogen fixation creates compounds that help a plant to grow in otherwise low nutrient conditions. [16]
Rainforests are forests characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropical rainforests or temperate rainforests, but other types have been described.
A mycorrhiza is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology, and soil chemistry.
In forestry and ecology, understory, or understorey, also known as underbrush or undergrowth, includes plant life growing beneath the forest canopy without penetrating it to any great extent, but above the forest floor. Only a small percentage of light penetrates the canopy so understory vegetation is generally shade-tolerant. The understory typically consists of trees stunted through lack of light, other small trees with low light requirements, saplings, shrubs, vines and undergrowth. Small trees such as holly and dogwood are understory specialists.
Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as lowland equatorial evergreen rainforest. True rainforests are typically found between 10 degrees north and south of the equator ; they are a sub-set of the tropical forest biome that occurs roughly within the 28-degree latitudes. Within the World Wildlife Fund's biome classification, tropical rainforests are a type of tropical moist broadleaf forest that also includes the more extensive seasonal tropical forests.
The Sundaland heath forests, also known as Kerangas forest, is a type of tropical moist forest found on the island of Borneo, which is divided between Brunei, Indonesia, and Malaysia, as well as on the Indonesian islands of Belitung and Bangka, which lie to the west of Borneo.
In hydrology, stemflow is the flow of intercepted water down the trunk or stem of a plant. Stemflow, along with throughfall, is responsible for the transferral of precipitation and nutrients from the canopy to the soil. In tropical rainforests, where this kind of flow can be substantial, erosion gullies can form at the base of the trunk. However, in more temperate climates stemflow levels are low and have little erosional power.
Rhizopogon is a genus of ectomycorrhizal basidiomycetes in the family Rhizopogonaceae. Species form hypogeous sporocarps commonly referred to as "false truffles". The general morphological characters of Rhizopogon sporocarps are a simplex or duplex peridium surrounding a loculate gleba that lacks a columnella. Basidiospores are produced upon basidia that are borne within the fungal hymenium that coats the interior surface of gleba locules. The peridium is often adorned with thick mycelial cords, also known as rhizomorphs, that attach the sporocarp to the surrounding substrate. The scientific name Rhizopogon is Greek for 'root' (Rhiz-) 'beard' (-pogon) and this name was given in reference to the rhizomorphs found on sporocarps of many species.
Forest dieback is a condition in trees or woody plants in which peripheral parts are killed, either by pathogens, parasites or conditions like acid rain, drought, and more. These episodes can have disastrous consequences such as reduced resiliency of the ecosystem, disappearing important symbiotic relationships and thresholds. Some tipping points for major climate change forecast in the next century are directly related to forest diebacks.
Dicymbe is a genus of 20 species of canopy trees in the family Fabaceae, within subfamily Detarioideae. It is found throughout the Guyana Shield region and parts of W Amazonia. Certain species within the genus are strongly associated with ectomycorrhizal fungi.
Plant litter is dead plant material that have fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer or O horizon. Litter is an important factor in ecosystem dynamics, as it is indicative of ecological productivity and may be useful in predicting regional nutrient cycling and soil fertility.
A mycorrhizal network is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individual plants together. Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times.
Soil carbon storage is an important function of terrestrial ecosystems. Soil contains more carbon than plants and the atmosphere combined. Understanding what maintains the soil carbon pool is important to understand the current distribution of carbon on Earth, and how it will respond to environmental change. While much research has been done on how plants, free-living microbial decomposers, and soil minerals affect this pool of carbon, it is recently coming to light that mycorrhizal fungi—symbiotic fungi that associate with roots of almost all living plants—may play an important role in maintaining this pool as well. Measurements of plant carbon allocation to mycorrhizal fungi have been estimated to be 5 to 20% of total plant carbon uptake, and in some ecosystems the biomass of mycorrhizal fungi can be comparable to the biomass of fine roots. Recent research has shown that mycorrhizal fungi hold 50 to 70 percent of the total carbon stored in leaf litter and soil on forested islands in Sweden. Turnover of mycorrhizal biomass into the soil carbon pool is thought to be rapid and has been shown in some ecosystems to be the dominant pathway by which living carbon enters the soil carbon pool.
An ectomycorrhiza is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. The mycobiont is often from the phyla Basidiomycota and Ascomycota, and more rarely from the Zygomycota. Ectomycorrhizas form on the roots of around 2% of plant species, usually woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine and rose families. Research on ectomycorrhizas is increasingly important in areas such as ecosystem management and restoration, forestry and agriculture.
Ectomycorrhizal extramatrical mycelium is the collection of filamentous fungal hyphae emanating from ectomycorrhizas. It may be composed of fine, hydrophilic hypha which branches frequently to explore and exploit the soil matrix or may aggregate to form rhizomorphs; highly differentiated, hydrophobic, enduring, transport structures.
Cenococcum geophilum Fr., synonym Cenococcum graniforme (Sow.) Ferd. and Winge, is an Ascomycete fungal species and is the only member in the genus Cenococcum. It is one of the most common ectomycorrhizal fungal species encountered in forest ecosystems. The geographic distribution of the species is notably cosmopolitan; it is found in ecosystems with a wide range of environmental conditions, and in many cases in high relative frequency. Because of its wide distribution and abundance in forest soils, it is one of the most well-studied ectomycorrhizal fungal species. While the species has long been known to be sterile and not produce asexual or sexual spores, cryptic sexual stages may exist. The hyphae produced by C. geophilum are characterized by their thick (1.5-8 um), straight and jet black appearance with little branching. They usually form monopodial (unbranched) ectomycorrhizas. The mantles of C. geophilum ectomycorrhizas are usually thick with few to many emanating hyphae.
A canopy root, also known as an arboreal root, is a type of root that grows out of a tree branch underneath an epiphytic mat. These adventitious roots form in response to moist, dark, nutrient-rich conditions that are found in “canopy soils”. Canopy roots have been found in species of maple, poplar, alder, myrtle, beech, and spruce, among many others. They are structurally similar to roots found on the forest floor and likely serve a similar purpose for water and nutrient uptake, though their specific functions are still being studied.
Gilbertiodendron dewevrei is a species of tree in the family Fabaceae, native to tropical rain forests in Central Africa. It is often the dominant tree species of the Guineo-Congolian rainforest. The timber is traded as limbali, and is used for construction, flooring and railway sleepers. It is also used for making boats, furniture, tool handles and joinery and for making charcoal.
The Guineo-Congolian region is a biogeographical region in Africa straddling the Equator and stretching from the Atlantic Ocean through the Congo Basin to the Congo / Nile divide in Rwanda and Burundi. Formerly, this region was largely covered in rain forest, on both well-drained sites and in swamp forests, but little undisturbed primary forest now remains, having been replaced in many areas by savanna and secondary-growth forest.
Rhizopogon salebrosus is a mushroom species within the Rhizopogon sub-genus Amylopogon. R.salebrosus is a monotropoid mycorrhiza that is of vital importance to the ecology of conifer forests, especially in the Pacific Northwest region of North America. Although it is native to North America, R. salebrosus has been found in Europe and its range is generally limited to mountainous regions with sufficient precipitation. The mycoheterotrophic plant, Pterospora andromedea is often found in an obligate association with R. salebrosus in western parts of the U.S. Eastern populations of P. andromedea are typically symbiotic with another Rhizopogon sub species, R. kretzerae.
Anthonotha fragrans is a medium to large sized tree commonly found in the rainforest environments of West and Central Africa; it belongs to the Fabaceae family. Its sapwood exudes a white to creamy exudate.