Importance of epiphytes in ecosystems
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Ecological importance and adaptations of epiphytes

Definition

An epiphyte is an organism that grows on another living organism. Highly diverse, occurring from forest floor to tree crowns. These plants only receive physical support from the host, but not nutrients, although they may injure it. Fully autotrophic plants are referred to as true epiphytes, but there are also epiphytic bacteria, fungi, algae, lichens, mosses and ferns.

Over 30.000 types of epiphytes have been identified worldwide, of these 24.000 in the tropics. These are approximate numbers, there are many more species that are not listed and even more are undiscovered. There is also the problem of defining epiphytes in terms of wrappers, climbers and others, which makes it difficult to estimate the number, depending on whether the plants maintain a connection to the ground or not.

There has been a large number of new reports of epiphytic species, in some cases for species in genera or even families with no previous records of epiphytic members.

General characteristics

Epiphytes obtain energy through photosynthesis. Roots are primarily developed for attachment, so special structures such as cups and bowls may be developed to collect rainwater.

Advantages:

  • they can grow where there is enough light, as they need a lot for growth and reproduction, without developing a large trunk. Epiphytes occupy sites of very variable light exposure, the highest percentage is found at intermediate levels;
  • high in the canopies they are better exposed to pollinators and wind, that disperses the seeds and brings nutrients;
  • they are removed from ground herbivores.

Most epiphytes have adapted to lack of water and nutrients. Epiphytes add new dimensions to forests by creating new niches that can be exploited by a range of other species. The structures in which the water collects provide water for various animals in the canopy, as well as a place for shelter and reproduction.

Plant-water-nutrient relations

Water scarcity is the most exposed abiotic constraint in the epiphytic habitat. Differences in water (and nutrient) availability lead to define two functional groups: continuously and intervally supplied. Structures like water tanks reduce the problems of irregular water supply, providing a reliable supply of moisture and nutrients.

With no root contact to the soil, epiphytes lack access to the most important nutrient source. Sources are atmospheric inputs (rain, dust, mist), nutrients released from ground‐rooted plants through decomposition, nitrogen fixation and remains of animals (mineral and organic matter). Nutrient scavenging in epiphytes shows in unusual morphological structures (phytotelmata, litter‐trapping leaves, bromeliad trichomes, orchid velamen radicum), many of these primarily enable water uptake.

Animals associated with epiphytes are mostly ants as they are the most abundant insects in the canopy of trees. Epiphytes benefit from an increased nutrient supply, they derive a significant part of its nitrogen and other molecules from the debris deposited by ants inhabiting its cavities. Mostly in tropical cloud forests branches may be covered by a soil layer, sometimes decimetres in depth. Canopy soils are of organic origin and their N and P content is often higher than that on the forest floor. Thus epiphytes rooting in such soils are no more nutrient‐limited than ground‐rooted plants. The notion that epiphytes are nutrient‐stressed is not generally valid, at least not in case of nitrogen, because mycorrhizal associations are present and necessary in dystrophic soils.

Photosynthesis

Depending on the species, plants are able to maintain a positive carbon balance in the absence of water for several days to weeks. Crassulacean acid metabolism (CAM photosynthesis) is very common in epiphytes. A majority of all CAM taxa worldwide are epiphytic. There is an increase in CAM species from wetter to drier forests and an increase from shaded to exposed sites within a forest. Even in relatively moist forests the percentage of CAM species among epiphytes is higher than among vascular plants. This photosynthesis is relevant for keeping the water supply and also for photo‐protection due to maintaining the carbon fluxes through decarboxylation.

Mortality

  1. Water stress: along with nutrient deficiencies may lead to reduced rates of CO2 exchange, decreased vegetative growth and low fertility.
  2. Temperature: most vascular epiphytes are tropical plants. To an extent, vascular epiphytes can endure frost and drought, but not both stresses simultaneously. This can be understood from their global distribution: epiphytes are found in habitats with occasional subzero temperatures and in arid tropical scrublands, but their growth in the temperate zone is restricted to humid climates with mild winters. Some species of hemiparasitic mistletoes, which share the same microenvironment with true epiphytes, are more tolerant to frost and are found in boreal forests.
  3. Light: whether photoinhibition can be lethal for entire groups is unclear. But tree or branch fall is a large cause of mortality for epiphytes, because those attached to a fallen tree or branch have low chances for survival on shady ground.
  4. Herbivory: a biotic cause of mortality. Some sub‐populations can be wiped out almost completely by a certain herbivore.

Definitions

  • Obligate epiphyte (holoepiphyte): spends its whole life cycle with no contact to the ground, always grows on another plant for structural support, but derives no nutrients from the host.
  • Opportunistic (accidental) epiphyte: terrestrial species that rarely grow epiphytically. Such plants experience more favourable abiotic conditions than terrestrial conspecifics. Examples in the temperate zone in Europe are herbs such as Oxalis acetosella, Geranium robertianum and trees such as Sorbus aucuparia, all species which can occasionally be found in forests growing on old trees with accumulations of organic material.
  • Facultative epiphytes: can use different substrates simultaneously, such as trees, rocks, or soil. They are ecotypes of the same species present on different substrates.
  • Primary hemiepiphyte: plant that begins its life cycle in a tree crown and secondarily becomes rooted in the ground (e.g., strangler fig).
  • Secondary hemiepiphyte: plant that begins its life cycle as a terrestrial seedling, ascends a tree, and can later lose root connections with the ground, e. g.: lianas, woody climbing plants and vines.
  • Lithophytes: plants that grow in or on rocks. There are many epiphytes among lithophytes.

Parasitic and semi-parasitic plants that grow on other plants are not true epiphytes, but they are epiphytes based on their habitat. E.g. mistletoe are hemiparasites that draw water and nutrients from the host tree via root-like structures.

The distinction between vines, terrestrial herbs with accidental occurrences on trees and epiphytes is difficult. The distinction of facultative and opportunistic epiphyte is tough, as we lack quantitative information. E.g.: many species of Ficus are hemiepiphytic, although some may primarily grow as lithophytes.

Classification

Epiphytic bacteria

Epiphytic bacteria are defined as populations that can survive and multiply on the surface of plants. They occupy ecological niches on the phylloplane that could be occupied by pathogens.

Epiphytic fungi

Contribute to leaf litter decomposition and play an important role in recycling carbon and nutrients in ecosystems. Epiphytic fungi depend on nutrients deposited on leaves from the atmosphere or exude them from leaves.

Epiphytic algae

Some species of algae live on other plants. They are a normal part of the environment, but if excess nutrients cause them to bloom, they can damage the host plant by smothering it or competing for light. Epiphytic algae play a significant role in shallow ecosystems, contributing to material circulation, energy flow and the maintenance of food webs.

Lichens and mosses

Some species in temperate and tropical forests grow on trunks and branches, some in the form of curtains in higher lying forests. In tropical forests, lichens and mosses in the undergrowth colonise the leaves of shrubs, young trees and sedges. Other small epiphytes grow on the lower parts of trunks or on ferns.

Many bryophytes are obligate epiphytes. Trees stabilise temperatures and provide microhabitats for attachment. Epiphytic bryophytes have the ability to store high amounts of precipitation water, causing a delayed release of water.

Vascular epiphytes

There are over 27.000 species of vascular epiphytes (including primary hemiepiphytes), which occur mostly in the tropics. Including larger ferns and flowering plants, ranging from orchids to trees that begin their life cycle as seeds in the tree canopy.

Groups of vascular epiphytes

  1. Nest-shaped epiphytes

Bromeliads and ferns. They have very short stems, narrow leaves that form a rosette or dense clusters. The tropics of Asia and Africa are dominated by ferns, while the tropics of Central and South America are dominated by specialised bromeliads. Ferns that form nests belong to the genus Asplenium. The rosettes of some species form the shape of a shallow bowl in the form of a nest. Twigs, leaves, fruit, flowers and bark are caught in these bowls. They gradually break down into humus from where plants absorb water and nutrients.

2. Bromeliads

The most obvious epiphytes in tropical forests, some species also grow in the shade near the ground, others in deserts and high in the Andes. Special reservoirs formed by bromeliads serve as a breeding area for many insects and their larvae, as well as for smaller frogs. The secretions of these animals and their corpses are an important part of nutrients for the plant. Bromeliad flowers are pollinated by hummingbirds, the flowers are tubular and contain nectar. They are mostly strongly coloured. The abundance and diversity of of Bromeliaceae is large in the rainforests on american continents, where its ecological importance is in adding to total epiphyte biomass and its influence on canopy fauna. Epiphytism is largely found in subfamilies Bromelioideae and Tillandsioideae, Tillandsia being the most species-rich genus, almost all species are epiphytes, followed by Vriesea.

3. Epiphytic ferns that do not form nests

Less specialised than nest-forming ferns (mainly the genus Lycopodium). Their stems and roots attach themselves to the humus of nest-making epiphytes, to the fibrous trunks of tree ferns, or to mats of mosses. Tmesipteris and Psilotum are considered descendants of one of the first land plants. Genuses: Lycopodium,Tmesipteris, Psilotum, Hymenophyllum, Trichomanes, Pyrrosia.

4.  Epiphytic orchids

Most orchids are epiphytes. In the family Orchidaceae, 69 % of all species are epiphytes. They grow in tropical forests, arctic tundra and deserts. There are approximately 10 times more species in the tropics than in the temperate and polar regions. In the temperate zone they are mostly terrestrial. They are usually attached by their roots directly to the bark of tree trunks or branches. The roots are specialised – they have several layers of dead cells on their surface, which quickly absorb and store water during rains. This dead, absorbent layer is the velamen radicum and from this the living tissue draws water. Living cells of all parts of plants, including roots, contain chlorophyll and carry out photosynthesis. Many have CAM type of photosynthesis. The variety and sophistication of the flowers evolved along with their pollinators – insects.

purple orchid

5. Epiphytic shrubs and stranglers

Are hemiepiphytes, but in cases of collapse of the host tree they become independent. There is a competition between the epiphyte and the tree for light because the crown of the epiphyte outgrows and shades the crown of the supporting tree.

There is a wide range of small epiphytic shrubs in tropical and subtropical forests. Classified in many families, the most represented are Ericaceae (Vaccinium and Virea rhododendrons). These shrubs mostly grow on the humus of epiphytes that form nests or on moss-covered branches – they grow in the canopy and their roots have contact with the ground. The leaves can be fleshy and store water, the thick waxy cuticle reduces water loss.

Species: strangler figs (Ficus) in tropical forests, Schefflera, Clusia, Fagraea, Metrosideros, Griselinia.

Origin and age

2 theories:

  1. the ancestors of light-loving epiphytes are plants on forest floors. Variants of these plants, which needed more light, gradually moved higher up the trunks. Thus epiphytes would have existed since flowering plants became dominant in rainforests, for about 100 million years.
  2. Epiphytes originate from plants of dry, terrestrial habitats. Since deserts are geologically younger than rainforests, botanists assume that epiphytes are not such an old group. This theory is supported by the absence of any clear fossil evidence from older periods. However, the fossil remains of epiphytes would be mainly leaves and other parts, which does not provide evidence of an epiphytic lifestyle.

Future studies

Studies have shown that the current understanding of epiphyte biology is highly biased:

  1. a strong taxonomic bias, with most research on epiphyte physiology focusing on few groups, particularly bromeliads and orchids;
  2. plants occurring at rather extreme sites in the periphery of the forest canopy were much more likely to be studied than those in the mid‐ and understorey.

Future studies should include a broader spectrum of species, both taxonomically and ecologically.

The most obvious abiotic constraint for growth and vegetative function of vascular epiphytes is water shortage, while other factors such as nutrient availability and irradiation, are less researched. Even less is known about other possible constraints: substrate instability, carbon gain/loss, dispersal, competition or herbivory.

A majority of published studies deal with one particular organ (mostly leaves). There is still a need to link the physiology of single organs to entire individuals and further to the structure and dynamics of populations and communities. Entire life cycles of epiphytes need to be researched to identify mechanisms.

Sources:

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