Forests are habitats where trees are usually the main form of vegetation, and can occur in many regions and climates. Forests can occur as tropical rainforests, temperate forests, boreal forests, and so on, and can form vertical layers of vegetation, an example of which is canopy layers, commonly known as forest canopies.
Vertical Forest Layers
The species composition of a forest is unique to it, and while some forests can have hundreds of tree species, others can have only a few. The species composition in forests is subject to spatial and temporal changes. Despite the wide variability of forests, they exhibit certain basic structural characteristics such as the distinction between several vertical layers, which can include –
The forest floor layer – this comprises the soil at the bottom of the forest, which can occur as blanketed deposits of fallen trees, twigs, decaying leaves, moss, materials from animals, and more. The recycling of materials in the forest begins at the forest floor, with many organisms such as bacteria, fungi, earthworms, insects, breaking down forest waste and converting them into reuseable materials.
The herb layer – this layer is predominantly vegetated by herbaceous, soft-stemmed plants, for example grass, ferns, etc. In forests where the canopies are thick, little light percolates into the herb layer and species adapted to the shade predominate.
The shrub layer – this layer comprises woody vegetation growing near the ground and can include brambles and bushes occurring due to the passage of light adequate to their growth.
The understory layer – this layer consists of young and small trees that have not grown as tall as the taller trees, and thus do not form part of the top canopy of trees.
The canopy layer – this layer is when the upper crowns of the taller trees in a forest meet and form a thick layer over the forest.
The emergent layer – this layer is composed of the tallest trees whose crowns emerge over the canopy layer (L. Klappenbach, 2016).
Fig: The Canopy Layer relative to some other vertical layers in a forest
Various species can form distinct habitats in these different layers of a forest. Forest canopies can be home to many undiscovered endemic species occupying their own ecological niche. A good knowledge is available on the role that canopy layers play in the functioning of forests. The structure of canopy layers for example, affect the cycling of nutrients, weather patterns, provide habitats, etc.
Trees can form many different layers, forming multiple layers, and often defining the canopy layer in a given area of vegetation can be difficult. The canopy layer is frequently considered as the uppermost continuous network of tree crowns, comprising all of the plant and animal habitats in that layer or network.
The maximum height of the canopy layer varies considerably between forests. Even within forests, the heights can exhibit considerable variability. For example, in the tropical forests of Monteverde, Costa Rica, the height of the canopy layer can exhibit great variability of between 15 m to 40 m.
There is at most times no single distinct canopy layer. Different species of trees utilize sunlight differently at different levels in terms of vertical positioning. As such trees can grow as upper canopy trees such as maples, oaks and chestnuts. Other than these various other plants can grow at the lower levels.
Various vines and shrubs can grow at differing levels of shade under the upper canopy. This multiple canopy layer system is formed due to temporal factors, competition between plants, stochastic events and also certain factors that determine species composition, involving ecological niche partitioning. This can involve a certain degree of niche stratification at the various canopy layers, which occurs at all layers of the vertical forest structure.
The chief property exhibited by the canopy layer is attributed to the fact that it is situated at the top of the forest cover. Sunlight reaches the canopy layer before any part of the forest other than the emergent layer. In the canopy layer, it can thus be warmer and drier than the rest of the forest below it. Also, in the event of rainfall, canopy layers are the first to get drenched and also absorb the most moisture.
The canopy layer in fact acts to reduce most of the sunlight and the wind reaching the lower layers of the forest, although water vapour is abundantly present at the lower layers, adding to the humidity of most forests. The canopy layer also acts to prevent sunlight from drying the forest floor, often leading to a moist forest floor.
The primary structure of the canopy layer of a forest area is composed of trees, which can sometimes be made up of the same species or can be heterogeneous. However, in canopy layers, there can be numerous plant and animal species that can live on the structures formed by the trees. The trees here can exhibit a wide variety of characteristics.
Many plants specially adapted to live in these tree habitats can be found in canopy layer habitats. Some of the types of plants include climbers, hemi-parasites, epiphytes and hemi-epiphytes. Climbers can include plants such as vines, palms or lianas; while an example of hemi-parasites is mistletoe.
Epiphytes can be plants such as ferns, mosses, lichens, orchids and bromeliads; while hemi-epiphytes can include plants such as strangler figs. These plants can take root in the soil layers that are positioned in branches, although no accurate estimates exist of all the plant species that form habitats within canopy layers.
A wide variety of animal life can also exist in canopy layers in the form of communities of birds, insects, amphibians, reptiles and mammals. These form essential constituents of the forest’s food web in addition to the plants. Many of these animals function to disperse seeds from the trees forming the canopy layer.
However, not all animal species forming a habitat in these arboreal habitats have been fully documented. Usually extensive equipment such as climbing ropes, canopy cranes, walkways and canopy rafts are required to study these arboreal habitats.
In rainforest ecosystems, more plants and animals form habitats in canopy layers than in any other habitat. In these forests, about 99 per cent of the incident sunlight falls on the canopy layer. As such epiphytes, which are plants that grow on other plants are plentiful in these arboreal habitats.
Also lianas are vines that attempt to reach the precious sunlight entering the forest by wrapping around and climbing up the tree trunks in these rainforests. These create walk ways for animals to reach the upper reaches of the canopy layer from the humid lower levels.
Fig: Lianas or Tree Vines are a common feature of tropical rainforests
Canopy layers represent some of the most complex habitats on Earth, and canopy layers can be considered separate ecosystems on their own, as many of the species adapted to these arboreal habitats would find it difficult to adapt to the lower layers of the forest. With shade, rainwater, and food in the form of fruits and plant products, canopy layers provide prosperous ecosystems for many species. Most animal species in these arboreal habitats are invertebrates that act as food for those higher up in the food web.
At the top are trees that are considerably taller than the rest of the trees at the canopy layer. These protruding trees out of the forest form the emergent layer that is the only layer that is situated above the canopy layer.
These trees suffer the highest damage due to climatic factors such as wind such that frequently their leaves are dry and adapted to have less surface area and waxy coating to keep evaporation to a minimum. Like canopy layers, emergent layers also offer habitats to a wide range of species, many emerging from the canopy layers. Life however, is not as abundant in the emergent layers as in the canopy layers.