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- Canopy layer consisting of the original mature fruit trees.
- Low-tree layer of smaller nut and fruit trees on dwarfing root stocks.
- Shrub layer of fruit bushes such as currants and berries.
- Herbaceous layer of perennial vegetables and herbs.
- Rhizosphere or ‘underground’ dimension of plants grown for their roots and tubers.
- Ground cover layer of edible plants that spread horizontally.
- Vertical layer of vines and climbers.
Two additional layers have since been proposed by John Kitsteiner:
- Aquatic/Wetland Layer
- Mycelial/Fungal Layer
Most of these vegetation layers are determined by the height range the plants' photosynthetic organs (leaves, needles, etc.). Taller species will have part of their shoot system in the underlying layers. In addition to the above-ground stratification there are the root and mycelial layers. The vertical layer is an outlier, as its growth is not strongly linked to its height.
After Hart and Mollison met, Hart's body of work was embraced by the permaculture community. Though largely used in the development of forest systems, layering has also been applied to other ecosystems. Layering is particularly helpful when designing a plant guild.
When using layers to design a site, consideration must be given to the amount of sunlight available to lower layers. Most of the traditional vegetable crops grown today, such as carrots, are sun loving plants not well selected for the more shady forest garden system. Hart favoured shade tolerant perennial vegetables.
Layers in natural forests
These layers of vegetation can range from a height of about five to forty-five metres. In a mature forest the top the crowns of the different species of trees form a more or less closed canopy. These layers creates special ecological conditions in the underlying layers. The density of the trees determines the amount of light inside the forest. The force of heavy rainfall is reduced by the canopy and the passage of rainwater is fed more slowly downwards. The tree layer can be further subdivided into the upper tree layer or canopy and the lower tree layer or understory.
The canopy refers to the highest layer of vegetation in a forest or woodland, made up of the crowns of its tallest trees. Dominant and co-dominant canopy trees form the uneven canopy layer. Canopy trees are able to photosynthesize relatively rapidly due to abundant light. The canopy layer provides protection from strong winds and storms, while also intercepting sunlight and precipitation, leading to a relatively sparsely vegetated understory layer.
Forest canopies are home to unique flora and fauna not found in other layers of forests. The highest terrestrial biodiversity resides in the canopy of tropical rainforests. Many rainforest animals have evolved to live solely in the canopy, and never touch the ground.
The understory refers to those trees above the shrub layer and below the canopy. Plants in the understory comprise an assortment of seedlings and saplings of canopy trees together with specialist understory shrubs and herbs. Young canopy trees often persist in the understory for decades as suppressed juveniles until an opening in the forest overstory permits their growth into the canopy. In contrast understory shrubs complete their life cycles in the shade of the forest canopy. Some smaller tree species, such as dogwood and holly, rarely grow tall and generally are understory trees.
Forest understories receive less intense light than plants in the canopy and such light as does penetrate is impoverished in wavelengths of light that are most effective for photosynthesis. Understory plants therefore must be shade tolerant -- they must be able to photosynthesize adequately using such light as does reach their leaves. They often are able to use wavelengths that canopy plants cannot. In temperate deciduous forests towards the end of the leafless season, understory plants take advantage of the shelter of the still leafless canopy plants to "leaf out" before the canopy trees do. This is important because it provides the understory plants with a window in which to photosynthesize without the canopy shading them. This brief period (usually 1-2 weeks) is often a crucial period in which the plant can maintain a net positive carbon balance over the course of the year.
As a rule, forest understories also experience higher humidity than exposed areas. The forest canopy reduces solar radiation, so the ground does not heat up as rapidly as open ground. Consequently, the understory dries out more slowly than more exposed areas do. The greater humidity allows fungi and other decomposers to flourish. This drives nutrient cycling, and provides favorable microclimates for many animals and plants, such as the pygmy marmoset.
The shrub layer is the stratum of vegetation within a habitat with heights of between one and a half to about five metres. Young trees are also part of this layer. It may be divided into the first and second shrub layers (low and high bushes). The shrub layer needs sun and only a little moisture, unlike the moss layer, which requires a lot of water. This stratum only receives light filtered by the canopy. i.e. it is preferred by semi-shade or shade-loving plants that would not tolerate bright sunlight. In the shrub layer, which consists of young trees and bushes, birds like the blackbird, song thrush, robin or blackcap are at home. They build their nests protected in the bushes and are therefore referred to as bush nesters. In addition to shrubs, such as elder, hazel, hawthorn, raspberry and blackberry, clematis may also occur. At the edge of a wood, the shrub layer acts as a windbreak close to the trees and protects the soil from drying out.
The herbaceous stratum contains non-woody vegetation, or ground cover, growing in the forest with heights of up to about one and a half metres. The herbaceous layer consists of various herbaceous plants, grasses, dwarf shrubs and young shrubs. In forests, early flowering plants appear first before the canopy fills out. Thereafter, the amount of light available to plants is significantly reduced and only those that are suited to such conditions can thrive. By contrast, grassland consists of moss and herbaceous layers. Sometimes, a shrub layer builds up as part of a process of reforestation (succession).
Growing on the surface of the forest floor is vegetation of up to about 0.15 metres in height in what is variously described as a moss, soil or cryptogam layer. The ground itself is covered by a layer of dead plant and animal material. In this layer and the underlying few centimetres of the topsoil live innumerable small soil organisms such as bacteria, fungi, algae and microorganisms, which break down the dead organic substances and work them into the soil. In places the ground is covered by lichens and mosses.
Also known as the rhizosphere, the underground area of a plant habitat is the root layer. It consists of the plants' roots and related elements such as rhizomes, bulbs and tubers. It is a narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. Protozoa and nematodes that graze on bacteria are also more abundant in the rhizosphere. Thus, much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots.
- Lowman, M.D. and M.W. Moffett. 1993. The ecology of tropical rain forest canopies. Trees 8:104-107.
- Kramer, D. M., G. Johnson, O. Kiirats, G. E. Edwards. 2004. New fluorescence parameters for the determination of Q redox state and excitation energy fluxes. Photosynthesis Research 79:209-218
- "Microbial Health of the Rhizosphere". http://uwstudentweb.uwyo.edu/T/Twhite/. Retrieved 5 May 2006.[dead link]
- "The Soil Food Web". USDA-NRCS. http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/?cid=nrcs142p2_053868. Retrieved 3 July 2006.
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