Shade tolerance
Encyclopedia
In ecology
, shade tolerance is a plant
's abilities to tolerate low light
levels. The term is also used in horticulture
and landscaping
, although in this context its use is sometimes sloppy, especially with respect to labeling of plants for sale in nurseries
.
Shade tolerance is a relative term, and its use and meaning depends on context. One can compare large trees to each other, but when comparing understory
trees and shrub
s, or non-woody plants, the term takes on a different meaning. Even in a specific context, shade tolerance is not a single variable or simple continuum, but rather a complex, multi-faceted property of plants, since different plants exhibit different adaptation
s to shade
. In fact, the same plant can exhibit varying degrees of shade tolerance or even of requirement for light, depending on its past history or stage of development.
is scarce, life can actually be easier in the shade. In direct sunlight, plants face desiccation
and exposure to UV rays, and must expend energy producing pigment
s to block UV light, and waxy coatings to prevent water loss.
Plants adapted to shade have the ability to utilise far-red light (about 730nm) more effectively than plants adapted to full sunlight. Most red light gets absorbed by the shade intolerant canopy plants, but more of the far-red light penetrates the canopy, reaching the understory. The shade tolerant plants found here are capable of photosynthesis using light at such wavelengths.
On the other hand, when less light is available, less energy
is available to the plant. Whereas in sunny and dry environments water can be a limiting factor
in growth and survival, in shade, energy (in the form of sunlight) is usually the limiting factor.
The situation with respect to nutrients is often different in shade and sun. Most shade is due to the presence of a canopy of other plants, and this is usually associated with a completely different environment--richer in soil
nutrient
s--than sunny areas.
Shade tolerant plants are thus adapted to be efficient energy-users. In simple terms, shade-tolerant plants grow broader, thinner leaves
, to catch more sunlight relative to the cost of producing the leaf. Shade tolerant plants are also usually adapted to make more use of soil nutrients than shade intolerant plants.
One can also argue for the distinction between merely "shade tolerant" plants and "shade-loving" or sciophilous plants. Those are the species that actually are dependent on a degree of shading that would eventually kill most other plants, or at least stunt them badly.
, during the period of time when it would still be hazardous for trees to leaf out. As an extreme example of this, winter annuals sprout in the fall, grow through the winter, and flower
and die in the spring.
Just like with trees, shade-tolerance in herbaceous plants is diverse. Some early-leafing out plants will persist after the canopy leafs out, whereas others rapidly die back. In many species, whether or not this happens depends on the environment, such as water supply and sunlight levels.
Although most plants grow towards light, many tropical vine
s, such as Monstera deliciosa
and a number of other members of the family Araceae
, such as members of the genus Philodendron
, initially grow away from light; this is a dramatic example of sciophilous growth, which helps them locate a tree trunk, which they then climb to regions of brighter light. The upper shoots and leaves of such a Philodendron grow as typical light-loving photophilous plants once they break out into full sunshine.
The Eastern Hemlock
, considered the most shade tolerant of all North American tree species, is able to germinate, persist, and even grow under a completely closed canopy. Hemlocks also exhibit the ability to transfer energy to nearby trees through their root
system. In contrast, the Sugar Maple
, also considered to be highly shade tolerant, will germinate under a closed canopy and persist as an understory species, but only grows to full size when a gap is generated.
Shade-intolerant species such as willow
and aspen
cannot sprout under a closed canopy. Shade-intolerant species often grow in wetland
s, along waterways, or in disturbed areas, where there is adequate access to direct sunlight.
In addition to being able to compete in conditions of low light intensity, shade bearing species, especially trees, are able to withstand relatively low daytime temperatures compared with the open, and above all high root competition especially with subordinate vegetation. It is very difficult to separate the relative importance of light and below ground competition, and in practical terms they are inextricably linked.
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, shade tolerance is a plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
's abilities to tolerate low light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
levels. The term is also used in horticulture
Horticulture
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
and landscaping
Landscaping
Landscaping refers to any activity that modifies the visible features of an area of land, including:# living elements, such as flora or fauna; or what is commonly referred to as gardening, the art and craft of growing plants with a goal of creating a beautiful environment within the landscape.#...
, although in this context its use is sometimes sloppy, especially with respect to labeling of plants for sale in nurseries
Nursery (horticulture)
A nursery is a place where plants are propagated and grown to usable size. They include retail nurseries which sell to the general public, wholesale nurseries which sell only to businesses such as other nurseries and to commercial gardeners, and private nurseries which supply the needs of...
.
Shade tolerance is a relative term, and its use and meaning depends on context. One can compare large trees to each other, but when comparing understory
Understory
Understory is the term for the area of a forest which grows at the lowest height level below the forest canopy. Plants in the understory consist of a mixture of seedlings and saplings of canopy trees together with understory shrubs and herbs...
trees and shrub
Shrub
A shrub or bush is distinguished from a tree by its multiple stems and shorter height, usually under 5–6 m tall. A large number of plants may become either shrubs or trees, depending on the growing conditions they experience...
s, or non-woody plants, the term takes on a different meaning. Even in a specific context, shade tolerance is not a single variable or simple continuum, but rather a complex, multi-faceted property of plants, since different plants exhibit different adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s to shade
Shade
Shade is the blocking of sunlight by any object, and also the shadow created by that object. Shade also consists of the colors grey, black, white, etc...
. In fact, the same plant can exhibit varying degrees of shade tolerance or even of requirement for light, depending on its past history or stage of development.
Basic concepts
Except for some parasitic plants, all plants need sunlight to survive. However, in general, more sunlight does not always make it easier for plants to survive. Where waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is scarce, life can actually be easier in the shade. In direct sunlight, plants face desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
and exposure to UV rays, and must expend energy producing pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s to block UV light, and waxy coatings to prevent water loss.
Plants adapted to shade have the ability to utilise far-red light (about 730nm) more effectively than plants adapted to full sunlight. Most red light gets absorbed by the shade intolerant canopy plants, but more of the far-red light penetrates the canopy, reaching the understory. The shade tolerant plants found here are capable of photosynthesis using light at such wavelengths.
On the other hand, when less light is available, less energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
is available to the plant. Whereas in sunny and dry environments water can be a limiting factor
Limiting factor
A limiting factor or limiting resource is a factor that controls a process, such as organism growth or species population, size, or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism...
in growth and survival, in shade, energy (in the form of sunlight) is usually the limiting factor.
The situation with respect to nutrients is often different in shade and sun. Most shade is due to the presence of a canopy of other plants, and this is usually associated with a completely different environment--richer in soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
nutrient
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
s--than sunny areas.
Shade tolerant plants are thus adapted to be efficient energy-users. In simple terms, shade-tolerant plants grow broader, thinner leaves
Leaves
-History:Vocalist Arnar Gudjonsson was formerly the guitarist with Mower, and he was joined by Hallur Hallsson , Arnar Ólafsson , Bjarni Grímsson , and Andri Ásgrímsson . Late in 2001 they played with Emiliana Torrini and drew early praise from the New York Times...
, to catch more sunlight relative to the cost of producing the leaf. Shade tolerant plants are also usually adapted to make more use of soil nutrients than shade intolerant plants.
One can also argue for the distinction between merely "shade tolerant" plants and "shade-loving" or sciophilous plants. Those are the species that actually are dependent on a degree of shading that would eventually kill most other plants, or at least stunt them badly.
Herbaceous plants
In temperate zones, many wildflowers and non-woody plants persist in the closed canopy of a forest by leafing out early in the spring, before the trees leaf out. This is partly possibly because the ground tends to be more sheltered and thus the plants are less susceptible to frostFrost
Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...
, during the period of time when it would still be hazardous for trees to leaf out. As an extreme example of this, winter annuals sprout in the fall, grow through the winter, and flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
and die in the spring.
Just like with trees, shade-tolerance in herbaceous plants is diverse. Some early-leafing out plants will persist after the canopy leafs out, whereas others rapidly die back. In many species, whether or not this happens depends on the environment, such as water supply and sunlight levels.
Although most plants grow towards light, many tropical vine
Vine
A vine in the narrowest sense is the grapevine , but more generally it can refer to any plant with a growth habit of trailing or scandent, that is to say climbing, stems or runners...
s, such as Monstera deliciosa
Monstera deliciosa
Monstera deliciosa is a creeping vine native to tropical rainforests of southern Mexico south to Colombia.Common names include Ceriman, Swiss Cheese Plant , Fruit Salad Plant, Monster fruit, Monsterio Delicio, Monstereo, Mexican Breadfruit, Monstera, split-leaf philodendron, Locust and Wild Honey,...
and a number of other members of the family Araceae
Araceae
Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe or leaf-like bract. Also known as the Arum family, members are often colloquially...
, such as members of the genus Philodendron
Philodendron
Philodendron is a large genus of flowering plants in the Araceae family, consisting of close to 900 or more species according to TROPICOS . Other sources quote different numbers of species. According to S.J. Mayo there are about 350-400 formally recognized species whereas according to Croat there...
, initially grow away from light; this is a dramatic example of sciophilous growth, which helps them locate a tree trunk, which they then climb to regions of brighter light. The upper shoots and leaves of such a Philodendron grow as typical light-loving photophilous plants once they break out into full sunshine.
Trees
In forests where rainfall is plentiful and water is not the limiting factor to growth, shade-tolerance is one of the most important factors characterizing tree species. However, different species of trees exhibit different adaptations to shade.The Eastern Hemlock
Eastern Hemlock
Tsuga canadensis, also known as eastern or Canadian hemlock, and in the French-speaking regions of Canada as pruche du Canada, is a coniferous tree native to eastern North America. It ranges from northeastern Minnesota eastward through southern Quebec to Nova Scotia, and south in the Appalachian...
, considered the most shade tolerant of all North American tree species, is able to germinate, persist, and even grow under a completely closed canopy. Hemlocks also exhibit the ability to transfer energy to nearby trees through their root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
system. In contrast, the Sugar Maple
Sugar Maple
Acer saccharum is a species of maple native to the hardwood forests of northeastern North America, from Nova Scotia west to southern Ontario, and south to Georgia and Texas...
, also considered to be highly shade tolerant, will germinate under a closed canopy and persist as an understory species, but only grows to full size when a gap is generated.
Shade-intolerant species such as willow
Willow
Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere...
and aspen
Aspen
Populus section Populus, of the Populus genus, includes the aspen trees and the white poplar Populus alba. The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by...
cannot sprout under a closed canopy. Shade-intolerant species often grow in wetland
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....
s, along waterways, or in disturbed areas, where there is adequate access to direct sunlight.
In addition to being able to compete in conditions of low light intensity, shade bearing species, especially trees, are able to withstand relatively low daytime temperatures compared with the open, and above all high root competition especially with subordinate vegetation. It is very difficult to separate the relative importance of light and below ground competition, and in practical terms they are inextricably linked.