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 shrubs, 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 adaptations to shade. In fact, the same plant can exhibit varying degrees of shade tolerance or even of requirement for light, depending on its history or stage of development.
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. In direct sunlight, plants face desiccation and exposure to UV rays, and must expend energy producing pigments to block UV light, and waxy coatings to prevent water loss.
Plants adapted to shade have the ability to utilise far-red light (about 730 nm) 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 nutrients--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.
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 frost, 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 vines, 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.
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, 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 wetlands, 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.
- C.D. Canham. "Different Respones to Gaps Among Shade-Tolerant Tree Species". Ecology, Vol. 70, No. 3, pp. 548–550. (Jun 1989)