Shade tolerance 5132315 212137324 2008-05-13T16:19:15Z 167.197.117.136 because [[Image:Tsuga canadensis2.jpg|right|thumb|Eastern Hemlock is a shade tolerant tree.]] '''Shade tolerance''' is an [[ecological]] concept that refers to [[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 [[nursery (horticulture)|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]]. ==Basic concepts== All but a few plants need sunlight to survive. However, in general, more sunlight does not always make it easier for plants to survive. Where [[water]] is scarce, life can actually be easier in the shade. In direct sunlight, plants face [[desiccation]] and exposure to [[UV ray]]s, and must expend energy producing [[pigment]]s to block UV light, and waxy coatings to prevent water loss. 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 their sun-loving counterparts. ==Non-woody 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 [[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 non-woody 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 [[Philodendron]] genus) initially grow away from light; this helps them locate a tree trunk, which they then climb to regions of brighter light. ==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]], 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 [[disturbance (ecology)|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. ==References== * C.D. Canham. "Different Respones to Gaps Among Shade-Tolerant Tree Species". ''Ecology'', Vol. 70, No. 3, pp. 548-550. (Jun 1989) [[Category:Plant physiology]]