- Shade tolerance
Shade tolerance is an
ecological concept that refers toplant s' abilities to tolerate lowlight levels. The term is also used inhorticulture andlandscaping , 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 comparingunderstory trees andshrub 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 differentadaptation s toshade .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 facedesiccation and exposure toUV ray s, and must expend energy producingpigment 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 alimiting 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, andflower 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 asMonstera deliciosa (and a number of other members of thePhilodendron 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 togerminate , persist, and even grow under a completely closed canopy. Hemlocks also exhibit the ability to transfer energy to nearby trees through theirroot system. In contrast, theSugar 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 andaspen cannot sprout under a closed canopy. Shade-intolerant species often grow inwetland 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.
References
* C.D. Canham. "Different Respones to Gaps Among Shade-Tolerant Tree Species". "Ecology", Vol. 70, No. 3, pp. 548-550. (Jun 1989)
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