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transpiration

transpiration, in botany, the loss of water by evaporation in terrestrial plants. Some evaporation occurs directly through the exposed walls of surface cells, but the greatest amount takes place through the stomates, or intercellular spaces (see leaf). Transpiration functions to effect the ascent of sap from the roots to the leaves (thus supplying the food-manufacturing cells with water needed for photosynthesis) and to provide the moisture necessary for the diffusion of carbon dioxide into and oxygen out of these cells. The rate of transpiration is almost always far greater than the above functions would seem to warrant; in most plants 200 to 1,000 lb (90–450 kg) of water are transpired for each pound of solid material added to the plant. Various factors influence the transpiration rate. Photosynthesis, induced by light, has the effect of increasing the water pressure in the guard cells that border each stomate and that, in expanding, pull apart to widen the stomate aperture and thereby increase water loss. Low humidity promotes the diffusion of water vapor from the air passages inside the leaf into the outside air. A lack of water in the soil cuts down the water supply to the cells, thus limiting expansion of the guard cells. Therefore the rate is highest on a bright, dry day and lowest at night or in drought conditions. Morphological factors such as reduced leaf surfaces, a heavy cuticle layer on the leaves, low numbers of stomates, and stomates recessed below the other epidermal cells also lower the rate; desert plants such as conifers and cacti conserve water in these ways. Plants also lose some water by guttation, a process whereby water is exuded directly through pores called hydathodes. The reaction of a plant to excessive water loss is wilting and, eventually, death.

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transpiration

transpiration The loss of water vapour by plants to the atmosphere. It occurs mainly from the leaves through pores (stomata) whose primary function is gas exchange. The water is replaced by a continuous column of water (and dissolved nutrients) moving upwards from the roots within the xylem vessels. The flow of this column of water is known as the transpiration stream, which is maintained by root pressure and a combination of cohesive and adhesive forces in the xylem vessels according to the cohesion–tension theory (see cohesion). See also potometer.

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transpiration

transpiration The loss of water vapour from a plant to the outside atmosphere. It takes place mainly through the stomata of leaves and the lenticels of stems. Its function is disputed. It may reduce leaf temperature, but its absence from some tropical plants would suggest that this is not essential. It may also be important in mineral absorption and translocation. However, it may be merely an inevitable concomitant of gaseous exchange which, to be efficient, requires open stomata; as gases are exchanged, water is lost.

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transpiration

transpiration The loss of water vapour from a plant to the outside atmosphere. It takes place mainly through the stomata of leaves and the lenticels of stems. Its function is disputed. It may reduce leaf temperature, but its absence from some tropical plants would suggest that this is not essential. It may also be important in mineral absorption and translocation. However, it may be merely an inevitable concomitant of gaseous exchange which, to be efficient, requires open stomata; as gases are exchanged, water is lost.

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transpiration

transpiration The removal of moisture from the soil by plant roots, its translocation up the stem to the leaves, and its evaporation through the stomata. The flow of water through the plant is known as the transpiration stream. It reduces leaf temperatures, and is thought to be important for mineral absorption and translocation within plants. The process imposes a number of environmental requirements upon plants; for example, wilting and desiccation result from an insufficient supply of water.

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transpiration

transpiration In plants, the loss of moisture as water vapour from leaf surfaces or other plant parts. Most of the water entering plant roots is lost by transpiration. The process is speeded up in light, warm, and dry conditions. The flow of water from the roots to the stomata is called the transpiration stream. See also photosynthesis; respiration; stoma

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Transpiration

Transpiration

Why do organisms transpire water?

Transpiration as an ecological process

Effects of human activities on transpiration

Resources

Transpiration refers to the evaporation of water from a biological surface, such as leaves, skin, or lungs. In its most common usage, however, transpiration refers to the loss of water from plant foliage (such as stems, fruits, and flowers), occurring through microscopic pores known as stomata. Transpiration is a component of a larger process known as evapotranspiration, which is the evaporation of water from a landscape, including both inorganic surfaces such as soil and bodies of water, and biological surfaces such as foliage.

Why do organisms transpire water?

Most transpiration by plants involves water that evaporates from the moist membranes of a tissue known as spongy mesophyll, which occur in the minute cavities beneath the tiny leaf pores called stomata. Stomata can be closed tightly using bordering cells known as guard cells. However, in most plants, stomata are kept open much of the time. This is done so that carbon dioxide, which is needed for photosyn-thesis, can diffuse into the leaf, and oxygen, a waste product, can diffuse out. Therefore, transpiration by plants can be viewed as an unavoidable consequence of having moist mesophyll membranes exposed to the atmosphere. It is important to understand that, in general, any area of vegetation with a multilayered canopy of foliage will evaporate much larger quantities of water than can an equivalent non-foliated area, such as the surface of a lake or moist soil.

A similar explanation can be developed for animals, who must also maintain moist respiratory surfaces open to the atmosphere; for example, in the lungs, from which water can freely evaporate. Transpiration can be an important problem for both plants and animals, because it may be difficult to replenish their water losses in relatively dry environments, so that dehydration can occur. If severe, dehydration can kill plants and animals. In some respects, then, transpiration can be regarded as a necessary evil that organisms must endure in order to exchange oxygen and carbon dioxide with the atmosphere.

However, there can sometimes be important benefits of transpiration. It takes about 540 calories of thermal energy to evaporate 1 gal (3.8 l) of water at an ambient temperature of 68° F (20° C). This is a rather large amount of energy, and as a result transpiration can be an important way by which some animals and plants cool themselves. For example, when humans are hot, they sweat to distribute water onto their exposed skin, which is then cooled by the subsequent evaporation.

Transpiration as an ecological process

Transpiration is an ecologically important process. In areas where forests are common, evapotranspiration almost entirely occurs as transpiration, and this process can account for a substantial part of the manner by which the landscape deals with water inputs through rain and snow. For example, in typical forested landscapes of northeastern North America, evapotranspiration accounts for about 15 to 40% of the annual inputs of water with precipitation, the remainder draining to groundwater, or flushing from the system as stream flow.

In any regions with a seasonal climate, rates of evapotranspiration vary greatly during the year. Consider, for example, the case of a landscape in a temperate climate, covered with a seasonally deciduous, angiosperm forest. During winter, very little transpiration occurs because plant tissues are frozen. However, there can still be some physical evaporation of water from the surface of snow and ice, occurring by the direct vaporization of solid water, a process known as sublimation. During the springtime, unfrozen water is abundant, but the trees do not yet have foliage and this greatly reduces the rates of transpiration. During the growing season, air temperatures are warm and the trees are fully foliated, so transpiration occurs in large rates. A mature tree, on average, may lose hundreds of gallons of water through its leaves on a hot, dry summer day. During this time of the year, so much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds water inputs by rainfall. As a result, the soil is dried by the demands of plant roots for water, to the extent that streams may cease to flow by late summer. Once the trees drop their leaves in the autumn, transpiration rates decrease greatly, the water-depleted soil becomes recharged by rainfall, and streams again flow.

Effects of human activities on transpiration

The influence of an intact forest on evapotranspiration is affected by ecological disturbances, such as clear-cutting and wildfires. In general, these sorts of stand-level disturbances greatly reduce the transpiration component of evapotranspiration for several years, a change that influences other hydrological processes, such as the timing and amounts of stream flow, which may then have effects on flooding and erosion. In addition, on sites that do not drain well, substantial decreases in transpiration can increase the height of the water table.

An ecological study done at Hubbard Brook, New Hampshire, involved the clear-cutting of all trees on a 39.5-acre (16-hectare) watershed, which was then kept clear of regenerating plants for two additional years. The subsequent disruption of transpiration increased the amount of stream-water flow by an average of 31% over the three-year period. Many other studies of forestry have come to similar conclusionsclear-cutting increases stream flow by decreasing transpiration. However, because disturbed forests regenerate quickly through ecological succession, the amount of foliage on the site quickly recovers, and the effect of disturbance on stream flow tends to be rather short lived. In general, the pre-cutting stream flow volumes are substantially re-attained after three to five years of revegetation has occurred, because of the recovery of

KEY TERMS

Evapotranspiration The evaporation of water from both inorganic and biological surfaces on a landscape.

Stomata Pores in plant leaves that function in exchange of carbon dioxide, oxygen, and water during photosynthesis.

Watershed The expanse of terrain from which water flows into a wetland, waterbody, or stream.

the transpirational surface area of plant foliage. (Of course, the regenerated foliage occurs on relatively short grasses, herbs, and shrubs, rather than on taller trees as in the initial forest.) The largest increases in stream flow usually occur in the first year after the forest is harvested, with progressively smaller effects afterward.

Resources

BOOKS

Babe, Robert E. Culture of Ecology: Reconciling Economics and Environment. Toronto, Ontario, Canada: University of Toronto Press, 2006.

Kruger, Linda E. Understanding Community-Forest Relations. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Service, 2003.

Tietenberg, Thomas, H. Environmental and Natural Resource Economics. Boston: Pearson/Addison Wesley, 2006.

Tudge, Colin. The Tree: A Natural History of What Trees Are, How They Live, and Why They Matter. New York: Crown Publishers, 2006.

Bill Freedman

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Transpiration

Transpiration

Transpiration is the process by which plants give off water vapor from their leaves to the atmosphere . The process is an important stage in the water cycle, often more important in returning water to the atmosphere than is evaporation from rivers and lakes. A single acre of growing corn, for example, transpires an average of 3,500 gal (13,248 l) of water per acre (0.4 ha) of land per day. Transpiration is, therefore, an important mechanism for moving water through the soil , into plants, and back into the atmosphere. When plants are removed from an area, soil retains more moisture and is unable to absorb rain water. As a consequence, runoff and loss of nutrients from the soil is likely to increase.

See also Erosion; Flooding; Soil conservation; Soil fertility

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Transpiration

Transpiration

Transpiration refers to the evaporation of water from a biological surface, such as leaves, skin, or lungs. In its most common usage, however, transpiration refers to the loss of water from plant foliage, occurring through microscopic pores known as stomata. Transpiration is a component of a larger process known as evapotranspiration , which is the evaporation of water from a landscape, including both inorganic surfaces such as soil and bodies of water, and biological surface such as foliage.


Why do organisms transpire water?

Most transpiration by plants involves water that evaporates from the moist membranes of a tissue known as spongy mesophyll, which occur in the minute cavities beneath the tiny leaf pores called stomata. Stomata can be closed tightly using bordering cells known as guard cells. However, in most plants stomata are kept open much of the time. This is done so that carbon dioxide , which is needed for photosynthesis , can diffuse into the leaf, and oxygen , a waste product, can diffuse out. Therefore, transpiration by plants can be viewed as an unavoidable consequence of having moist mesophyll membranes exposed to the atmosphere. It is important to understand that, in general, any area of vegetation with a multilayered canopy of foliage will evaporate much larger quantities of water than can an equivalent non-foliated area, such as the surface of a lake or moist soil.

A similar explanation can be developed for animals, who must also maintain moist respiratory surfaces open to the atmosphere, for example in the lungs, from which water can freely evaporate. Transpiration can be an important problem for both plants and animals, because it may be difficult to replenish their water losses in relatively dry environments, so that dehydration can occur. If severe, dehydration can kill plants and animals. In some respects, then, transpiration can be regarded as a necessary evil that organisms must endure in order to exchange oxygen and carbon dioxide with the atmosphere.

However, there can sometimes be important benefits of transpiration. It takes about 540 calories of thermal energy to evaporate 1 gallon of water at an ambient temperature of 68°F (20°C). This is a rather large amount of energy, and as a result transpiration can be an important way by which some animals and plants cool themselves. For example, when humans are hot, they sweat to distribute water onto their exposed skin, which is then cooled by the subsequent evaporation.

Transpiration as an ecological process

Transpiration is an ecologically important process. In areas where forests are common, evapotranspiration almost entirely occurs as transpiration, and this process can account for a substantial part of the manner by which the landscape deals with water inputs through rain and snow. For example, in typical forested landscapes of northeastern North America , evapotranspiration accounts for about 15-40% of the annual inputs of water with precipitation , the remainder draining to ground-water , or flushing from the system as streamflow.

In any regions with a seasonal climate, rates of evapotranspiration vary greatly during the year. Consider, for example, the case of a landscape in a temperate climate, covered with a seasonally deciduous, angiosperm forest. During winter, very little transpiration occurs because plant tissues are frozen. However, there can still be some physical evaporation of water from the surface of snow and ice , occurring by the direct vaporization of solid water, a process known as sublimation. During the springtime, unfrozen water is abundant, but the trees do not yet have foliage and this greatly reduces the rates of transpiration. During the growing season, air temperatures are warm and the trees are fully foliated, so transpiration occurs in large rates. During this time of the year, so much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds water inputs by rainfall. As a result, the soil is dried by the demands of plant roots for water, to the extent that streams may cease to flow by late summer. Once the trees drop their leaves in the autumn, transpiration rates decrease greatly, the water-depleted soil becomes recharged by rainfall, and streams again flow.


Effects of human activities on transpiration

The influence of an intact forest on evapotranspiration is affected by ecological disturbances, such as clear-cutting and wildfire . In general, these sorts of stand-level disturbances greatly reduce the transpiration component of evapotranspiration for several years, a change that influences other hydrological processes, such as the timing and amounts of stream flow, which may then have effects on flooding and erosion . In addition, on sites that do not drain well, substantial decreases in transpiration can increase the height of the water table.

An ecological study done at Hubbard Brook, New Hampshire, involved the clear-felling of all trees on a 39.5-acre (16-hectare) watershed , which was then kept clear of regenerating plants for two additional years. The subsequent disruption of transpiration increased the amount of streamwater flow by an average of 31% over the three-year period. Many other studies of forestry have come to similar conclusions—clear-cutting increases streamflow by decreasing transpiration. However, because disturbed forests regenerate quickly through ecological succession , the amount of foliage on the site quickly recovers, and the effect of disturbance on streamflow tends to be rather short lived. In general, the pre-cutting stream flow volumes are substantially re-attained after three to five years of revegetation has occurred, because of the recovery of the transpirational surface area of plant foliage. (Of course, the regenerated foliage occurs on relatively short grasses , herbs, and shrubs, rather than on taller trees as in the initial forest.) The largest increases in stream flow usually occur in the first year after the forest is harvested, with progressively smaller effects afterwards.


Resources

books

Freedman, B. Environmental Ecology. 2nd ed. San Diego: Academic Press, 1995.

Kimmins, J. P. Forest Ecology. New York: Macmillan, 1987.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Evapotranspiration

—The evaporation of water from both inorganic and biological surfaces on a landscape.

Stomata

—Pores in plant leaves which function in exchange of carbon dioxide, oxygen, and water during photosynthesis.

Watershed

—The expanse of terrain from which water flows into a wetland, waterbody, or stream.

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