Plant Transpiration: Amazon Rainforest
Studies indicate that transpiration from plants account for about 10 percent of the moisture in the atmosphere

What Is Transpiration?

Transpiration is an important mechanism in both the water and carbon cycles. It involves the discharge of water from a tree, or plant, in the form of water vapor. Although the climate science remains unclear, it’s possible that transpiration may help to offset some of the effects of global warming by exerting a cooling effect on the transpiring tree, and the air around it.

What’s more, this cooling effect may counteract a significant percentage of the global warming caused by extra carbon dioxide in the atmosphere. Even better, transpiration is a major contributor to cloud formation, whose albedo keeps Earth’s temperature several degrees lower than it would be otherwise . See, for instance: How Do Clouds Affect Climate?

How Does Transpiration Work?

Transpiration begins when water is absorbed from the soil by the plant’s roots and transported to the leaves and stems through the plant’s xylem tissue (sapwood). 1 Thousands of tiny pores called stomata, located mostly on the underside of the leaf surface, allow the water vapor to escape.

Of all the water absorbed by plants, only about 5 percent is retained by the plant for growth and metabolism. The rest is lost through transpiration. 2 In most active plants, water is continuously evaporating from the leaves, while being replenished by fresh absorption of moisture from the soil. Yet another reason why soil is so important to the planet.

Once transpiration is under way, it results in a continuous column of water in the plant’s xylem from the roots to the leaves, where it evaporates into the atmosphere. Due to the tendency of water molecules to stick to one another, this column of water is drawn upwards by the evaporation taking place via the stomata, in a process known as the Cohesion Theory of Sap Ascent in plants.

To put it more simply, transpiration is like sucking on a soda straw: it causes a negative pressure which sucks the column of water to the leaf surface.

How Plant Transpiration Works
How transpiration works: water moves up from the roots, through the plant and evaporates through the leaves via open stomata.

Transpiration has two main benefits: it cools the plant (rather like sweating cools the human body), and it pumps water, nutrients and minerals to the leaves for photosynthesis. Scientists estimate that up to 98 percent of a plant’s energy is devoted to transpiration. 3 Studies indicate that transpiration accounts for about 10 percent of the moisture in the atmosphere, with evaporation from the oceans, and other bodies of water providing about 90 percent. 4

What Is Evapotranspiration?

This rather confusing term describes the loss of water caused both by (a) evaporation from the soil, wetlands, paddy fields and the like, and (b) transpiration from the leaves of plants and trees. How much of evapotranspiration is evaporation and how much is transpiration? Scientists don’t know exactly. Precise figures are hard to come by.

Evapotranspiration Infographic
Water cycle of the Earth’s surface, showing transpiration of plants and evaporation from the Earth’s land and ocean that make up evapotranspiration. Image © CC BY 4.0

According to one estimate: over a growing season, between seventy and eighty percent of all evapotranspiration is made up of transpiration. 5 Evapotranspiration plays an important role in the water cycle being responsible for about 15 percent of all water vapor in the atmosphere. Without this contribution, clouds couldn’t form and precipitation would never fall. 6

Why Do Plants Transpire?

There are four reasons why plants transpire.

  • To cool down

Transpiration uses up a lot of the plant’s energy. For example, it takes almost 600 calories to evaporate 1 gram of water. So, a large oak tree – which can transpire 151,000 litres a year – may burn as many as 150 million calories a year on transpiration.

By using all this energy (heat), transpiration exerts a huge cooling effect on the plant and surrounding air. This is one reason why urban planners around the world are embracing the idea of ‘sponge cities‘ – new, green infrastructure projects that combat urban flooding and urban heat island effect.

Plants need to cool down for several reasons. When ambient temperatures get too high, metabolic systems begin to slow down, sometimes causing growth and flowering to stop altogether. In cases of extreme heat, plants can become severely stressed and even die.

Transpiration is an evaporative cooling system that brings down the temperature of plants, but because it results in water loss through the stomata, it is carefully regulated by neighboring guard cells that control the opening and closing of the stomata to protect the tree or plant from dehydration. Even so, those plants that are able to manage by keeping their stomata slightly, rather than fully, open, will have a better “water use efficiency” (a lower ratio of water lost to carbon dioxide gained), which will make it better able to survive periods when soil moisture is scarce.

  • To drive the “circulatory” system

In humans, the mechanism that drives circulation is the heartbeat; in plants and trees, it’s transpiration. Water vapor that evaporates from the leaf surface, draws more water up to replace it. And not just water, but minerals and other nutrients too. Moisture absorbed from the soil typically contains a variety of nutrients that are necessary for plant growth and development. During transpiration, as water evaporates, these nutrients remain behind and are then distributed throughout the plant via the xylem.

  • To permit the absorption of carbon dioxide 

Plants derive their energy from photosynthesis – a chemical process which takes place inside every plant and tree during daylight hours. For photosynthesis to occur, a plant needs access to water, light and carbon dioxide (CO2). It gets water from the soil, light from the sun and carbon dioxide (CO2) from the atmosphere. The CO2 enters the plant through the latter’s stomata. So, when the plant transpires during the day, it also permits the entry of CO2 and facilitates the chemical reactions that drive photosynthesis. (For details of how plants emit CO2, see: What is Plant Respiration?)

  • To maintain water pressure

Transpiration is essential to maintaining water pressure (known as turgor pressure) within the cells of the plant, so that they can keep the plant upright without the need for any skeletal structure. Although only about 5 percent of the water taken up by roots stays in the plant, that amount is essential for plant structure and function. For example, rigid leaves have much higher transpiration rates than wilting foliage.

How Much Water Do Plants Transpire?

During a growing season, an acre of corn transpires roughly 3,000-4,000 gallons (11,500-15,000 litres) of water each day,  4 and a large eucalyptus tree can transpire 20,000 gallons (75,000 litres) per year. In the Amazon Rainforest, each canopy tree transpires around 760 litres (200 gallons) of water annually, equivalent to about 76,000 litres (20,000 gallons) of water per acre of canopy. Much the same occurs in the Congo rainforest in central Africa.

What Factors Affect The Rate Of Transpiration?

There are 7 main factors that determine how much water is transpired.

1. Plant type

Some plants (e.g. cacti) hold onto their water and have low transpiration rates. In the case of cacti, it’s because they have only a limited number of stomata. Trees and crops are on the other end of the spectrum and can transpire large amounts of water vapor every day. A large maple tree, for instance, can transpire 190-230 litres (50-60 gallons) of water per hour.

Larger boundary layer reduces transpiration

The boundary layer around a plant is the thin layer of usually moist air which hugs the surface of the leaf. Similar to the ocean-atmosphere exchange of CO2, any water vapor leaving the stomata must pass through this boundary layer to reach the atmosphere. The thicker the boundary layer, the less transpiration is likely to occur. In order to achieve this lower rate of transpiration and increase their water use efficiency, some plants boast a variety of structural features, including stomata that are sunk low into the leaf surface, or leaf hairs that slow down air movement – both of which will significantly increase the boundary layer.

Thicker cuticles reduce transpiration

A plant’s cuticle is the waxy layer on its visible tissue (like a leaf), which functions as a barrier to water movement out of the stomata. The cuticle is made of wax, so water doesn’t pass through it very easily. The thicker the layer of cuticle on a leaf surface, the less transpiration occurs. Generally speaking, plants from hot, arid climates have thicker cuticles than plants from cool, moist zones.

2. Temperature

Warm air is able to hold more water vapor than cool air. Therefore, in warmer ambient temperatures, transpiration tends to increase, while in cooler temperatures it declines. Also, at warmer temperatures, plants open their stomata wider and release more water vapor.  

3. Relative humidity (RH)

In a nutshell, as humidity in the surrounding air increases, transpiration decreases. Technically speaking it’s all about relative humidity – how much water vapor is the surrounding air holding, compared to the maximum it can hold. The smaller the amount of moisture in the air, compared to its potential, the more transpiration will occur. Whereas if the air is almost full of water, transpiration will be extremely slow or even cease altogether.

4. Wind

In general, the more wind, the more transpiration. This is because wind blows away the boundary layer, that still layer of water vapor hugging the surface of leaves, making it easier for transpiration to occur.

5. Water availability

If the soil is dry and plants can’t get enough water, they will conserve it by closing their stomata. Transpiration will therefore cease. Plants can’t transpire in very dry soils without wilting because the water in the xylem that is sucked up and out through the leaves is not being replaced by the soil water. When this happens, the leaf loses its rigidity and the stomata to close, or else the plant will wilt.

6. Soil type

Soil type and soil profile exerts a strong influence on how much water can be held in the soil and how easy it is for plants to draw water out of it. Also, in ecosystems where the ground is covered in vegetation or foliage, the rate of transpiration is considerably higher than the rate of evaporation from the soil.

7. Light

Stomata are programmed to open during daylight so that the plant can ingest the necessary carbon dioxide to fuel its growth, via photosynthesis. Since photosynthesis only occurs in the light, stomata (in most plants) are closed in the dark. Happily, stomata are most sensitive to blue light, the colour that predominates at dawn, so they open promptly at the first hint of sunrise.

Is Transpiration Affected By Global Warming?

Yes. But nobody knows the exact picture. It seems that rising temperatures and rising levels of carbon dioxide both have an impact on transpiration, although scientists and climate modellers have yet to obtain sufficient data to determine the extent of that impact. See also: Is more CO2 good for plants?

• At present, it is well documented through outdoor experiments (involving whole tree chambers or free air carbon enrichment) as well as field studies, that increasing atmospheric carbon dioxide causes greater and faster growth in both plants and trees. It’s a bit like adding fertilizer to the soil. The point is, when there’s more CO2 in the air, plants can photosynthesize more easily, which gives them more energy for growth and development.

As far as transpiration is concerned, more CO2 in the air means that plant stomata do not need to open as much. This means that less water needs to be transpired per leaf. And with increased photosynthesis but reduced water loss, the plants’ water-use becomes more efficient.

Thus, on the face of it, more carbon dioxide in the atmosphere is good for plants – they grow faster, use less water and become more water-efficient. And if it is good for plants, then surely it can’t be bad for the planet.

It’s true, water loss per leaf is lower, but plant growth leads to more leaves, so the total amount of transpiration actually increases. This leads to a cooling effect, but how much of a cooling effect? (Answer: we don’t know.) Is the cooling effect enough to offset the increase in the greenhouse effect caused by rising greenhouse gas emissions of CO2? (Answer: absolutely not.)

• Here’s another question. In view of the growing importance attached to trees in the mitigation of climate change 7 might not the increased tree growth (due to its greater CO2 absorption via photosynthesis), soak up a significant percentage of the increased CO2 emissions? (Answer: we don’t really know.)

Climate models still can’t give us the answers we need, although they are getting better. And as they improve, they are producing some unlikely results.

• For example, in 2010, a study by NASA found that additional plant growth in a world with doubled CO2 levels, creates a new negative feedback – a cooling effect – in the climate system that may help to limit the rise in temperature. The warming effect of climate change caused by the elevated CO2 was found to be 1.94 degrees Celsius, but the cooling effect of the extra plant transpiration and photosynthesis reduced this by 0.6 degrees Celsius. 8

• The NASA model contradicts an earlier study conducted by scientists at the Carnegie Institution for Science. 9 This found that a doubling of atmospheric CO2 raised global temperatures because it caused plant stomata to shrink, which led to reduced transpiration and a greatly reduced cooling effect. Researchers estimated that this effect contributed a whopping 16 percent to the overall warming effect.

• Another study has found that increasing humidity and higher CO2 levels both tend to reduce transpiration and counteract the additional transpiration caused by CO2-induced plant growth. 10

• A 2019 study of a South American tree (Alchornea glandulosa) native to southern Brazil, from Minas Gerais to Rio Grande do Sul, concluded that the tree might be able to cope with increases in air temperature, although reductions in transpiration per leaf could cause higher leaf temperatures over the coming century. Interestingly, photosynthesis and photosynthetic capacities were shown to have very high temperature tolerances. 11

• Another 2019 study, this time into the C4 forage species Panicum maximum, found that higher CO2 led to a 10 percent reduction in total transpiration, a 25 percent rise in photosynthesis, and a 75 percent greater water-use efficiency. Higher temperatures had no effect on transpiration, while in an environment combining elevated CO2 and temperature, transpiration was unaffected. 12

• In another recent study, scientists sought to answer the question: “Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?” They found that for most locations and under most RCPs (Representative Concentration Pathways – scenarios proposed by the IPCC’s Fifth Assessment Report, 2013), the closure of stomata caused by elevated CO2, has a larger effect on reducing transpiration than higher temperatures have on raising transpiration. 13

Net Effect of Global Warming

Climate change involving increased levels of atmospheric carbon dioxide and higher global temperatures may be partially offset by a small but noticeable cooling effect from the plant growth caused by the CO2. However, the quality of computer-generated climate data needs to improve significantly before we can understand the scale and value of this cooling. For more, see: 7 Effects of Climate Change on Plants.

References

  1. Handbook of Plant Science. Keith Roberts, ed. (2007). John Wiley & Sons. []
  2. “Modern Plant Physiology.” CRC Press. Rajiv Kumar Sinha. ISBN 9780-8493-1714. []
  3. Plant Survival: Adapting to a Hostile World. Brian Capon. Timber Press. Portland, Oregon. []
  4. Evapotranspiration and the Water Cycle.” U.S. Geological Survey. [][]
  5. “Components of Evapotranspiration. (Chapter 6, Crop Water Use) ” Plant & Soil Sciences eLibrary. []
  6. “Evapotranspiration.” North Carolina Climate Office. climate.ncsu.edu []
  7. “Tree planting ‘has mind-blowing potential’ to tackle climate crisis.” The Guardian. Damian Carrington. 4 Jul 2019. []
  8. Importance of carbon dioxide physiological forcing to future climate change.” Long Cao, Govindasamy Bala, Ken Caldeira, Ramakrishna Nemani, George Ban-Weiss. PNAS May 25, 2010 107 (21) 9513-9518. []
  9. Importance of carbon dioxide physiological forcing to future climate change.” Long Cao, Govindasamy Bala, Ken Caldeira, Ramakrishna Nemani, George Ban-Weiss. PNAS May 25, 2010 107 (21) 9513-9518. []
  10. Evapotranspiration Response to Climate Change.” R.L. Snyder, R. Moratiel , Zhenwei Song, A. Swelam, I. Jomaa, T. Shapland. Snyder, R.L., Moratiel, R., Zhenwei Song, Swelam, A., Jomaa, I. and Shapland, T. (2011). Acta Hortic. 922, 91-98. []
  11. Contrasting responses of stomatal conductance and photosynthetic capacity to warming and elevated CO2 in the tropical tree species Alchornea glandulosa under heatwave conditions. ” Sophie Fauseta, Lauana Oliveira, Marcos S.Buckeridge. Christine H.Foyer. David Galbraith, Rakesh Tiwari, Manuel Gloora. Environmental and Experimental Botany. Volume 158, February 2019, Pages 28-39. []
  12.  “Increasing atmospheric CO2 and canopy temperature induces anatomical and physiological changes in leaves of the C4 forage species Panicum maximum.” Eduardo Habermann, Juca Abramo Barrera San Martin, Daniele Ribeiro Contin, Vitor Potenza Bossan, Anelize Barboza, Marcia Regina Braga, Milton Groppo, Carlos Alberto Martinez. PLOS ONE. February 19, 2019. []
  13. Warming and Elevated CO2 Have Opposing Influences on Transpiration. Which is more Important?” Kirschbaum, M.U.F. & McMillan, A.M.S. Curr Forestry Rep (2018) 4: 51. []
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