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42: Transpiration 42: Transpiration JOHN ROBERTS Centre for Ecology and Hydrology, Wallingford, Oxfordshire, UK The transpiration process is the uptake of water by plant roots, transport through the plant and evaporation from the leaf through pores called stomata. Evaporation of water from the leaf is determined by atmospheric conditions such as radiation, temperature and humidity deficit but the plant can limit transpiration by partial or complete stomatal closure. Generally stomata open in response to increasing radiation but tend to close with increasing air humidity deficit and reduced availability of soil moisture. Because the stomata have to be open in daylight for the entry of carbon dioxide into the leaf for the photosynthesis process, water loss is an inevitable consequence. Nevertheless transpiration itself has important roles. Nutrients are brought into the plant when water is taken up from the soil and evaporation of transpired water prevents leaf temperature reaching supra-optimal levels. There are numerous ways that transpiration might be measured. These include measurements of soil water changes below vegetation or changes in atmospheric humidity above vegetation. Alternatively measurements can be made on individual plants or leaves. In most circumstances the source of water for transpiration is the soil and principally the surface soil layers where most roots are found. In the future, increased levels of atmospheric CO2 are expected to reduce transpiration through reduction in stomatal aperture. WHAT IS TRANSPIRATION? There is a continuous stream of water from within the leaves of plants down through the plant to the roots and soil. Water molecules bind together and these bonds have Transpiration is the process by which water is evaporated substantial strength. There is a continuous column of water from within a plant. Essentially, water is evaporated through from within the leaf drawing up water from the soil. small holes (known as stomata) in the leaves, and this The cohesive forces of water molecules mean that water draws water up through the plant (in microscopic tubes columns down through the plant can be maintained under termed xylem) from the soil. This “transpiration stream” significant tensions. In very tall trees, these tensions can be brings water to the plant to be used in photosynthesis, to considerable. Suctions equivalent to 5 MPa (50 bars) have produce carbohydrates, and to maintain turgidity (rigidness) commonly been reported for actively transpiring tall trees. in the cells and tissues. However, very little of the water is Transpiration from vegetation is usually reported as a actually used in photosynthesis and to maintain turgidity. depth of water (mm), in the same way that rainfall and Most of the water sucked up from the soil is evaporated evaporation are reported. Transpiration might range from through the stomata. A primary purpose of the stomata is to very low or zero in completely water-stressed vegetation, exchange carbon dioxide and oxygen with the atmosphere sparse crops, or vegetation in winter. The highest values in addition to regulating the loss of water from the leaves. of transpiration might be up to rates estimated as the The movement of water up from the soil through the plant potential transpiration rate (see Chapter 41, Evaporation plays a key role in bringing minerals from the soil into and Modeling: Potential, Volume 1). In this case, it would through the plant. In situations where leaves experience a be expected that the vegetation completely covered the high radiation loading, leaf temperatures can be critically ground, there was no shortage of soil water and climatic high. Cooling of the leaf by the dissipation of heat during conditions are optimal, for example, high summer or the evaporation of transpired water is another important role tropical conditions. Transpiration can exceed the potential for transpiration. rate if extra energy is available as advection. Encyclopedia of Hydrological Sciences. Edited by M G Anderson. 2005 John Wiley & Sons, Ltd. 2 HYDROMETEOROLOGY Table 1 Annual transpiration of global forest types Annual Vegetation Location transpiration (mm) Reference Tropical rainforest Manaus, Brazil (2◦57’S: 59◦ 57’W) 1030 Shuttleworth (1988) Southern European Evora, Portugal (38◦ 32’N: 8◦ 01’W 207 David et al. (2004) evergreen oak Temperate coniferous Thetford, UK(52◦ 25’N: 0◦ 39’E) 352 Gash and Stewart (1977) forest Boreal coniferous Saskatchewan, Canada (53◦ 55’N: 104◦ 41’W 204 Saugier et al. (1997) forest In summer conditions in the United Kingdom, maxi- ) 400 1 −1 − Reserva Ducke mum transpiration rates of up to 4 mm day have been s 2 measured routinely in forests although much higher rates − Thetford forest ∼8mmday−1 have been observed in fast-growing short 300 rotation coppice plantations. In forests with 750 trees ha−1, a transpiration rate of 4 mm day−1 would mean that 50 kg 200 of water per day would be lost on average from each tree. In a wheat or barley field with around 250 stems m−2,tran- spiration loss through each stem would be of the order of 100 150 gm. An insight into the range of annual forest transpiration that might be encountered can be achieved by comparing Stomatal conductance (mmol m 0 0 5 10 15 20 values from various studies carried out in a range of Specific humidity deficit (g kg−1) forest types occurring from boreal to tropical regions. The annual transpiration (with associated information) of forest Figure 1 The decline in leaf stomatal conductance with air types occurring in different global regions are given in humidity deficit in Piptadenia suaveolens, an upper canopy Table 1. As expected the highest annual total is found tree species at the Reserva Florestal Ducke, Manaus, Brazil in tropical rainforest in Brazil. The high annual total (unpublished data from John Roberts) and the upper canopy of Scots pine (Pinus sylvestris L.) at Thetford is largely a consequence of the evergreen canopy and Forest, UK (Redrawn from Beadle et al., 1985, Journal of therefore year-round transpiration with no limitations of Applied Ecology 22, 557–571, by permission of British solar radiation, air temperature, or available soil moisture. Ecological Society) Potential evaporation rates in this area of the Amazon basin would enable much higher transpiration, but the reduction Transpiration from the temperate coniferous forest is of canopy conductance in response to an increased air vapor higher than both the southern European and boreal forests. pressure deficit (see Figure 1) means that daily transpiration Although not constrained by water stress, daily transpiration is often around 3.5 mm, barely different from transpiration is likely to be limited by a strong decline in stomatal con- of Scots pine (Pinus sylvestris) measured at Thetford Forest ductance with increasing vapor pressure deficit (Figure 1). on summer days. The annual transpiration of around 325–350 mm year−1 Annual transpiration from the evergreen oak (Quercus shown for Thetford Forest was shown to be very similar for rotundifolia) woodland in Portugal is 207 mm. Although many woodlands (both broadleaf and coniferous) in Europe this estimate does not include losses from ground vegetation by Roberts (1983). Roberts identified a number of fac- beneath the trees, the low transpiration rate is largely a tors that might contribute to this similar transpiration. Few consequence of the sparse open canopy of the woodland. forests are limited by water stress, and daily transpiration Shortage of water is probably not an issue as the trees had is constrained by probable links between stomatal/surface access to groundwater. The annual transpiration from the conductance and air humidity deficit. Furthermore, the pres- boreal forest is also low. Although the growing season is ence of understory vegetation below an open tree cover will short (∼140 days), a major constraint on transpiration is have a significant role in eliminating tree transpiration dif- probably soil temperature, which will still be cold enough to ferences between dense and open forests. One factor that limit root water uptake, and probably also mineral nutrients has been shown to be important in determining transpiration throughout the growing season. This was considered as a from forests and woodlands is the age of the trees. There major factor in producing low stomatal conductances in the is now substantial evidence from studies both on trees and jack pine (Pinus banksiana) in the boreal forest. catchments that as trees age their transpiration declines. TRANSPIRATION 3 MesophyII Intercellular cell spaces rw Substomatal + + ri = rc(rc ri rw) cavity rl + + rc(rc ri rw) rs rc Cuticle Guard cell ra Cuticular Stomatal evaporation evaporation Figure 2 Pathways for water loss from one surface of a leaf showing the boundary layer (ra), cuticular (rc), variable stomatal (rs), intercellular space (ri ), wall (rw ), and leaf (rl ) resistances. The total leaf resistance is the parallel sum rl for upper and lower surfaces (Redrawn from Jones (1992), Plants and Microclimate, Second Edition, Cambridge University Press) Practically all of the water lost from vegetation will have trees, and herbaceous plants. Figure 2 shows a cross-section been taken from the soil. The amount of water taken up through a stomatal apparatus, with the stomatal opening from the soil and used in metabolic processes, for example occurring on the underside of the leaf. Figure 2 shows photosynthesis, is trivial. Some water is lost from plants as the resistances met as water vapor is transpired from one transpiration does not come directly from the soil but from surface of the leaf. Evaporation of water from cell walls is storage in the body of the plant. Generally, this stored water regarded by some to be the site of the first resistance (rw) is a smaller fraction of the total daily transpiration than in the water loss pathway. The transfer resistance within water coming directly from the soil.
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