Transpiration and water flux regulation

How is this defined?

The actual evapotranspiration (AET, mm) of plant cover is the sum of all the water vapour fluxes to the atmosphere : AET
=
tree transpiration
+
soil evaporation and understory herbaceous transpiration
+
evaporation of water intercepted through the canopy

Climate variables which determine AET of plant surfaces are : :

• variables known for forcing that impose climatic demands (see the page Meteorology),
• precipitation affecting soil water availability and consequently the tree's system for regulation (stomatal regulation) in conditions of water scarcity or water deficit.

Tree transpiration is the predominant term in actual evapotranspiration. When the stand is closed and the leaf area index is high, soil and understory evapotranspiration is often low, even negligible, as evaporative demand is reduced below the canopy.

How is AET measured? What unit is used for this measurement?

There currently exists a proven method for measuring the water vapour flux above a canopy: the eddy covariance method. This method combines a three-dimensional sonic anemometer and a fast infra-red gas analyzer.

Another method, known as water balance, is used to estimate the AET considering changes in soil water content and rainfall from below the canopy. This method does not provide accurate temporal resolution as the time required is generally a week or longer. In addition, the high spatial variability of the soil water content due to the tree's uptake requires a high number of replicates.

Trees transpiration (T) can also be assessed through measuring sap flow in the trunk. Several repetitions (5-10) are required to account for inter-tree variability which is linked to crown properties or to species composition.
Transpiration is expressed in litres (or kg) of water per time period. Similar to precipitation, stand transpiration is usually expressed in mm (= 1 L/m²).

How is the water flow controlled?

Transpiration is directly dependent on the energy intercepted by tree crowns. Potential evapotranspiration (PET), which drives transpiration, quantifies the atmospheric demand (see the page meteorology). PET is a function of global radiation, temperature, air humidity and wind speed. We have shown that a tree's transpiration in a closed stand with high leaf area index and no water stress can reach a maximum of 75% of PET. When the leaf area index is lower (below 6), transpiration decreases. For broad-leaf stands during leaf expansion, leaf yellowing and the leaf fall phase, transpiration is less than maximal.
In the case of drought, relative extractable soil water (REW) decreases. When REW drops below 0.4, trees consequently experience water stress which induces stomatal closure and reduces transpiration. It is interesting to know that this threshold of soil water deficit can be applied to a large number of species and soil types.
Another component of AET is the soil layer evapotranspiration, which is equal to the sum of soil evaporation and understory transpiration. Here, we call understory the herbaceous layer. The woody understory layer, i.e. the understory as forest managers often refer to as coppice (e.g. composed of hornbeams in an oak or beech stand) is integrated in the leaf area index of the stand. Herbaceous sub layer evapotranspiration depends principally on the energy available below the tree's canopy. As such, the BILJOU© model is designed to calculate global radiation which reaches the sub layer. This calculation involves both the stand's global incident radiation (see the page meteorology and the stand's leaf area index.

How does transpiration vary with time?

Variance in transpiration runs parallel with PET and LAI, particularly during the growing season for deciduous canopies. In addition, T is regulated by stomata closure during soil water deficit periods (see the page water balance).

How does transpiration vary spatially?

In a forest stand, transpiration varies considerably between trees, as we have shown in sap flow measurements on the opposite figure. Variation in transpiration from one tree to another relates mainly to the trees' social status, the dominant trees showing higher transpiration rates due to their higher leaf area and the fact that their crowns are located in more favourable microclimatic conditions than smaller trees.
In a forest stand, there may also exist spatial variations relative to soil heterogeneities and gaps in the canopy.
However, modelling for water balance at stand scale requires constant conditions. Therefore, a heterogeneous plot should be divided in more homogeneous sub-elements. Forest managers generally achieve this by setting basic management units.

Some key figures

During the course of a sunny day with high soil water conditions, stand transpiration typically reaches 2 to 4 mm / day (= 20 - 40 m3/ha/day). When a stand is closed, tree age becomes less a factor in transpiration. In other words, if there are 2,000 trees (i.e. a young stand) per hectare, each tree will transpire on average between 10 and 20 litres of water per day. If there are 200 trees (i.e. an older stand), transpiration of one tree will be around 100 to 200 litres per day.

Useful references

Aussenac G., Granier A. (1979) Etude bioclimatique d'une futaie feuillue (Fagus silvática L. et Quercus sessiliflora Salisb.) de l'Est de la France II. Etude de l'humidité du sol de l'évapotranspiration réelle. Annales des Sciences Forestières, 36, 265-280.

Bréda N., Cochard H., Dreyer E., Granier A. (1993) Water transfer in a mature oak stand (Quercus petraea): seasonal evolution and effects of a severe drought. Canadian Journal of Forest Research, 23, 1136-1143

Granier A, Biron P, Bréda N, Pontailler J Y, Saugier B (1996) Transpiration of trees and forest stands: Short and longterm monitoring using sapflow methods. Global Change Biology 2(3): 265-274

Granier A, Loustau D, Bréda N (2000) A generic model of forest canopy conductance dependent on climate, soil water availability and leaf area index. Annals of Forest Science 57: 8,755-765

Lagergren F, Lindroth A (2002) Transpiration response to soil moisture in pine and spruce trees in Sweden. Agricultural and Forest Meteorology 112(2): 67-85

Vincke C, Bréda N, Granier A, Devillez F (2005a) Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. I. Trees and forest floor daily transpiration. Annals of Forest Science 62(6): 503-512