Measurement in leaves (see Sestak et al. 1971;
Pearcy et al. 1991):
• Porometer: this is a vessel onto which a leaf is pressed or into which a leaf is sealed. The changes in air humidity and C02 concentration are measured and from this a flux is calculated. Advantage: simple to use. Disadvantage: the response of the plant, i.e. the change in stomatal conductance and not the transpiration rate is actually measured under artificial conditions.
• Cuvettes: these are controlled chambers in which all or part of a leaf is enclosed. The temperature and the humidity can be controlled so that it is possible to determine the plant's responses to specific conditions. Advantage: experiments are possible in the field. Disadvantage: complicated machinery. The measured transpiration rate does not correspond to the transpiration rate under undisturbed conditions.
Measurement on single plants (see Pearcy et al. 1991):
• Xylem flux: the water in the xylem is heated at a point and the distribution of temperature in the stem is measured. There are different approaches, either the temperature dissipation is held constant and the current is regulated (Cermak method) or the energy input is held constant and the change in temperature is measured (Granier method). Advantage: it allows measurement of the natural rate of transpiration of the plant. Disadvantage: the method is best suited for trees, small shoots are difficult to measure: extrapola tion to the whole vegetation surface is difficult.
Measurements of canopies (see Aubinet et al.
• Eddy correlation method:
Turbulence over the surface of vegetation is determined from the vertical profile of wind speed. The size of the air parcels that enter or leave the vegetation is then determined by combining wind profiles with measurements of temperature and humidity. From knowledge of the chemical composition of the air packages (concentration of gases, e.g. C02) the water and C02 fluxes may be assessed over a large area of vegetation (1-4 km2). Advantage: integrated measurement of fluxes for whole ecosystems. Disadvantage: can only be used under particular topographical conditions (small slope, homogeneous canopy and unstable layers).
Measurements at landscape scales (see Lloyd et al. 2001):
• CBL-balancing (CBL=convective boundary layer): The lower layer of the atmosphere may be used as a cuvette, i.e. during the day changes in concentration in gases in this zone may be measured from an aeroplane and knowledge of the flux balances with the environment and the troposphere can be obtained at landscape scale from 50-100 km2. Advantage: quantification of flux balances from varied landscapes. Disadvantages: technically and meteorologically demanding and limited to particular meteorological conditions.
ample, in the shadow of woods. It is called equilibrium evaporation (Eeq). In this case, the transport of water vapour from the boundary layer away from the plant is not taking place and evaporation will saturate the non-turbulent air space immediately. Water vapour transport to the atmosphere is only possible if the temperature of the surface rises as a consequence of radiation.
Under natural conditions both limiting cases occur but usually a mixture of transpiration imposed by saturation deficit and in equilibrium with radiation occurs.
where Q is the decoupling factor which quantifies the connection of the vegetation to the at-
0.1 1 10 20 Height of vegetation (log m)
H Fig. 2.2.20. A Dependence of the uncoupling factor Q on the height of the vegetation as a measure of the roughness of the surface (after Kelliher et al. 1995). B In boreal forest the ratio of latent to sensible heat flux is affected by the form of tree canopy. Picea obovata x excelsa (narrow crown) is more densely covered with needles and therefore warms more than Picea excelsa (broad crown) or Betula pubescens. The thick ground vegetation with Vacci-nium myrtillus does not have a very rough surface and thus warms more strongly. Gutulia, central Norway (Photo E.-D. Schulze). C Canopy of a Eucalyptus marginata stand with an average height of 100 m in south-western Australia. Eucalyptus marginata reaches a height of 140 m. In contrast to boreal forests, the leaf surface is almost uniformly distributed in the canopy, although Eucalyptus forms leaf bundles at the ends of braches. Picture taken from the "Glocester tree" near Pemperton. (Photo E.-D. Schulze)
mosphere, /.Ecq is the equilibrium evaporation and /Eimp the imposed evaporation [see Eqs. (2.2.21) and (2.2.22)].
The sensitivity with which transpiration reacts to changes of stomata, (dE/E)/(dGs/Gs), is directly determined by Q:
The above-mentioned relations are very important in the regulation of water loss from plant
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