13.1.2  Small-scale variation in CO2

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Plants growing in their natural environment experience a range of CO2 concentrations above and below the mean ambient tropospheric value. This is most frequently observed in vegetation with a closed canopy which reduces both rate and extent of exchange between the air in and below the canopy and the air above it. The variation in CO2 concentration arises because of respiratory release of CO2 from plants and soil, as well as photosynthetic consumption of CO2 within the canopy. This effect is most pronounced during low aerodynamic conductances for CO2 transfer into and out of the canopy, as occurs when wind speeds are low and/or when the surrounding atmosphere is ‘stable’. This happens when there is a negative gradient in temperature within and away from the canopy. Similarly, there can be large vertical variations in CO2 (and water vapour) concentrations within canopies themselves.

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Figure 13.1 Changes in CO2 mole fraction over a typical day in and above an Amazonian forest. Measurements 1 m above the ground (solid circles) show that CO2 concentrations are higher near the ground than in or above the canopy (51 m). This is because of respiratory release of CO2 from soil and also because of the low exchange of air between this level and the planetary boundary layer (PBL). During morning and early afternoon, CO2 concentrations within the canopy decline substantially due to canopy photosynthesis plus exchange of air between the rainforest canopy and PBL. (Based on Grace et al. 1995 plus unpublished data)

Diurnal changes in ecosystem respiration and photosynthesis and in rates of heat and mass transfer into and out of canopies therefore result in large variations in CO2 concentration. A typical example is shown for a rainforest in Brazil (Figure 13.1; Grace et al. 1995). In this case, even above the canopy, CO2 concentration rises to 450–500 µmol mol–1 pre-dawn and falls to values close to 350 µmol mol–1 by afternoon. Close to the canopy floor, concentrations are usually much higher than at higher levels in the canopy, especially during the day. This effective ‘decoupling’ with CO2 concentrations lower in the canopy occurs because of a significant daytime release of CO2 from soil respiration (in this case at a rate of about 6 µmol m–2 s–1). Low wind speeds near the forest floor and associated low aerodynamic conductances for CO2 transfer are additional factors. This means that young seedlings in dense forests often experience ambient CO2 concentrations substantially above those measured in the free
troposphere.

When gradients in CO2 concentration are measured within canopies, if the extent of mixing of CO2 within the canopy is also known (from measurements of turbulence) it is possible to deduce the distribution of photosynthesis and respiration within the canopy.

It is also possible to measure CO2 water vapour and heat fluxes into and out of canopies directly using eddy correlation (or eddy covariance). This technique involves simultaneous high-frequency measurement (typically about 20 times per second) of fluctuations in the vertical component of wind speed and associated fluctuations in CO2 and H2O concentrations and temperature (Moncrieff et al. 1996). When there is a net flux of CO2 into the canopy (i.e. photosynthesis exceeds respiration) the concentration of CO2 in parcels (or eddies) of air leaving the canopy is less than that of parcels entering, hence ‘eddy covariance’. The technique measures the vertical velocity and CO2 concentrations of all air parcels entering and leaving a canopy and, when averaged over a long enough period (typically 30 min to 1 h), calculates the rate of removal (or production) of CO2 and other entities by that canopy.

In order to calculate the rate of physiological exchange of CO2 by vegetation and soil it is necessary to take into account the variations in CO2 that also occur within the canopy (Figure 13.1). When expressed on a ground area basis these fluctuations can be very large, especially an hour or two after sunrise (as much as 60 µmol m–2 s–1 ) and in some cases may be greater in magnitude, and of a different sign, to the flux measured above the canopy by eddy covariance (Grace et al. 1995).

A steady decline in CO2 concentration, often observed a few hours after sunrise, is attributable to high rates of photosynthesis removing some of the CO2 that has built up over-night, plus onset of turbulent conditions and a rapid increase in the height of the atmospheric/planetary boundary layer. The atmospheric boundary layer (ABL, or PBL) is the layer of air directly above the earth’s surface in which the effects of the surface (friction, heating and cooling and changes in trace gas concentrations) are perceived directly on time scales of less than a day, and in which significant fluxes of momentum, heat or matter are carried by turbulent motions on a scale of the order of the depth of the boundary layer or less. A convective boundary layer (CBL) occurs when strong surface heating (due to solar radiation) produces thermal instability or convection in the form of thermals and plumes, and when upside-down convection is generated by cloud-top radiative cooling (Garratt 1992). A simple and informative summary of the general properties of the CBL is given by Raupach et al. (1992). In the absence of complete cloud cover, the CBL over land shows a strong diurnal development, the height of the CBL typically increasing from 100–500 m in early morning to 1–2 km in mid-afternoon. A stable layer, capped by a radiation inversion, usually develops near the ground when solar heating of soil surfaces ceases around dusk.

Diurnal patterns in the height of the CBL thus interact with the rates of ecosystem photosynthesis and respiration in determining the CO2 concentrations to which plants are exposed. When the rate of CO2 fixation by photosynthesis is high compared to the rates of CO2 release from respiration and the rate of transport of CO2 into the CBL from the troposphere above, then the CO2 concentrations experienced by plants growing on the earth’s surface are often below that of the troposphere above the CBL. By measuring the rate of change in CO2 concentrations within the CBL and measuring (or modelling) the CBL growth rate (and hence the rate of entrainment of tropospheric CO2 into the CBL) it is possible to deduce the rate of ecosystem gas exchange at a ‘regional’ level. This gives the average value of carbon (or water vapour/
temperature) exchange over an area of some hundreds of square kilometres (Raupach et al. 1992).

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