The thermal balance of plants is closely connected to the chemical composition of and the physical transport processes in the atmosphere, which are part of the discipline of meteorology (textbooks: Lutgens and Tarbuck 2000; Wallace and Hobbs 1977). Variations in solar energy balance are responsible for the climatic conditions in the boundary layer near the ground, compared to the free atmosphere. Gregor Kraus (1911) was the first scientist to describe this phenomenon quantitatively on limestone sites near Wurzburg, Germany, and thus founded a new discipline of micrometeorology (textbook: Jones 1994).
Water vapour, C02 and 02 are the most important gases for the plant, independent of several other trace gases (ozone, nitric oxide, ammonia, methane and others) which influence the plant (see Chap. 1.9). Here, we discuss the energy balance in the context of water vapour and C02 in the atmosphere. Both gases are important for the existence and growth of plants.
Through the formation of clouds, water vapour influences
• absorption and reflection in the atmosphere and thus the solar radiation reaching the earth's surface;
• evaporation from the earth's surface as being dependent on the saturation deficit and on the plant cover;
• density of the atmosphere and the transport processes in it (formation of clouds) via the temperature dependence of saturation, which is the basis for precipitation.
Carbon dioxide influences
• the thermal balance of the lower atmosphere by absorption and radiation of long-wave radiation;
• photosynthesis, as it is the substrate for the process.
The interaction between the optical characteristics of the atmosphere and its constituent gases is explained in Fig. 2.1.2 A (Mitchell 1989; IPCC 1996). The solar radiation entering the earth's atmosphere occurs in the short-wave range at about 6000 K with maximum radiation at about 0.6 ^m wavelength (visible light). Mean radiant energy at the upper limit of the atmosphere is 1370 Wm-2 (solar constant, measured in the stratosphere). Because the earth is not flat, but a sphere, the mean radiation flux during the day, averaged over the illuminated hemisphere, is about 340 Wm"2 (WBGU 1997). This radiation is balanced by the long-wave radiation (thermal radiation, Ij), which by itself is a balance between thermal radiation of the atmosphere and thermal radiation from the stratosphere which operates at a temperature of 255 K. Long-wave radiation follows the Stefan-Boltzman law:
where a =5.67X10"8 (Wm"2K"4), the Stefan-Boltzman constant, and T the temperature in Kelvin. Without an atmosphere, there would be no re-radiation from the atmosphere and thus the average temperature on earth would be -18 °C.
Atmospheric gases, particularly water vapour and C02, have the effect that part of the incoming solar radiation in the short-wave and near-IR range is absorbed and reflected (Fig. 2.1.2B). In the atmosphere the short-wave solar radiation (UV radiation) is absorbed particularly by ozone. H20 and C02 absorb in the near-IR, thus limiting the incoming radiation to a narrow radiation window with a maximum in the visible range. The incoming energy is balanced by emission of long-wave IR from the earth's surface; this is limited by water vapour and C02. There is only a narrow emission window between 8 and 14 ^m wavelength in which the earth's surface absorbs or emits heat.
Reflection and absorption processes are additive in determining the energy balance of the earth (Fig. 2.1.2 C; Mitchell 1989). Short-wave radiation is absorbed in the molecules of the atmosphere and clouds and reflected. Some of the short-wave radiation is reflected from the earth's surface, dependent on the type of vegetation cover.
The thermal balance is determined by the sum of processes of reflection and absorption (Table 2.1.1). A distinction is made between the radiation balance and the energy balance. The radiation balance comprises the sum of short-and long-wave radiation fluxes and their reflection whilst the energy balance of radiation is the sum of all thermal fluxes and incorporates thermal transport, latent heat of evaporation and the fluxes of heat into the soil. The radiation balance is normally not at equilibrium (i.e. it differs from zero), but the energy balance must be zero as the sum of all processes.
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