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Modification of Radiation and Temperature by Biotic Factors

Radiation climate and temperature at the soil surface are particularly affected by vegetation cover:

• Reflection: The composition of the vegetation determines its reflection characteristics. The reflection coefficient varies between 30%, e.g. for birch forests, and 15% for conifers. The low reflection of conifer forests affects the climate at the limit of boreal forests; more radiant energy is absorbed, and thus more energy kept as heat in the stand, than on forest-free areas. The occurrence of evergreen conifers may, thus, cause a shift in the conifer tree limit to the north; the effect is compensated by the shading of roots by the evergreen canopy, which delays thawing in spring and thus shortens the growing season.

• Radiation absorption: The largest part of the incoming radiation is absorbed in the canopy, independent of the height of vegetation (Fig. 2.1.7 A). The radiation balance depends on the leaf area and leaf orientation. The leaf area is quantified by the leaf area index (LAI), defined as the sum of the projected leaf surface per soil area. With the idealised assumption of a statistically random distribution of leaves in the canopy, the decrease of radiation (extinction) follows the LambertBeer law of absorption:

Fig. 2.1.7. A Reduction of photosynthetically active radiation (PAR) with increasing leaf area index for dicotyledonous shrubby plants (horizontal leaf position, extinction coefficient k=07), deciduous forests, wheat field and grass meadows (vertical leaf position, extinction coefficient k=<0.5; after Larcher 1994). B Relative reduction of the light intensity of infrared light (IR), net radiation balance (net) and the photosynthetically active radiation (PAR: 400-700 nm) with the leaf area index. (Jones 1994)

Fig. 2.1.7. A Reduction of photosynthetically active radiation (PAR) with increasing leaf area index for dicotyledonous shrubby plants (horizontal leaf position, extinction coefficient k=07), deciduous forests, wheat field and grass meadows (vertical leaf position, extinction coefficient k=<0.5; after Larcher 1994). B Relative reduction of the light intensity of infrared light (IR), net radiation balance (net) and the photosynthetically active radiation (PAR: 400-700 nm) with the leaf area index. (Jones 1994)

where I is the radiation flux at a defined site in the vegetation, I0 is the incoming radiation above the canopy, LAI is the leaf area index above the measuring point, and k is the extinction coefficient [and corresponds thus to the absorption (1-s) in the radiation balance, Eq. (2.1.2)], k varies between 0.5 for vertical leaves (e.g. grasses) and 0.7 for horizontal leaves (e.g. clover). Several species (particularly legumes) are able to move their leaves and thus the LAI is actively and variably regulated (see also Fig. 2.1,13). The "clumped" distribution of needles in conifers results in decreased absorption of light with depth in the canopy, and thus to an increased LAI (see Fig. 2.1.8).

The extinction of the visible, short-wave light in the canopy (which depends on the LAI) is faster than extinction of the long-wave energy flux (Fig. 2.1.7B; Jones 1994). Thus long wave radiation (heat) penetrates deeper into the vegetation than the photosynthetically active photon flux. Vegetation is only able to increase the leaf area until all visible light is absorbed, i.e. the maximum LAI is deter mined by the incoming radiation (available light) and the angle of the leaf. However, even with complete absorption of visible light, still a part of the IR radiation reaches the soil.

• Climate within the vegetation: With changes of radiation in the canopy (Fig. 2.1.8; Mon-teith 1973) not only the temperature, but all other meteorological factors change, particularly the saturation deficit and the turbulence. Because of mixing in the atmosphere within the canopy, temperature extremes are less than on a vegetation-free soil surface, during the day as well as at night. The gradient in the stand depends on the roughness of its surface. In coniferous forests absorption of light is less steep (related to the height of the canopy) than in cereal crops. Consequently, wind penetrates deeper into the canopy of conifers; temperatures and vapour pressures are more uniform during the day in a coniferous forest than in a cereal field. Because of the difference in coupling to the atmosphere, only in the cereal canopy is there a significant decrease in C02 concentration. During the night the gradients of temperature, humidity and C02 are reversed. Differences depend mainly on the roughness of the surface and thus on coupling to the atmosphere (see g>

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