Opportunistic Metabolic Activity in situ

The photosynthetic activity of lichens has been monitored in situ in various climatic regions (Kappen 1988). Figure 2.11 depicts diurnal courses for lichens in two xeric habitats, the

Photosynthesis

Photosynthesis

FIGURE 2.11 Series of diurnal courses of lichen water content (WC) and CO2 exchange of Lecidella crystallina (on an area basis) from southern Africa in spring (September 16-20, 1990) and of Umbilicaria spodochroa (on chlorophyll basis) from Norway in winter (January 5-9, 1994). (After Lange, O.L., Meyer, A., Zellner, H., and Heber, U., Funct. Ecol., 8, 253,1994; Kappen, L., Schroeter, B., Hestmark, G., and Winkler, J.B., Botanica Acta, 109, 292, 1996.)

FIGURE 2.11 Series of diurnal courses of lichen water content (WC) and CO2 exchange of Lecidella crystallina (on an area basis) from southern Africa in spring (September 16-20, 1990) and of Umbilicaria spodochroa (on chlorophyll basis) from Norway in winter (January 5-9, 1994). (After Lange, O.L., Meyer, A., Zellner, H., and Heber, U., Funct. Ecol., 8, 253,1994; Kappen, L., Schroeter, B., Hestmark, G., and Winkler, J.B., Botanica Acta, 109, 292, 1996.)

Namib desert in spring (Lange et al. 1990c, 1994) and south-exposed rocks of southern Norway in winter (Kappen et al. 1996). It may be noted here that frozen water is able to maintain CO2 exchange in winter if irradiance is sufficient.

Different types of diurnal courses of gas exchange of lichens result from the existence of several possible sources of moisture (dew, fog, and rainfall, each one alone or in combination). Losch et al. (1997) have proposed a simplified scheme for the seasonal variation of the periods of photosynthetic activity of nonvascular autotrophs with regard to the climatic conditions. In subpolar and polar regions, the most productive and extended periods of activity occur in spring, summer, and fall. In temperate regions, the periods of activity are regular but rather short and occur during all seasons, although they are somewhat limited in

FIGURE 2.12 Schematic description of typical diurnal photosynthetic activity in the seasonal course for bryophytes and lichens in different climatic regions. NP, net photosynthesis; DR, dark respiration. (After Losch, R., Pause, R., and Mies, B., Bibl. Lichenol, 67, 145, 1997.)

summer. In hot arid regions, the periods of activity are brief but frequent, since they potentially occur throughout the year or every day during the wet season, depending on the regions (Figure 2.12). Examples of field studies in different environments are those by Hahn et al. (1989, 1993), Bruns-Strenge and Lange (1991), Sancho et al. (1997), and in the Antarctic (see Chapter 14 of this volume). The daily periods of moistening may last from midnight to up to 3 h in sunlight if dewfall is involved. In arid regions, the length of the period of lichen hydration is directly influenced by the exposure and compass direction of the site. In warm and temperate climates, shaded habitats allow the longest periods of hydration, but in frigid climates, hydration is combined with insolation of the habitat (Kappen et al. 1980, Kappen 1982, 1988,1998b, Nash and Moser 1982, Pintado et al. 1997).

A long-term investigation of the lichen Ramalina maciformis in the Negev desert revealed that thalli were active most days of the year: Dewfall caused 306 days of metabolic activity (of which carbon balance was positive on 218 days and negative on 88 days), whereas rainfall produced activity on only 29 days per year (Kappen et al. 1979). In high mountains and polar regions, melting snow can extend the productive period of lichens and bryophytes over several days or weeks (Kappen et al. 1995, 1998b). In Antarctica, the lichen Usnea aurantiaco-atra was active for a total of 3359 h within 268 days of 1 year in the wet maritime region (Schroeter et al. 1997), whereas the period of activity was reduced to one-fifth of this value within 120 days in the Antarctic dry valleys for cryptoendolithic microorganisms (see Table 2.4)

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