Desert grass

Yet another example of natural chemical bonding is solidification of desert soils to build bird houses in Egypt. Nile silt was mixed with desert grass and river water to make tower shapes on concrete roofs (Fig. 1.2). Holes made in these towers provide shelter to the birds, which eventually are sold for food. The minerals from such rich soils consolidate when they are mixed with water and are dried in the intense sun of the desert. 

Bird houses on banks of Nile River made from silt and desert grass. 

Figure 8-20. Responses of net C02 uptake rate to photosynthetic photon flux for species differing hi mesophyll surface area per unit leaf area (Am s/A). Curves were obtained at ambient CO2 concentrations, optimal temperatures, and the AmesIA indicated in parentheses. [Sources for the C4 desert grass Pleuraphis rigida, Nobel (1980) for the C3 desert composite Encelia farinosa, Ehleringer et al. (1976) and for the C3 maidenhair fern Adiantum decorum and the C3 moss Mnium ciliare, Nobel (1977).]...

Nobel, P.S. 1980. Water vapor conductance and COz uptake for leaves of a C4 desert grass, Hilaria rigida. Ecology 61 252-258. 

Martin, S. C. (1975). Ecology and management of southwestern semi-desert grass-shrub ranges the status of our knowledge. RM-156, Ft. Collins, Colorado, USDA/Forest Service Research Paper. 

We can see that the Tropical Rain Green Forest ecosystems occupy about 1/5 of the African equatorial belt, whereas about 1/2 of this area is Woody and Tall Grass Savanna ecosystems. The rest of the area are occupied by various Dry Steppe and Dry and even Extra-Dry Desert ecosystems, like the Sahara, with annual rainfall less than 200 mm. As it has been mentioned above, the amount of precipitation is of high significance for exposure pathways of pollutants. 

Dry desert tropical ecosystems occupy 4.5 x 106 km2, or 3.0% of total land area of the Earth. These ecosystems have dry periods during 7-10 months a year. Not only trees, but also numerous grasses cannot grow in such severe conditions. The vegetation... 

Jahren et al, 2001). Today most C4 plants are tropical grasses, and most CAM plants are submerged aquatic plants and desert succulents. Most other kinds of plants use the C3 photosynthetic pathway. There is the potential to recognize these various metabolic pathways from the isotopic composition of organic carbon in paleosols and in fossil plants, and in the fossils of animals which ate the plants (Ceding et al, 1997 MacFadden et ah, 1999 Krull and Retallack, 2000). 

Helonias (helonias) Hemerocallis (day lily) Hesperocallis (desert lily) Hippeastrum (hippeastrum) Hosta (plantain lily) Hyacinthoides (hyacinthoides) Hyacinthus (hyacinth) Hymenocallis (spider lily) Hypoxis (star grass)... 

The smaller Cw values are also connected with water deficiency in Steppe and Desert ecosystems. However, the concentration of various chemical species in rainwater of background regions is higher that that in the Forest ecosystem belt. The major reason is the wind deflation of the soil s surface owing to lack of tree species and only a lean protective layer of grasses and half-shrubs. A large mass of soil particles becomes entrained into the air migration. The most characteristic example is connected with the Yellow sand phenomenon (see Box 7). 

The biggest problem is that the materials applied to the lawn do not necessarily stay there when it rains or the lawn is overwatered. They can go into groundwater or into streams. The result can be loss of aquatic vegetation and animal life in streams and lakes from the herbicides and insecticides, as well as eutrophication, which often involves growth of undesirable species, from the fertilizer. The result may well be a loss in biodiversity. Biodiversity can also be lost if native wild flowers and shrubs are replaced by lawn, as often happens. (The lawn is a biological desert of a few species of Eurasian grasses.305)... 

Wliile most researchers in the field agree on the relative importance of biospheric feedbacks operating at high northern latitudes, the discussion becomes more interesting and diverse as the subtropics are concerned. Climate reconstructions and data on fossil pollen compiled by Jolly et al. (1998), Hoelzmann et al. (1998), Pe-tit-Maire (1996), and Anhuf et al. (1999) indicate that North Africa was much greener in the mid-Holocene than today. The Saharan desert was, presumably to a large extent, covered by annual grasses and low shrubs. The Sahel reached at least as far north as 23 °N, more so in the western than in the eastern part. 

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