**4. Climatic approach**

tropical rainforests. Semi-deserts can be found near deserts on this imaginary line, just a

If this most general definition of deserts is accepted, several habitats, which are not referred to as deserts in the habitual language use must be considered to be deserts. In order to quantify the scale, we can use biomass per area as a static indicator as well as net primary production per area per time as a dynamic indicator. Biomass of tropical rainforests is characterized by

"ideal existence status" of the biosphere. In the case of deserts, NPP is between 0 and 3 g/

• areas without precipitation (orographical, e.g., Gobi, Tibet, or cool, dry deserts, e.g.,

• areas with much larger evapotranspiration than precipitation due to the heat (southern

• permanently frozen areas (Greenland, Antarctica, and peaks of high-altitude mountains) • sheer, unweathered rock surfaces where water runs off (barren, rocky areas in mountains)

even less than that in the Sahara, however, NPP has a semi-desert value of 125 g/m2

• Photic zone of open oceans (where biomass has a desert value of only 3 g/m2

• Abyssal water of open oceans, caves (special ecosystem with low productivity)

• Inner part of the rock masses of the Earth (where microbial life may often exist)

**5.** Continuous physical disturbance, which prevents plants from settling in

.

biomass, this is currently the

, which is

/

. In the case of semi-deserts, NPP amounts to

/year net primary production (NPP) and 45,000 g/m2

**1.** Lack of liquid water usable for plants (arid areas without precipitation)

/year and biomass amount to 20–700 g/m2

little bit nearer the other endpoint of the scale.

/year and biomass is between 0 and 20 g/m2

Desert or semi-desert conditions can be caused by:

**3. Vegetation approach**

2 Community and Global Ecology of Deserts

Namib, Atacama)

year) **3.** Lack of light

**4.** Lack of air and light

• Coastal tidal zone

• Shifting sand areas

• Fast-running reaches of rivers

and central part of Sahara)

**2.** Lack of access to minerals in the lithosphere

2200 g/m2

3–150 g/m2

m2

Vegetation is normally able to evapotranspirate water equivalent to 20 mm of precipitation at a temperature of 10°C. A month is considered to be climatically arid if its monthly average temperature exceeds double the monthly amount of precipitation. If each of the 12 months of the year is arid in multi-year average, the area is considered to be a desert from a climatic point of view. If less than 2 months of the year are not arid, we speak about semi-desert climate. Similarly, if the multi-year average of the monthly average temperatures is below 0°C for 12 months, we speak about ice desert from a climatic point of view; if the period with an average temperature above 0°C lasts up to 2 months, the climate is tundra. However, the climatic approach may be misleading because the typical vegetation type of a certain area does not depend on the climate only.

Plants and animals have been able to adapt to areas with different environmental (among them climatic) conditions better and better during the evolution (on a historic time scale). In the second part of the Cambrian period (approx. 542–488 million years ago), hot tropical conditions were dominating most of the Earth, however, 100% of land could be considered a desert, only traces of some coastal invertebrates indicate terrestrial life. Also, during the Ordovician (488–443 million years ago), the climate was hot in several areas, the first plants (liver mosses and hyphae) appeared on land, however, this type of vegetation could have reached rather desert than semi-desert level. The first vascular terrestrial plants appeared in the Silurian (443–416 million years ago), however, they began to form vegetation mainly on the waterside, the continents' interior kept being a desert for the most part from a vegetation point of view. During the evolution of terrestrial life, more and more various adaptation modes have appeared in response to the various climatic conditions, and this process is still going on. At the same time, climatic regulation capacity and generally self-regulation capacity, biodiversity, total biomass, and productivity of the biosphere have also shown an increasing tendency (excluding fall-backs caused by climatic variation).


**Table 1.** Comparison of frequency distribution of plant life-forms (based on [11]).

Vegetation types have reached various levels regarding adaptation to the abiotic conditions of the given habitat, which is also shown by the distribution of plant life-forms. Long-living K-strategists and phanerophyte life-form are dominating where vegetation has been able to adapt efficiently to existing conditions. However, where the habitat is rather a "struggle zone", shorter-living r-strategists, herbaceous annual, or rosette plants are dominant (see **Table 1**).

Thus, deserts and semi-deserts are "front lines" of the expansion of biosphere, and therefore, their research (similarly to tropical rainforests) is highly important regarding life and future of mankind.
