**3. Vegetation approach**

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 2200 g/m2 /year net primary production (NPP) and 45,000 g/m2 biomass, this is currently the "ideal existence status" of the biosphere. In the case of deserts, NPP is between 0 and 3 g/ m2 /year and biomass is between 0 and 20 g/m2 . In the case of semi-deserts, NPP amounts to 3–150 g/m2 /year and biomass amount to 20–700 g/m2 .

Desert or semi-desert conditions can be caused by:

	- areas without precipitation (orographical, e.g., Gobi, Tibet, or cool, dry deserts, e.g., Namib, Atacama)
	- areas with much larger evapotranspiration than precipitation due to the heat (southern and central part of Sahara)
	- permanently frozen areas (Greenland, Antarctica, and peaks of high-altitude mountains)
	- sheer, unweathered rock surfaces where water runs off (barren, rocky areas in mountains)
	- Photic zone of open oceans (where biomass has a desert value of only 3 g/m2 , which is even less than that in the Sahara, however, NPP has a semi-desert value of 125 g/m2 / year)
	- 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)
	- Coastal tidal zone
	- Fast-running reaches of rivers
	- Shifting sand areas

At the borders of the semi-desert category, there are tundra, open grasslands (e.g., rocky, saline, and mountain) and several other habitat types besides classic tropical and temperate semi-deserts.
