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The natural geographical occurrence, carbon assimilation, and structural and biochemical diversity of species with C-4 photosynthesis in the vegetation of Mongolia was studied. The Mongolian flora was screened for C-4 plants by using C-13/C-12 isotope fractionation, determining the early products of (CO2)-C-14 fixation, microscopy of leaf mesophyll cell anatomy, and from reported literature data. Eighty C-4 species were found among eight families: Amaranthaceae, Chenopodiaceae, Euphorbiaceae, Molluginaceae, Poaceae, Polygonaceae, Portulacaceae and Zygophyllaceae. Most of the C4 species were in three families: Chenopodiceae (41 species), Poaceae (25 species) and Polygonaceae, genus Calligonum (6 species). Some new C-4 species in Chenopodiaceae, Poaceae and Polygonaceae were detected. C-4 Chenopodiaceae species make up 45% of the total chenopods and are very important ecologically in saline areas and in cold arid deserts. C-4 grasses make up about 10% of the total Poaceae species and these species naturally concentrate in steppe zones. Naturalized grasses with Kranz anatomy,of genera such as Setaria, Echinochloa, Eragrostis, Panicum and Chloris, were found in almost all the botanical-geographical regions of Mongolia, where they commonly occur in annually disturbed areas and desert eases. We analyzed the relationships between the occurrence of C-4 plants in 16 natural botanical-geographical regions of Mongolia and their major climatic influences. The proportion of C-4 species increases with decreasing geographical latitude and along the north-to-south temperature gradient; however grasses and chenopods differ in their responses to climate. The abundance of Chenopodiaceae species was closely correlated with aridity, but the distribution of the C-4 grasses was more dependent on temperature. Also, we found a unique distribution of different C-4 Chenopodiaceae structural and biochemical subtypes along the aridity gradient. NADP-malic enzyme (NADP-ME) tree-like species with a salsoloid type of Kranz anatomy, such as Haloxylon ammodendron and Iljinia regelii, plus shrubby Salsola and Anabasis species, were the plants most resistant to ecological stress and conditions in highly arid Gobian deserts with less than 100 mm of annual precipitation. Most of the annual C-4 chenopod species were halophytes, succulent, and occurred in saline and arid environments in steppe and desert regions. The relative abundance of C-3 succulent chenopod species also increased along the aridity gradient. Native C-4 grasses were mainly annual and perennial species from the Cynodonteae tribe with NAD-ME and PEP-carboxykinase (PEP-CK) photosynthetic types. They occurred across much of Mongolia, but were most common in steppe zones where they are often dominant in grazing ecosystems.