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Floristic composition, morphological functional types, and altitudinal distribution pattern for C-4 species were studied in Yunnan province, South-western China. 159 species, in 6 families and 60 genera, were identified with C-4 photosynthesis. 93 % of these C4 species were found in Monocotyledoneae, e.g. Cyperaceae (18 species), Gramineae (129 species), and Commelinaceae (q species), the other 7 % was in Dicotyledoneae, e.g. Amaranthaceae (5 species), Portulacaceae (4 species), and Chenopodiaceae (2 species). Hence C-4 plants mainly occurred in very few families in the tropical region. Compared with those in semi-arid grasslands and and deserts in North China, more C-4 grasses and much less Chenopodiaceae C4 species occurred in the tropical region. This indicates the physiological responses Of C-4 plants from the two families are very different. Chenopodiaceae C-4 species may be more fit semi-arid and and environments, while C-4 grasses are more fit the moist tropical conditions. There was a strong relationship between C-4 distribution and altitude in the tropical region. Altitudinal distribution pattern for C-4 species in the region was consistent with altitude, climate, and habitats.