In the development of dynamic global vegetation model, under the limit of manpower, material and financial resources, the studies on species level not only were unlikely, but also were unnecessary. Plant function groups differentiated the plants of different regions from ecological aspect but not from system develop aspect, consequently the number of plants was put down. Now plant function groups have been the important units studying global vegetation change and the ecosystem function of biodiversity. The different photosynthesis approaches of plant, C₃, C₄ and CAM, showed the different response to water, temperature and light from leaf tissue structure to physiological function and from ecological adaptability to geographical distribution. Therefore plant photosynthetic groups were some perfect plant function groups analyzing global vegetation change. Using Northeast China Transect (NECT) as our study flat roof, we analysed the relations of plant photosynthetic function groups (C3 and C4 plants) and community biomass and environment changes in the grassland communities that Leymus chinensis is an edificatory of communities. Our sampling area are located in the steppe segment (125°～115°E) of NECT using 43.5°N as middle axes. From east to west, they are in turn the Changchun agricultural area (123°～125°E) in the center of Northeast China Plain, the meadow and meadow steppe area (122°～124°E) in the south of Songnen Plain, the Kerqing sandlot steppe area (119°～123°E) in the middle of Songliao Plain, the type steppe area (113°～119°E) in the center of Inner Mongolia Altiplano and the desert steppe area (108°～113°E) in the northeast of Ulanzabu Altiplano. All studied zone covered 1000km. Although our sampling area are basically a same latitude, but from east to west the sampling site is gradually elevatory in height above sea level (from 240m to 1480m) and is farther and farther from sea, hence the water and heat condition of sampling area is worse and worse. The maximal value of annual average temperature is 5.9℃ of Tongliao weather station in Kerqing sandlot area, the minimal value is 1.1℃ of Abaga weather station in the west of sampling area. The annual precipitation decreases gradually from 500～600mm of Changchun weather station locating in the east of studied zone to 200～300mm of Abaga weather station locating in the west of studied zone. From east to west, the annual precipitation has decreased 50%.
The studied results showed: the relation between C₄ plants biomass and 4 environment factors including to annual precipitation, annual average temperature, soil water percent and elevation was some marked valley type functions (R2＞0.5, p＜0.05), and the C₄ plants biomass was higher in the two end of aridity and saline-alkali habitat. Under grazing disturbance, C₃ plants biomass was evidently decreased, and C₄ plants biomass was evidently increased. The changes of C3 and C4 plants biomass were an redeeming mechanism for some ecological loss, therefore in maintaining ecosystem primary productivity, not only the different plant growth-form function groups played an important role, but also the plant photosynthetic function groups exerted an important influence. In ungrazing plats, the ratio of C4 plant function groups biomass was only 12.82% of the total biomass of communities, but in grazing plats, it was increased to 42.49%. In the light of the variation of biomass, the variation coefficient of C4 plants (1.10 under ungrazing, 0.66 under grazing)was bigger than that of C₃ plants (0.16 under ungrazing, 0.48 under grazing), hence the biomass of C₄ plant function groups was more evident in response to environmental changes than of C3 plant function groups. In all samples, the discovered C4 plant species were Chloris virgata, Cleistogenes squarrosa, Arundinella hirta, Setaria lutescens, Kochia prostrata, Kochia sieversiana and Salsola collina, of which Chloris virgata, Arundinella hirta and Kochia sieversiana distributed in the saline-alkali Leymus chinensis meadow community in the Songnen Plain, Setaria lutescens distributed in the sandlot habitat, Salsola collina appeared in the Leymus chinensis+Stipa krilovii community and in the sandlot habitat, Kochia prostrata appeared in the type steppe, meadow steppe and sandlot habitat, Cleistogenes squarrosa distributed in the all sampled plats except the severe saline-alkali habitat. Under grazing disturbance, Cleistogenes squarrosa as an edificator was composed of the grazing succession types of all studied communities not including severe saline-alkali habitat. On the basic of above analyses, we brought forward the conclusions below: (1) C₄ plants biomass had some complex and sensitive response to environment change and its variation coefficient was bigger, therefore the change of C4 plant function groups could sensitively reflect the environment change; (2) the changes of C3 and C4 plants biomass had evident compensation, and in NECT, a number of C4 plants were often discovered in some stress, for example in the arid or saline-alkali habitat; (3) C₄ plants species was fewer, only 7 species was appeared in the all studied communities, accounting for 9.72% of the total species, Hereby C4 plants was fit for the fundaments of plant functional groups conception. The above conclusions showed C4 plants as an important plant function groups may be used to evaluate and forecast the change of the grassland vegetation or ecosystem in Chinese temperate zone.