In natural condition, the diurnal and seasonal changes in the photosynthetic rates of Aneurolepidium chinense and Stipa grandis were measured by means of a simulation chamber. The chief results obtained are as follows:
1.The types of diurnal changes in the photosynthetic rates of Aneurolepidium chinense and Stipa grandis all showed double peak curves. The high peak appeared in the morning and afternoon respectively. But the peak values in the morning always were higher than those of the afternoon.
2.The trends of seasonal changes in the diurnal net photosynthesis of two the communities were similar; The diurnal net photosynthetic rate increased rapidly from the period of turning green to the beginning of July, their maxmum values were 31.68 and ll.5gCO2/m2d respectively, and after then they gradually decreased.
3.The photosynthesis abilities of the plants, the values of the peaks and their appearance times, the fall range of photosynthesis at noon and the characteristic of the seasonal changes etc. of the two communities were closely correlated with the characteristics of the plants growth and environmental conditions.
4.The instantaneous photosynthetic rate, diurnal net photosynthesis, LAI and biomass of the Aneurolepidium chinense community were higher than those of the Stipa grandis one,but under poor(drought) environmental conditions, the increase of the photosynthesis of Stipa grandis was less than under good conditions.
5.Stipa grandis community was more drought resistant than Aneurole pidium chinense commumity, but its productivity increase under good conditions was less.
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