Abstract: Differences in environmental adaptability and relative competitive ability of co-existing species are two key factors driving community dynamics and succession. Photosynthetic responses are closely linked to adaptability and competitive ability. Therefore, it is useful to study the differences in photosynthetic characteristics of co-existing species to help understand the mechanisms of community succession. In this study, 10 co-existing plant species in middle secondary successional stages in Loess Plateau vegetation were studied. The species studied were Heteropappus altaicus, Artemisia scoparia, Artemisia sacrorum, Artemisia giraldii, Oxytropis glabra, Lespedeza dahurica, Bothriochloa ischaemum, Agropyron cristatum, Cleistogenes chinessis, and Cleistogenes songorica. Their photosynthetic rate, transpiration rate and water use efficiency under different water treatments (severe, moderate and mild water deficit) were measured in August, September and October, with the aim of categorizing the differences among these plant species in photosynthetic response to variation in soil moisture. The results showed that: 1) Under all water conditions, the diurnal variation of net photosynthetic rate of C. songorica, B. ischaemum, O. glabra and L. dahurica showed a diurnally unimodal curve, while A. cristatum and H. altaicus showed bimodal curves with an obvious midday depression of photosynthesis. The diurnal peak of C. songorica had the highest amplitude, while that of A. cristatum had the lowest. 2) The diurnal variation in transpiration rate of most species followed a single-peaked curve under all water conditions, with the main exception being C. chinessis and H. altaicus. Again, the peak of C. songorica had the largest amplitude, while O. glabra had the lowest. 3) There were significant inter-species and growing season differences in daily average net photosynthetic rate and transpiration rate (P<0.05), while differences in daily average water use efficiency were not obvious (P>0.05). Among the 10 co-existing species, C. songorica had the highest daily average net photosynthesis and transpiration rates. 4) With respect to the ecological successional sequence, the daily average water use efficiency of the early-succession species were relatively low, while values for the post-succession species were relatively high, meaning that the water use performance of the late-succession species was higher. Considering the arid conditions in this region of the Loess Plateau, and their likely impact on the succession process, the logical expectation is that late-succession species will, through their water use characteristics, gradually occupy a dominant position in their plant communities, and thus gradually replace the early-succession species in the arid and semi-arid hilly region of the Loess Plateau.

Key words: co-existing species, photosynthetic characteristics, soil moisture, net photosynthetic rate, transpiration rate