中文摘要：

羊草作为欧亚大陆草原东缘干旱半干旱区域的优势种,现生产力已逐年降低.为明确未来降雨量变化和CO2 浓度升高对羊草草地变化的影响,我们模拟3个降雨量梯度（正常降雨量40%）和两个CO2 浓度（380±20,760±20μmol/mol）进行了光合特性与生物量的研究.结果表明,降雨量变化和CO2 浓度的交互作用显著影响羊草的净光合速率等光合特性,地下生物量和根冠比,而不影响地上生物量.在目前CO2 浓度时随着降雨量的增加,以及在CO2 浓度升高时的降雨量由低降雨量增至正常降雨量,羊草的净光合速率、气孔导度、蒸腾速率和地上、地下及总生物量均显著增加,但在CO2 浓度升高,降雨量由正常降雨量增至高降雨量时上述指标无显著变化.根冠比在目前CO2 浓度时差异不显著,在CO2 浓度升高下低降雨量时显著增加.在降雨量相同条件下,CO2 浓度的升高使胞间CO2 浓度、水分利用效率显著增加,气孔导度和蒸腾速率显著降低.低降雨量时CO2 浓度升高对羊草净光合速率和生物量的促进作用显著高于正常降雨量和高降雨量,而高降雨量时CO2 浓度升高对羊草生物量没有促进作用.以上结果暗示着在目前CO2 浓度下,随着降雨量的增加,羊草生物量随之增加,在未来CO2 浓度升高的背景下,高降雨量年份对生物量的积累并无显著的促进作用,CO2 浓度升高可以补偿低水分条件对多年生根茎型禾草--羊草生长发育所造成的不利影响.

英文摘要：

Leymus chinensis is a dominant,rhizomatous perennial C3 species found in the grasslands of Songnen Plain,Northern China,where its productivity has decreased year by year due to environmental changes.As a dominant species growing in arid and semiarid regions,precipitation is a key factor limiting plant productivity. To determine how this species’productivity responds to different precipitation regimes and increased CO2 lev-els,we conducted an experiment that measured photosynthetic parameters,along with the accumulation and partitioning of biomass.Plants were subjected to combinations of three precipitation gradients (normal precipi-tation,normal±40%)and two CO2 levels (380±20 and 760±20 μmol/mol)in environment-controlled cham-bers.The interaction between increased CO2 and precipitation had significant effects on photosynthetic parame-ters,belowground biomass and root∶shoot ratio,but no effect on aboveground biomass.Net photosynthetic rate,stomatal conductance,transpiration rate,aboveground and total biomass rose with increases in precipita-tion and CO2 concentration,but no significant change was observed when precipitation increased from normal to high under CO2 enrichment.There was no significant difference in the ratio of root to shoot among precipita-tion regimes at the low CO2 condition,but it changed significantly with CO2 enrichment and low precipitation. Water use efficiency and intercellular CO2 concentration increased significantly when precipitation was low,but decreased when precipitation was high under CO2 enrichment.The effect of elevated CO2 on photosynthesis and biomass accumulation was more obvious at low precipitation than at normal or high precipitation.The results suggest that at ambient CO2 levels,net photosynthetic rate and biomass of L.chinensis increase with precipita-tion,but that these measures are not further affected by additional precipitation when CO2 is elevated.Further-more,CO2 may partly compensate for the negative effect of low precipitation on the growth and development o