温带草原不同土地利用方式下优势种植物和生态系统的气体交换

Dominant species and ecosystem gas exchange in temperate grassland under different land use patterns

在内蒙古温带草原围封、放牧和割草3种处理下的样地内,对生态系统尺度和大针茅(Stipa grandis)、冷蒿(Artemisia frigida)、羊草(Leymus chinensis)3种优势种植物叶片尺度上的气体交换和水分关系进行了测定,对比研究了植物碳水对环境的响应。结果表明,在优势种单株尺度和生态系统尺度上,大气-植被CO2交换因草地利用方式的不同而具有不同的表现。在生态系统层面,放牧样地的群落净CO2气体交换量和总初级生产力都与围封样地和割草样地有差异,群落总初级生产力受生态系统呼吸的影响。在放牧处理下,群落净CO2气体交换量日变化表现为生态系统对碳的吸收,而围封和割草则以碳释放为主。单叶光合速率出现负值并随时间推移而恢复的现象,应是植物对干旱高温、高光照的特殊反应。生态系统水分利用效率没有明显不同,但各样地的蒸散能力有趋势上的变化;对于同种植物,放牧样地植物单叶水分利用效率的日变化波动幅度最大,围封样地最小。

Aims Ecosystem carbon uptake or release is influenced by land uses. However, few quantitative studies have been made to give accurate explanations. Based on field measurements of gas exchange and water relations at the leaf scale and the ecosystem scale in temperate grasslands locating in northern China, we aim to 1） clarify the difference in carbon exchange between the leaf scale and the ecosystem scale in different land uses; 2） understand how human activities influence the carbon pools at the ecosystem scale by analyzing changes in gross primary productivity （GPP） and ecosystem respiration （Reoo）; and 3） explore whether water condition in semi-arid steppe restricts CO2 absorption at the ecosystem level. Methods We selected three dominant spcies, Stipa grandis, Artemisia frigida and Leymus chinensis, based on quadrat surveys. Some ecophysiological indexes were measured using a modified LI-6400 in the sample plots of three land use patterns： enclosed plots, mowed plots and grazed plots. We calculated net ecosystem CO2 exchange （NEE）, GPP, Reco and water use efficiency （WUE） with the measured results. All data were analyzed using one-way ANOVA and t-test of SPSS. Important findings By comparing the carbon exchange of dominant species at the ecosystem scale, we found that carbon exchange varies with change of land use. NEE and GPP of the ecosystem in the grazed plots are different from those in the enclosed plots and the mowed plots, and GPP is more dependent on Reco. In grazed plots, the diurnal change of NEE shows C uptake, but in the other two plot t3~pes it appears to release C. The phenomenon that negative net photosynthetic rate （Pn） returns to positive values from midday to dusk should be a special reaction of plants living in grasslands to protect themselves under high light and temperature conditions. There is no difference in WUE at the ecosystem scale, but the tendency of evapotranspiration （ET） is not the same; nevertheless, the degree of fluctuation of WUE for the same species at the leaf scale is largest in the grazed plots and smallest in the enclosed plots.