黄土丘陵区典型小流域草地群落数量分类与结构特征及其影响因素

黄土丘陵区退耕还林(草)工程已实施20年，了解草地现状对该区草地生态系统保护与恢复调控措施选择具有重要意义。以安塞纸坊沟小流域内22个草地群落为研究对象，基于Ward聚类和非度量多维度排序(NMDS)方法，对草地类型、结构及其影响因素进行了分析。结果表明：(1)流域内草地共出现23科83种植物，其中禾本科、豆科和菊科物种重要值占比达75%—85%。草地可划分为狗尾草群丛、茵陈蒿群丛、甘草群丛、铁杆蒿群丛和白羊草群丛5类群丛。群丛间结构存在显著差异，狗尾草群丛盖度、地上生物量(AGB)、地下生物量(BGB)和根冠比均显著最低，但Shannon-Wiener指数和Pielou优势度指数均显著高于白羊草和铁杆蒿群丛。茵陈蒿、甘草与铁杆蒿群丛间AGB、BGB及多样性指数无显著差异。(2)功能群对群落结构产生显著影响，豆科矮草对AGB和BGB产生显著正效应，禾本科高草对BGB产生显著正效应，而菊科矮草对群落盖度与BGB产生显著负效应。(3)土壤有机碳(SOC)、全氮(TN)和全磷(TP)含量偏低(分别为6.21,0.82、0.53 g/kg),其中SOC和TN含量显著影响群丛物种组成、功能群和结...     

Composition of plant life forms of subtropical shrubland in China and its correlation with temperature and precipitation

Aims Shrubland is one of the most important vegetation types in subtropical region, but studies on its life-form composition and geographic patterns are scarce. In this paper, we addressed the following questions: what are the characteristics of the composition of plant life forms of subtropical shrublands in China? And what are the relation-ships between the life form compositions, geographic patterns of subtropical shrublands and hydrothermal factors?
Methods A total of 688 plots, designed with the stratified random sampling method, were systematically set and investigated in subtropical shrublands in China. Life forms were identified based on Raunkiaer’s (1934) life form systems. Life form spectrums were compiled at each sampling site and within the whole shrubland, respectively. Partial correlation analysis was applied to reveal the latitudinal, longitudinal and altitudinal patterns of each life form. Pearson’s correlation analysis was adopted to evaluate the relationships between each life form and hydrothermal factors. Further, redundancy analysis (RDA) was used to distinguish the effect of temperature and precipitation on geographical variations of life forms.
Important findings (1) In subtropical shrublands in China, 56.37% of the total species were woody species, among which 33.88% were deciduous species and 22.49% were evergreen species. The remaining 43.63% of the total species were herbaceous species, dominated by 35.15% perennials. Generally, phaenerophytes (Ph) was the most dominant component, while the second dominant one was hemicryptophytes (H). The life form spectrum showed a pattern of Ph (55.74%) > H (31.03%) > T (therophytes 8.80%) > Cr (cryptophytes 4.08%) > Ch (chamaephytes 1.34%). (2) Plant life forms mainly exhibited longitudinal and altitudinal patterns. With the increase of longitude, the percentage of Ph tended to increase while H, Cr and T tended to decrease. Furthermore, an increase in altitude would lead to an increase in H and Cr but a decrease in Ph, Ch and T. (3) Among all the investigated climatic factors, the mean temperature of the warmest month (MWT) and the mean precipitation of the coldest month (MCP) had the greatest impact on Ph, H, Cr and T. With an increase of MWT and MCP, Ph would increase while H, Cr and T would decrease. Taken together, MWT and MCP could explain 41.97% of the variation of life form spectrum, among which 28.35% was attributed to their joint-effect. The results suggested that Ph and H were mainly influenced by both temperature and precipitation, and that Cr was more sensitive to temperature gradients. Besides, T was more sensitive to precipitation, and Ch showed little fluctuation along hydrothermal gradients. In conclusion, MWT and MCP are the most important hydrothermic factors that determine the geographic patterns of life forms in subtropical shrublands in China.     

响应变量为多维时因果推断中混杂因子的控制和排序

研究可交换条件下多维结构回归模型中总体平均处理效应的混杂因子的控制和排序问题,利用矩阵的迹定义混杂因子的控制效率,通过控制效率来控制混杂因子,并给出混杂因子的排序,同时给出一个应用实例。