多年生草本年轮资料是反映草本物种生长状态的重要指标,已被应用于草本物种年际生长动态特征、生活史策略及气候响应敏感性研究当中,然而这方面研究在黄土高原还未引起足够的重视.在黄土高原不同地点采集了16种多年生草本物种的根部年轮样品,通过根部解剖结构识别年轮结构,并分析了这些物种的年轮宽度和导管大小随年龄的变化趋势特征.研究结果表明调查样品中共有14个草本物种(87.5％)具有清晰可识别的年轮结构,平均年龄为7年左右;草本年轮宽度随年龄均有不断减小的趋势,这主要是由于随年龄不断增强的水分胁迫导致的.自然草本物种导管直径(导水能力)随年龄有不断变大的趋势,生长具有可持续性;人工草本物种导水能力随年龄有不断减弱趋势,生长趋于衰退;而分布于黄土高原北缘至半荒漠地区的二色补血草导水能力则随年龄有先上升后下降的趋势,表现出输水效率和安全的权衡策略.本研究阐述了黄土高原多年生草本的年轮学价值和对胁迫生境条件下的生活史策略,能为黄土高原草地植被的生态恢复提供科学依据.%Recent studies have demonstrated that many forb species in temperate zones develop growth rings in the roots that represent annual growth increments,which are likely to reflect the variations in growth conditions,life history strategies,and climate response sensitivity of the species.However,there are few studies related to growth tings for herbaceous species in the Loess Plateau of China.Our objective was to analyze herb-chronological data from herbaceous vegetation in the Loess Plateau to determine the annual growth variation and life history strategies of perennial forb species under water shortage conditions.In our research,the main roots of 16 perennial forb species at 13 sample sites in the Loess Plateau were collected.Cross-sections (10-15 μm thick) were produced from the proximal end of sampled roots using a sledge microtome.Annual growth rings in the main roots were identified and measured by differentiating between earlywood and latewood in the secondary xylem.Maximum xylem diameter,an indication of hydraulic conductance efficiency,was also measured under a microscope.The changing trends in annual growth rings and maximum xylem diameters with increasing age were identified.The results indicated that most of the forb species (87.5％,14 species) in the Loess Plateau showed clearly demarcated growth rings in the root xylem that were most likely annual rings.The age distribution in the whole sample was relatively wide (2-14 years),and the average age of sampled forb species was about seven years.The width of annual growth rings of forb species demonstrated a linearly decreasing trend with increasing age,and this is mainly ascribed to the gradual decline in soil water availability over time.With increasing age,the changing trends in xylem diameter could be roughly classified into three types:The xylem diameter of native forb species (Potentilla spp.,Oxytropis racemosa,Cynoglossum divaricatum,etc) tended to increase monotonically over time,which indicated that an increase in water availability was associated with an increase in maximum xylem conductivity;by contrast,the xylem diameter of planted forb species (Medicago sativa,Sophora flavescens,and Glycyrrhiza uralensis) tended to decrease monotonically over time,which is evidence of hydraulic failure that corresponds with a loss of vascular transport capacity to the increasing demand for water resources;additionally,the xylem diameter of Limonium bicolor (distributed from the northwestern part of the Loess Plateau to semi-desert regions) increased at first and later decreased,resulting in a curvilinear growth trajectory over time,which indicated the tradeoff strategy between hydraulic conductance efficiency and safety for forbs under extremely dry conditions.Our results presented substantial evidence of the presence of growth rings in the secondary xylem of main roots in perennial forbs of the Loess Plateau,and analyzed the differentiated life history strategies of those species,and that has scientific value for soil moisture conservation and sustainable vegetation development in semi-arid regions.
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