根系分形结构影响根系的分布格局,是植物根系与胁迫生境相互适应的结果.采用全根挖掘和Win-RHIZO根系分析仪相结合的方法,按照垂穗披碱草种群密度设置Ⅰ(500-700株/m2)、Ⅱ(300-500株/m2)、Ⅲ(100-300株/m2)3个密度梯度,研究了尕海湿地不同密度条件下垂穗披碱草(Elymus nutans)的根系分形结构.结果表明:随着垂穗披碱草种群密度的降低,湿地群落的高度、盖度、地上生物量及根系分形维数呈逐渐减小趋势,地下生物量与分形丰度逐渐增大;垂穗披碱草根系分形维数与分形丰度在高密度(Ⅰ)样地和低密度(Ⅲ)样地均呈极显著负相关关系(P＜0.01),在中密度(Ⅱ)样地呈显著负相关关系(P＜0.05),垂穗披碱草根系分形维数和分形丰度存在着“此消彼长”的权衡关系.在高密度湿地群落垂穗披碱草倾向于密集型根系构型构建模式,在低密度湿地群落选择扩散型的根系生长模式,体现了密度制约下高寒湿地植物种群应对资源多重竞争的生态适应机制.%Fractal root systems affect the distribution pattern of root systems,and are important to understand the environmental adaption mechanism of roots under the habitation stress.The objective of this study was to examine the relationship between root fractal dimension and root fractal abundance of Elymus nutans in response to density in the northwest of China.The study site was located in Gahai Wetland,Gansu Province,China (102.08°-102.47° E,33.97°-34.32° N).The altitude of the study site is 3430-4300 m,whereas the average annual temperature is 2.3℃.Sixty samples (1 m × 1 m) were set up along the river bank to investigate the density of E.nutans in August 2016.Population density was categorized as high (Ⅰ,500-700 plant/m2),medium (Ⅱ,300-500 plant/m2),and low (Ⅲ,100-300 plant/m2).The density,height,coverage,and above-ground biomass of the plant communities were recorded from 1 m × 1 m grids in the three densities with six replications,The SC-900 Soil Compaction Meter was used to measure soil compaction in each grid with five replications.The XDB-12 Root Sampler was used to collect the roots of E.nutans,and samples from each grid were sieved completely to collect 60 roots,which were transferred to the laboratory.Afterward,the method of excavation of the whole root system was used to dig soil cores (30 cm × 30 cm × 50 cm) from six grids (30 cm × 30 cm) from the three density gradients.Then the sieve (mesh size=0.25 mm) was used to clean the soil core in the nearby river,and the roots were collected and transferred to the laboratory.Meanwhile,we adopted the method of stratifying the sampling (0-50 cm).In the laboratory,the root samples of E.nutans were scanned using Win-RHIZO to measure the root fractal dimension and fractal abundance,and the biomass was measured for different plots after being dried in an oven (at 105 ℃ for 12 h).Similarly,soil moisture content was measured by oven-drying (at 84 ℃ for 24 h).The results showed that as the population density changed from high-medium to low-medium,the coverage,height,above-ground biomass,and fractal dimensions of the plants displayed a pattern of steady decline,whereas below-ground biomass,root-shoot ratio,main root to lateral root ratio,fractal abundance of the population,and soil compaction increased.Root fractal dimension and root fractal abundance were negatively correlated (P＜0.05)across the alpine wetland.A highly significantly negative correlation (P＜0.01) was observed between root fractal dimension and root fractal abundance in the high density (Ⅰ) and low density (Ⅲ) wetlands of E.nutans population,whereas a less significant correlation (P ＜ 0.05) was observed in the medium density (Ⅱ) wetlands.With the change in density,the root fractal dimension of E.nutans decreased whereas the root fractal abundance increased,which reflected the trade-off mechanism of different root traits.
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