ObjectiveThe clonal growth of Populus euphratica plays an important role to the recruitment and persistence of the population in arid-climate region, and the change in branching pattern of clonal roots is a key process for P. euphratica to achieve the clonal growth in heterogeneous environment. This paper attempts to clarify the branching pattern of clonal root system of P. euphratica in heterogeneous riparian habitat, to estimate the correlations between branching traits of clonal roots including internal length (IL), branching intensity (BI), vertical-and horizontal-branching angle (VBA and HBA), and soil factors, and then to find the key factors driving changes of the branching pattern.

MethodWe investigated 16 clonal root branches over 10 m by manual digging and collected below-ground environmental factors correspondingly in a natural floodplain of Ejin Oasis, Inner Mongolia of northern China. Correlation and ordination of the branching traits of clonal root were analyzed using correlation analysis (CA) and principal component analysis (PCA). Redundancy analysis (RDA) and variation partitioning (VP) were applied to explore the relationship between root branching traits and soil factors.

Result(1) The results of CA and PCA showed that there was a high correlation between the four branching traits (P < 0.05), and the first axis, reflecting 66.88% variation of the branching traits, can represent the changes from guerrilla-branching type (with sparser and longer branches, smaller branching angle in vertical and horizontal) to occupation-branching type (with denser and shorter branches, steeper branching angle in vertical and wider branching angle in horizontal). The former more occur on the edge of the river, and the later more on the edge of the forest. (2) The results of RDA revealed that VBA was mainly affected by soil compactness positively (R²=0.64, P < 0.05), and IL was mainly affected by it negatively (R²=0.87, P < 0.05). Soil total carbon and total nitrogen had a major positive influence on BI and HBA, but soil sandy content had a negative effects on them. The depth of branching points of clonal root was positively controlled by capillary water depth (R²=0.62, P < 0.05). (3) VP results showed that the three set of soil variables including soil nutrient-related, water-related and physical factors explained a total of 68.3% of the variance of P. euphratica branching traits. The independent effects of soil physical factors (soil texture, compactness) were the highest (R²=0.12, P < 0.05). Nutrient- and water-related factors had relative smaller independent effects (R²=0.07, P < 0.05; R²=0.03, respectively), but they had a greatest together influences with soil physical factors to variance of the branching traits (R²=0.23, P < 0.05).

Conclusion(1) Guerrilla-branching type and occupation-branching type are two main branching pattern of clonal roots for P. euphratica to vegetative propagate in heterogeneous floodplain of arid region. (2) Soil physical factor is the most important factor to drive changes in branching pattern of the clonal roots. P. euphratica tends to develop an occupation-branching clonal roots in soil patches with less sandy and smaller compactness to establish its ramets. (3) The vertical branching angle of clonal roots of P. euphratica expresses a significant ecological plasticity, it may be an important adaptive trait of clonal roots for woody plant that should be paid attention in the future researches of clonal plant.