Physical environmental controls on riparian root profiles associated with black poplar (Populus nigra L.) along the Tagliamento River, Italy

Abstract

AbstractTree roots contribute to the resistance of riparian sediments to physical deformation and disintegration. Understanding reinforcement by roots requires information on root distributions within riparian soils and sediments. Continuous‐depth models or curves have been proposed to describe vertical root density variations, providing useful indicators of the types of function that may be appropriate to riparian trees, but have generally been estimated for terrestrial species or broad vegetation types rather than riparian species or environments.We investigated vertical distributions of roots >0.1 mm diameter of a single riparian tree species (Populus nigra L.) along the middle reaches of a single river (Tagliamento River, Italy), where Populus nigra dominates the riparian woodland. Root density (hundreds m−2) and root area ratio (RAR in cm2 m−2) were measured within 10 cm depth increments of 24 excavated bank profiles across nine sites. Sediment samples, extracted from distinct strata within the profiles, were analysed for moisture content, organic matter content and particle size. Statistical analyses identified two groups of wetter and drier profiles and five sediment types.Following loge‐transformation of root density and RAR, linear regression analysis explored their variation with depth and, using dummy variables, any additional influence of moisture and sediment type. Significant linear regression relationships were estimated between both root density and RAR and depth which explained only 15% and 8% of the variance in the data. Incorporating moisture and then sediment characteristics into the analysis increased the variance explained in root density to 29% and 36% and in RAR to 14% and 26%.We conclude that riparian tree root density and RAR are highly spatially variable and are poorly explained by depth alone. Complex riparian sedimentary structures and moisture conditions are important influences on root distributions and so need to be incorporated into assessments of the contribution of roots to river bank reinforcement. Copyright © 2016 John Wiley & Sons, Ltd.