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

Abstract

Tree 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.

Understanding processes of island development on an island braided river over timescales from days to decades

“…Figure 1, 2002 image shows sprouting trees deposited in 2000). In nearly all cases, the deposited trees that initiate pioneer islands in the study reach are black poplar (P. nigra), a facultative phreatophyte capable of rapid root growth in response to different water and sediment conditions (Holloway et al, 2017a(Holloway et al, , 2017b(Holloway et al, , 2017c, and the LiDAR data and field measurements show that deposited trees of this species can regenerate to produce a canopy that grows rapidly, particularly in the first 15 years following deposition ( Figure 6A). The median canopy height increased from 0.24 to 6.85 m (6.61 m growth) on the 2004PI surfaces, from 1.64 to 9.76 m (8.72 m growth) on the 2000PI surfaces and from 22.79 to 25.04 m (2.83 m growth) on the 1980EI surfaces between 2005 and 2013.…”

“…capable of resprouting) on bar surfaces during the falling limb of flood events, through an initial pioneer island phase of sprouting and sediment retention, to an established island phase as a result of the aggradation, enlargement and coalescence of pioneer islands. Since 2001, a number of refinements to the conceptual model have been introduced (Gurnell and Petts, ; Gurnell et al, , ), and empirical evidence has been presented to support specific aspects of the model, including for example, island morphological (Bertoldi et al, ), sedimentological (Gurnell et al ., ), soil (Mardhiah et al, ; Bätz et al, ), and vegetation development (Francis et al, ; Perona et al, ); living wood recruitment and dynamics (Bertoldi et al, ); early growth rates of tree seedlings, cuttings and deposited trees (Francis et al, ; Francis et al, ; Francis and Gurnell, ; Francis, ; Moggridge and Gurnell, ) and interactions among developing subsurface tree root and shoot biomass and sediment retention (Holloway et al, , , ). In this paper we integrate empirical evidence of the processes and forms that accompany island evolution over decadal to event timescales, focusing on a single island‐braided reach of the Tagliamento River, Italy.…”

Physical engineering of an island‐braided river by two riparian tree species: Evidence from aerial images and airborne lidar

“…Aggradation around vegetation of 0.5 m in the first 5 years of growth and 1.5 – 2 m over 25 years has been observed in the study reach (Gurnell et al, 2019). Hortobágyi et al (2018), following Eichel, Corenblit, & Dikau (2015), have described such periods of landform building as “biogeomorphic feedback windows”, providing trees with elevational protection from floods and finer, moisture‐retentive sediments within which to develop extensive root systems (Holloway et al, 2017; Holloway, Rillig, & Gurnell, 2017a, 2017b). Furthermore, the numerous flow pulses between the 2008 and 2012 floods likely provided opportunities for further sediment trapping as the surviving trees matured, providing an increasing potential for them to survive the 2012 event.…”

“…Building on knowledge of life history and ecological characteristics (Karrenberg, Edwards, & Kollmann, 2002), much research has focused on the role of Salicaceae species in initiating, building and sustaining landforms (Corenblit et al, 2009; Francis, Corenblit, & Edwards, 2009; Gurnell, Tockner, Edwards, & Petts, 2005) and thus creating a topographic signature of vegetation development across river beds and margins (Bertoldi, Gurnell, & Drake, 2011; Bywater‐Reyes, Wilcox, & Diehl, 2017). Research has also revealed the biomechanical properties of the Salicaceae that allow this family to survive, to retain fluvial sediments and to build and reinforce landforms (e.g., Imada, Yamanaka, & Tamai, 2008; Scippa, Trupiano, Rocco, Di Iorio, & Chiatante, 2008; Pasquale, Perona, Francis, & Burlando, 2012; Bankhead, Thomas, & Simon, 2017; Holloway, Rillig, & Gurnell, 2017; Holloway et al, 2017a, 2017b; Wintenberger, Rodrigues, Juge, & Villar, 2017).…”

Plant root and rhizome strength: Are there differences between and within species and rivers?

“…Each reach was approximately 2 km in length and the reaches were spaced approximately 4 km and 28 km apart. Previous research has separated these nine sites into two distinct groups – wet and dry sites – according to soil moisture conditions (Holloway et al, ) and has shown distinct differences in the vertical profiles of root density and root area ratio according to whether data were collected at the wet or dry sites. However, no significant difference was found in the relationship between root tensile strength and diameter according to moisture conditions, suggesting that root tensile strength is insensitive to this environmental property, despite the fact that root profiles show distinct differences.…”