Underground riparian wood: Buried stem and coarse root structures of Black Poplar (Populus nigra L.)

Abstract

Despite the potential importance of tree species in influencing the processes of wood recruitment, transport, retention, and decay that control river wood budgets, focus has been relatively limited on this theme within fluvial wood research. Furthermore, one of the least investigated topics is the belowground living wood component of riparian trees. This paper presents observations of the morphology and age of buried stem and coarse root structures of eight Populus nigra individuals located in the riparian woodland of two sites on the middle to lower Tagliamento River, Italy. This species was selected because of its wide distribution along European rivers and its frequent dominance of riparian woodland.Each tree was excavated by hand to expose a minimum of half of the root system with complete exposure of the main axis. Smaller roots were then removed and larger protruding roots cut back to permit access to the main axis. The excavated structures were photographed from multiple angles for photogrammetric modelling; the structure and character of the exposed sediments around the tree's main axis were recorded; and wood samples were taken from the main aboveground stem(s), sections of the main buried axis, and major roots for dendrochronological analysis. Results from these field observations and laboratory dating of the wood samples were combined to describe the belowground morphology of each tree and to draw inferences concerning the impact of fluvial disturbances. Common features of these excavated structures included: (i) rooting depths to below the bar surface where the original tree established, with many young roots also existing at depth; (ii) translocation of the main buried axis in a downstream direction; (iii) a main buried axis comprised mainly of stems that have become buried and then generated new shoots, including multistem patches, and adventitious roots; (iv) the presence of steps and bends in the main buried axis associated with the generation of coarse lateral roots, that reflect the sedimentary structure of the surrounding aggraded bank sediments; and (v) grafting of roots within and between some sampled trees.Overall, the sampled trees possessed extremely complex three-dimensional buried wood structures that permeate bank sediments and tie the tree and aggraded bank sediments to basal gravels. These properties and the considerable amount of underground wood that is present have great significance for anchoring trees and giving uprooted trees and root wads a propensity to snag once they enter the fluvial system. Furthermore, the ability of this underground biomass to sprout suggests that uprooted and remaining components of root networks following tree uprooting may sprout, generating new vegetation canopies that can trap mobile wood. Overall, this underground wood offers many traits that may tighten wood budgets, and it is likely that other riparian Salicaceae species with similar traits may have similar wood budget impacts.

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

“…The present paper attempts to provide such information, specifically focussing on one widely‐occurring riparian tree species: Populus nigra L . Complementary research on the architecture of the coarsest root components of individuals of this species is reported elsewhere (Holloway et al , , ), whereas the present paper employs the trenching method to explore root profiles more generally to answer the following research questions: Is there an association between riparian tree root properties and depth within the bank profile? Does the association between root properties and depth within the bank profile vary with moisture availability? Is there an association between root properties and properties of the surrounding sediment? …”

“…Therefore, establishing such distributions has been the main motivation for field investigations of riparian root system structure, including the distributions of root area ratio (RAR) (sectional area of roots per unit sectional area of bank), root diameters (since breaking stress is dependent on size) and root density (frequency per unit area of bank section) through riparian sediments. With the exception of one study in which coarse roots of over 1 cm diameter were observed in eroding banks from a boat (Rood et al , ), another in which whole tree root systems were removed from man‐made canal, dam and flood protection dikes (Vennetier et al , ; Zanetti et al , ), and two recent papers in which the coarse root architecture of eight individual mature Populus nigra trees was investigated and interpreted (Holloway et al ., , ), riparian root investigations have been conducted using the wall‐profile or ‘trenching’ method (Maeght et al , ), where exposed roots are investigated in excavated vertical sediment profiles. Some early excavations by Abernethy and Rutherfurd () and Simon and Collison () found a steep (near exponential) decline in root numbers with depth, although this only approached a steady curve at distances of several metres from the trunks of nearby trees, at which point numbers and area of roots became extremely low, and depth distributions of RAR varied dramatically with species and site.…”

Event‐scale floodplain accretion revealed through tree‐ring analysis of buried plains cottonwoods, Powder River, MT, USA

“…Rates of floodplain deposition (Hupp and Bazemore, 1993;Scott et al, 1997;Friedman et al, 2005;Holloway et al, 2017) and erosion (Simon and Hupp, 1987;Gärtner et al, 2001;Gärtner, 2007) have been estimated using dendrochronology. Specific dendrochronologic techniques have been developed to pinpoint the depth of germination of buried floodplain trees and to determine the age of germination (Hupp and Bazemore, 1993;Scott et al, 1997;Holloway et al, 2017). Specific dendrochronologic techniques have been developed to pinpoint the depth of germination of buried floodplain trees and to determine the age of germination (Hupp and Bazemore, 1993;Scott et al, 1997;Holloway et al, 2017).…”

“…Sigafoos (1964) noted that the new growth of the buried stem of a tree (we use the term 'stem' rather than the term 'trunk') anatomically resembled a root rather than a stem, displaying root-like characteristics such as increased vessel size (Nanson and Beach, 1977;Friedman et al, 2005), a reduction in ring width (Rubtsov and Salmina, 1983;Friedman et al, 2005), and increasingly diffuse annual ring boundaries (Friedman et al, 2005). Anatomical signatures of burial were not as prominent in black poplar (Populus nigra) (Holloway et al, 2017), and were not observed in a controlled study of mature pedunculate oak (Quercus robur) trees (Copini et al, 2015). These observations have been recorded for relatively few tree species including tamarisk (Tamarix ramosissima), sandbar willow (Salix exigua) (Friedman et al, 2005;Dean et al, 2011) and other willow (Salix) species (Rubstov and Salmina, 1983), green ash (Fraxinus pennsylvanica) (Sigafoos, 1964), and balsam poplar (Populus balsamifera) (Nanson and Beach, 1977).…”

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

“…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.…”