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 resprout, 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.