What lies beneath riparian black poplar (Populus nigra L.): Root distributions, associations and structures.

Abstract

Vegetation plays a central role in river dynamics and riparian forest is itself a rare and valuable habitat. Tree roots stabilise riparian sediments and are key to regeneration after disturbance. However, despite mechanistic understanding of these effects, poor knowledge of spatial variability and its controls limits its practical application. With findings from field investigations undertaken within a dynamic and near-natural riparian forest system dominated by black poplar (Populus nigra L.), this thesis describes observed root distributions and investigates dimensions of their variability and potential controls that can contribute to both scientific understanding of river dynamics and river management at individual tree to landscape scales. Following an introduction to the thesis (Chapter 1), critical literature review (Chapter 2), and descriptions of study sites and methods (Chapter 3), Chapter 4 presents observed root depth distributions, revealing a more complex picture than a simple decline with depth, with differences dependent on environmental variables that vary between and within study sites. Chapter 5 tests the hypothesis that root distributions are significantly associated with the complex sediment profiles found in active riparian systems. It emerges that commonly-used aggregate root metrics are less well-predicted by sediment variables than parameters describing the local root diameter composition. Further light is shed on the variability of root distributions in Chapter 6 by considering the development of gross subterranean tree structures, from which finer roots emanate. Analysis of root and buried stem system exposures demonstrates how these complex, often very massive structures are dependent on both local contemporary environmental conditions and the disturbance history of an individual tree. Finally, the significance of the research findings for a whole-system understanding of river dynamics, management, conservation and restoration, is explored. What can be reasonably assumed, and its limitations, is distinguished from what may be more dependent on local context, and why. Investigations pursuant of additional questions emerging from the research are also suggested, alongside preliminary results from supplementary further studies.