Structural complexity influences the ecosystem engineering effects of in‐stream large wood

Abstract

Large wood (LW) is an ecosystem engineer and keystone structure in river ecosystems, influencing a range of hydromorphological and ecological processes and contributing to habitat heterogeneity and ecosystem condition. LW is increasingly being used in catchment restoration, but restored LW jams have been observed to differ in physical structure to naturally occurring jams, with potential implications for restoration outcomes. This article examines the structural complexity and ecosystem engineering effects of LW jams at four sites with varying management intensity incorporating natural and restored wood. Our results reveal: (i) structural complexity and volume of jams was highest in the site with natural jams and low intensity riparian management, and lowest in the suburban site with simple restored jams; and(ii) that structural complexity influences the ecosystem engineering role of LW, with more complex jams generating the greatest effects on flow hydraulics (flow concentration, into bed flows) and sediment characteristics (D 50 , organic content, fine sediment retention) and the simplest flow deflector-style restored jams having the least pronounced effects. We present a conceptual model describing a continuum of increasing jam structural complexity and associated hydromorphological effects that can be used as a basis for positioning and evaluating other sites along the management intensity spectrum to help inform restoration design and best practice.

Channel-reach morphometric analysis on a headwater stream in a low-mountainous region: a case study from Mecsek Hills

“…Geomorphic (dis)connectivity in a middle-mountain context: Human interventions in the landscape. Furthermore, we consider the type we defined as stable WDJ comparable to the "active jam" type published by Cashman, M.J. et al (2021), which increases the jam's structural complexity and hydromorphological diversity.…”

Biogeomorphological response to river restoration of a suburban river with large wood: Creating a restoration vision and cost‐effectively monitoring the response trajectory using the citizen science MoRPh survey

“…Large wood is a natural component of most river systems and is increasingly being used in river restoration (Cashman et al, 2021;Grabowski et al, 2019;Roni et al, 2015). It is very appropriate to the…”

“…However, the trajectory would be more constrained and less complex than would occur in the absence of the above- In summary, the vision suggests a more constrained and spatiallyfixed development of flow patterns and landforms than would be expected in a more naturally-functioning system but a distinct increase in physical habitat diversity and turnover in comparison with pre-restoration conditions. Despite cost constraints, the restoration represents a far more ambitious approach in terms of the variety and scale of interventions than simply installing fixed log deflectors, which are reportedly the core of most wood-based restorations implemented in the United Kingdom to date (Cashman et al, 2019(Cashman et al, , 2021.…”

Professor Kenneth John Gregory (1938–2020): The career, contributions and legacy of an eminent geographer and fluvial geomorphologist

“…Cashman et al (2021) investigate how structural complexity influences the ecosystem engineering effects of instream large wood, considering four sites that represent the range from naturally formed to human‐designed examples, the latter associated with river restoration. One of these sites, which provided examples of naturally formed wood jams, was on the Highland Water in the New Forest, southern England, where Ken undertook pioneering work on large wood from the late 1970s until the early 1990s.…”