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Introduction to the Wood in World Rivers special issue

Large wood has gradually come to be recognized as a critical component of process and form in river corridors, reflecting increasing recognition of the complex interactions among water, sediment, living vegetation, and wood in forested regions (Gurnell, 2013; Montgomery et al., 2003; Ruiz-Villanueva et al., 2016b; Wohl, 2017). Downed, dead wood pieces larger than 10 cm in diameter and 1 m in length have been described as the third leg of the tripod of physical fluxes (Wohl et al., 2019), along with water (Poff et al., 1997) and sediment (Wohl et al., 2015), in natural river regimes. Large wood is also recognized as part of a continuum of interactions among river process and form and both living and dead vegetation (e.g., Gurnell et al., 2016; Gurnell & Grabowski, 2016; Hawley & MacMannis, 2019; Opperman et al., 2008). Research on the geomorphic and ecological effects of large wood was initially largely confined to the Pacific Northwest region of the United States as part of an increasing focus on old-growth forest ecosystems (e.g., Keller & Swanson, 1979; Swanson et al., 1976), although important precursors of this work were pioneering studies of interactions between forest and river dynamics (Hack & Goodlett, 1960) and the influence of vegetation on channel forms in small streams (Zimmerman et al., 1967). Simultaneous with the work in the Pacific Northwest, ecologists started to emphasize the role of large wood in creating structure, habitat diversity, and transient storage in smaller streams in the eastern United States (e.g., Bilby & Likens, 1980). The long history of deforestation and wood removal from river corridors in most of the world (Erskine & Webb, 2003; Harmon et al., 1986; Montgomery et al., 2003; Wohl, 2014) initially limited recognition of the historical importance of large wood in rivers. Over the past five decades, however, a rapidly growing body of field-based case studies (e.g., Ruiz-Villanueva et al., 2016b; Swanson et al., 2021) has documented the ubiquity of large wood influences, from headwaters to inland and coastal deltas and estuarine areas of large rivers (Hinwood & McLean, 2017; Kramer et al., 2017; Phillips, 2012), and on to coastal regions (Dugan & Hubbard, 2010; Gheskiere et al., 2005; Kennedy & Woods, 2012; Kramer & Wohl, 2015), the open ocean (Rama et al., 2014, 2016), and abyssal plains (Amon et al., 2017; Schwabe et al., 2015). Increased documentation of the geomorphic and ecological importance of large wood has led to increased emphasis on retaining and restoring large wood to river corridors (BoR, 2015; Grabowski et al., 2019; Roni et al., 2015). The first Wood in World Rivers Conference was held in October 2000 at Oregon State University (USA), with the stated intent of (i) synthesizing knowledge of the physical dynamics and ecological interactions of large wood in rivers and streams of different geographical regions, (ii) creating a framework for interpreting and applying the results of research at diverse sites, (iii) identifying management systems for wood in rivers, (iv) assessing physical and biological responses of large wood in stream restoration, and (v) exploring links between primary information on the physical and ecological dynamics of large, resource management, and the communities and cultures in which such management is applied (Gregory et al., 2003). This conference resulted in an edited volume published in 2003 and this volume remains a comprehensive and valuable source of understanding for diverse aspects of wood in rivers. The second Wood in World Rivers Conference took place in August 2006 at the University of Stirling in Scotland. Eleven papers resulting from this conference were published in an Earth Surface Processes and Landforms special issue in 2007 (Gurnell, 2007). The third conference was held in Padova, Italy in July 2015, with 15 papers published in a 2017 special issue in Geomorphology (Picco et al., 2017). The fourth conference, held in Valdivia, Chile in January 2019, is the primary source for papers in this special issue, although we have also brought in relevant papers published independently of the conference. Papers from the first Wood in World Rivers Conference emphasized the much greater historical abundance of large wood in most river corridors of the temperate latitudes (Montgomery et al., 2003). Subsequent geologic research indicated how the evolution of land plants and then woody plants influenced river channels and floodplains as the root systems of vascular plants helped to increase bank resistance and the development of logjams influenced channel avulsion and planforms (e.g., Gibling & Davies, 2012). For many years following the initiation of riverine large wood research in the Pacific Northwest, however, few riverine scientists conceptualized large wood as an inherent part of river corridors, with the exception of work in southern England (e.g., Gregory et al., 1985). The geographic scope of riverine large wood research has grown tremendously since circa 2000, thanks in part to the first Wood in World Rivers (WWR) conference. The geographic diversity of the four WWR conferences reflects the spread of studies on wood in rivers and the growth from a niche topic in the Pacific Northwest to a topic of global relevance in Received: 20 January 2021 Accepted: 21 January 2021