Joshua J. Latterell

Sources and dynamics of large logs in a temperate floodplain river

Large logs, important agents of biophysical heterogeneity in temperate floodplain rivers, have been virtually eliminated from modified systems. Our purpose was to quantify the sources and dynamics of large logs (> or = 1 m diameter) in the mainstem of a nearly pristine system: the Queets River, Washington, USA. Erosion of forests by the river supplies 0.40 logs x (100 m)(-1) x yr(-1) to the channel. Most (72%) are new logs entering the river for the first time as the river undercuts mature fluvial terraces dominated by large conifers. Retrospective airphoto analyses demonstrate that, over 63 years, the Queets River recruits 95% of new logs from a riparian corridor extending 265 m laterally on both banks, mostly through channel meandering. However, input rates are patchy, with 10% of the valley length supplying 38% of the new logs. As the river moves laterally, remnant logs are left on channel surfaces that later develop riparian forests and reenter the river when those forests erode. Remnant logs lying on the floodplain forest floor surface or buried in alluvium constitute 21% and 7% of the annual inputs from bank erosion, respectively. We estimate that 50% of logs deposited in the channel in a given year, including those underpinning logjams, are transported downriver within five years. Over the next 55 years, bank erosion reclaims an additional 23%, leaving 27% of the logs stable for > 60 years. Simulations indicate that recurrent transport is common, with half of the large conifers being deposited in > or = 3 locations and transported > or = 1.5 km prior to disintegrating. One in ten logs links distant reaches by occupying > or = 7 locations spanning > or = 12.0 km. Instream supplies are therefore a mixture of new and old logs from nearby and upstream forests, sustained by the recapture and transport of stockpiled remnant logs during periods when new inputs are low. We propose that patchy input rates and the periodic rearrangement of large logs are important drivers of temporal variation in river valley habitats, adding to the spatial complexity created by stable logs. These findings underscore the importance of extensive mature forests and connectivity in temperate floodplain rivers.

A Process-Based View of Floodplain Forest Patterns in Coastal River Valleys of the Pacific Northwest

Floodplains in the Pacific Coastal Ecoregion (PCE) stem from steep eroding mountain landscapes in a rain forest environment, and sustain a rich array of natural resources. Like floodplains elsewhere, many of the approximately 200 coastal river valleys are profoundly altered by flow regulation and land conversion for agriculture and urban development, and these activities have contributed to widespread declines in anadromous fishes and environmental quality. Some of the coastal river valleys, however, still retain many of their natural features, thereby providing important reference sites. Understanding fundamental biophysical processes underpinning natural floodplain characteristics is essential for successfully protecting and restoring ecological integrity, including inherent goods and services. This article examines factors underpinning the ecological characteristics of PCE floodplains, particularly riparian soils and trees. Drawing on over two decades of research and literature, we describe the spatial and temporal characteristics of physical features for alluvial PCE floodplains, examine the importance of sediment deposition and associated biogeochemical processes in floodplain soil formation, quantify vegetative succession and production dynamics of riparian trees, discuss how epiphytes, marine-derived nutrients, and soil processes contribute to tree production, describe the roles and importance of large dead wood in the system, the role of termites in its rapid decomposition, and show how large wood contributes to vegetative succession. These highly interconnected features and associated processes are summarized in a model of system-scale drivers and changes occurring over several centuries. Collectively, this integrated perspective has strong implications for floodplain rehabilitation, and we identify appropriate metrics for evaluating floodplain condition and functions. We draw heavily from our own experience on several well-studied rivers, recognizing additional studies are needed to evaluate the generality of concepts presented herein. As in any complex adaptive system, fundamental uncertainties remain and constraints imposed by the legacies of past human actions persist. Nevertheless, the evolving knowledge base is improving conservation strategies of lightly modified floodplains and is supporting the incorporation of emerging process-based perspectives into the rehabilitation of heavily modified systems.

Origins, Patterns, and Importance of Heterogeneity in Riparian Systems

Riparian systems epitomize heterogeneity. As transitional semiterrestrial areas influenced by water, they usually extend from the edges of water bodies to the edges of upland terraces. Riparian systems often exhibit strong biophysical gradients, which control energy and elemental fluxes, and are highly variable in time and space. These attributes contribute to substantial biodiversity, elevated biomass and productivity, and an array of habitats and refugia. Focusing on riparian systems of medium-sized floodplain rivers, we describe heterogeneity at multiple space and time scales, illustrate interactions among scales, and propose a conceptual model integrating major system components. We show how climatic and geologic processes shape an array of physical templates, describe how disturbances redistribute materials, and illustrate how soils and subsurface processes form and are sustained. Collectively, these processes strongly influence plant productivity and fluxes of channel-shaping large woody debris (LWD). Ultimately, riparian ecosystem function integrates climate (past and present), geologic materials and processes, soil development and attendant microbial transformations, subsurface characteristics, plant productivity, animal activities, and LWD—and the active, continuous and variable feedbacks between the individual components.