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.