THE ROLE OF FOUNDATIONAL TREE SPECIES IN THE SELF-ORGANIZATION OF FORESTED ALLUVIAL VALLEYS

The role of large riparian trees in catalyzing the self-organization of alluvial river valleys in the Pacific Northwest of North America exemplifies a phenomenon that may be general to temperate forest river valleys. The presence of large riparian trees in these environments initiates multiple biogeomorphic feedback mechanisms that define an ecosystem trajectory toward a self-reinforcing state by organizing fluvial processes and landforms in ways that structure and maintain forested river valley ecosystems. We propose that tree species belonging to a “large tree functional type” instigate a “forest-river wood cycle” having several pathways that reinforce the regeneration and growth of large trees while structuring river landscape. For example, fluvial wood from large trees become stable key pieces in wood jams which induce deposition and create elevated floodplain patches protected from fluvial erosion for hundreds of years on which forests can produce large trees and future key pieces. These stable forested islands create and maintain an anastomosing channel pattern with numerous secondary channels, and help create a floodplain patchwork characterized by varied elevations, geomorphic surfaces, and forest composition and age. This physical complexity promotes and reinforces biological diversity and productivity. Because large riparian trees control geomorphological and ecological conditions across a range of scales, processes, and spatial domains, we propose they be considered foundation species. We also propose loss of large trees triggers a divergent trajectory toward an alternate, self-reinforcing state with simplified ecological and physical structure, process, and diversity, and that loss of the largest riparian trees is an alternate or complementary explanation to direct human alteration (e.g., by levees or river training) in the historical anthropogenic simplification of riverine environments. Conversely, the large riparian tree functional type and the physical and ecological interactions it triggers can be critical to river restoration. Available evidence suggests that, while interactions and triggering mechanisms may differ with river scale and within and between biomes, large riparian trees play roles in the self-assembly of river valleys throughout the temperate zone.