A two end-member model of wood dynamics in headwater neotropical rivers

Geomorphic and ecological effects of instream wood have been documented primarily along rivers in the temperate zones. Instream wood loads in tropical rivers might be expected to differ from those in analogous temperate rivers because of the higher transport capacity and higher rates of wood decay in the tropics. We use data from four field sites in Costa Rica and Panama to demonstrate that wood loads are consistently lower in tropical rivers, despite substantial variations among tropical sites as a result of differences in mechanisms of wood recruitment. We develop a model of wood dynamics (recruitment, transport, and retention) based on differences in dominant wood recruitment mechanism. The steady-state end-member reflects sites where gradual recruitment of wood through individual tree fall creates a relatively consistent wood load through time and development of logjams is minimal. The episodic end-member reflects sites dominated by episodic mass recruitment via landslides or blowdowns. This facilitates formation of transient logjams, so that wood loads exhibit substantial spatial and temporal variation along the channel network. The model presented here should also apply to headwater streams in the temperate zone, although existing documentation suggests that jams are more persistent along streams in the temperate zone.