Large-river floodplains throughout the world contain understudied ecosystems, and societal pressure for river regulation necessitates the development of principles for conserving their ecological functions. For instance, the areal extent of tidal forestlands (swamps) historically present worldwide is substantially reduced today, and the provision of swamp habitat functions for endangered species has become an ecological restoration goal in places such as the Columbia River, U.S.A. Picea sitchensis (Sitka spruce) swamps, known as “tideland spruce,” naturally occurred along oligohaline and freshwater channels from southeastern Alaska to northern California, broadly congruent with the coastal fog belt. However, like most original forests of the temperate zone, few intact reference sites remain and the historical habitat functions are relatively unknown. The objectives of this study were to address uncertainties in an ecosystem conceptual model of Sitka spruce swamps, focused on salmon habitat function; document the reference condition of the ecosystem, including its controlling factors; and elucidate probable restoration trajectories by evaluating elements of the model at a restoration site. Ecosystem attributes studied in four remnant patches and a restoration site on the lower Columbia River floodplain included topography, inundation, hydraulic geometry, channel morphology, plant community, litterfall, large wood, soils/sediments, water properties, salmonids, and prey.
Results/Conclusions
The swamps occupy an elevation band of ~1-4 m NAVD88, with hummocky microtopography. The total recorded plant species richness is 75: 44 herbaceous species, 22 shrubs, and 9 trees. The dominant overstory evergreen tree species is Sitka spruce; subdominant Thuja plicata (Western redcedar). Alnus rubra (red alder) or subcanopy tree species exhibit highest relative frequencies and densities. Sitka spruce attain moderate heights, generally <40 m for trees up to 120 cm dbh, and seedlings frequently occur on “nurse logs.” The frequency of overbank inundation is 1.8-18.7%. Channel morphology is characteristic of a large-wood forced step pool type (2.5 channel widths/pool; 2.8 pools/100 m). Channel and floodplain sediments are mostly fines with mean total organic carbon 3.1% (channels) and 5.2% (floodplain). The mean floodplain sediment accretion rate is 0.5 cm/yr. Channel cross-sectional morphology is correlated with catchment-scale attributes (area; total length of channels). Chinook, coho, and chum salmon are present and most major prey taxa identified in salmon stomachs are collected in insect fallout traps, neuston tows, or benthic cores. Data suggest that the hydrologic regime, sediment processes, and salmon respond quickly to simple hydrologic reconnection restoration, while subsequent reestablishment of microtopography and the plant community could occur over centuries.