Terrestrial allochthonous inputs represent the primary energy source for small stream ecosystems, and activities that alter those inputs produce cascading effects through multiple trophic levels. Urban development affects riparian forests through direct removal or through modifications to species composition and structure. Our study evaluated stand composition and litterfall inputs to small streams across a range of urbanization. Heterogeneity was high, but a consistent pattern of deciduous dominance was observed in more-disturbed areas and a mosaic of conifer, mixed, and deciduous forest patches in less-disturbed areas. Such mosaics are a region-dependent pattern that results from Pacific Northwest forest succession. Riparian vegetation disturbance level increased as total impervious area in the watershed increased. Annual mean daily litterfall rates ranged from 0.0 to 2.5 g m−2 d−1 in 128 trap locations distributed over 13 stream reaches that represent the range of riparian vegetation conditions currently present in the Puget Lowland. Conifer, mixed, and deciduous plots in watersheds with disturbance produced 12.5 to 19.3 mg N m−2 d−1 and 0.9 to 1.3 mg P m−2 d−1. Deciduous riparian forests associated with moderate urban development produced organic matter loads similar to those of conifer forests, but the greater prevalence of leaf material produced 54% higher N loads and 40% higher P loads from litter inputs compared to conifer-dominated riparian areas. Conversion from coniferous to deciduous vegetation also produced subtle shifts in timing of inputs. Anthropogenic activities shape and maintain the current vegetation regime in the Puget Lowland, and this regime has profound implications for nutrient processes and aquatic productivity.
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Vol. 28 • No. 4