In precipitation‐limited regions, where water availability is a key driver of vegetation patterns, altered hydrology in urban areas may have a particularly strong effect on riparian plant communities. Urban water inputs can create perennial flows in formerly intermittent streams, potentially providing a water subsidy to riparian plants during dry seasons. However, during rainy seasons, increased storm flow magnitudes in urban areas can cause stream channel incision and associated lowered water tables. Channel incision may reduce riparian soil moisture throughout the year and limit the effect of dry‐season stream flow. We investigated whether channel incision and dry‐season stream flow were related to the composition of riparian vegetation communities along small streams in Sacramento, California, United States, and whether these two factors interacted. Sacramento has a Mediterranean climate with a long summer dry season. We sampled the riparian woody plant community along 66 stream reaches that created a gradient of channel incision severity across sites with and without dry‐season flow. The riparian vegetation community was more strongly related to channel incision than dry‐season flow. The two factors did interact in relation to the overall community composition based on multivariate analyses, but we did not detect interactions in univariate generalized linear mixed models predicting specific components of the woody vegetation community, with the exception of large native oak trees. We found that wetland‐associated species and non‐native species both became less prevalent with increasing channel incision. Conversely, native upland oak trees became more prevalent with increasing incision, although large oak trees only showed this pattern along flowing streams. Wetland‐associated seedlings were almost exclusively found along flowing streams. Synthesis. Our findings suggest that channel incision is a key driver of riparian woody vegetation composition. Although the effects of dry‐season flow were less pronounced, urban water subsidies may be essential for the establishment of wetland‐associated trees along historically intermittent streams. Varied hydrological conditions in precipitation‐limited cities are associated with divergent assemblages of woody species in riparian zones.
Riparian soil processes and vegetation are sensitive to water availability. Urbanization can alter riparian water availability by modifying stream flows and stream channel morphology. In cities, runoff from impervious surfaces tends to increase stormflow magnitudes, causing stream channels to incise, or downcut. This change in channel morphology has been linked to lowered water tables and drier conditions in temperate urban riparian zones, leading to shifts in riparian nitrogen (N) cycling and vegetation communities. In Mediterranean climates with distinct wet and dry periods, there is an additional dynamic to consider: runoff from urban water use can cause streams to flow when they would otherwise be dry. This dry‐season stream flow could create increased, rather than decreased, water availability in urban riparian zones. However, channel incision may counteract this effect. We asked whether dry‐season stream flow interacted with channel incision to influence riparian soil characteristics and understory vegetation along streams in Sacramento, California, which has a Mediterranean climate with an intense summer dry season. At 40 stream reaches that varied by severity of downcutting and presence of dry‐season flow, we sampled soils and vegetation on top of stream banks and at the margin of the low‐flow channel, an important location for nutrient cycling in dry climates. We measured soil moisture, organic matter, and ∂15N, as well as total and perennial understory vegetation cover. We found that channel characteristics associated with incision limited the influence of dry‐season stream flow on soil moisture, and this interaction appears to have lasting effects on soil organic matter and perennial vegetation on bank tops. At the stream margin, channel downcutting was associated with reduced soil organic matter and vegetation cover, while dry‐season flow was associated with increased vegetation cover. Values of soil ∂15N pointed to limited hydrologic linkage between stream flows and riparian bank soils along incised streams. Our findings suggest that channel incision could limit the ability of urban riparian ecosystems to mitigate low‐flow water quality. However, where streams are not incised in Mediterranean climates, dry‐season flows from urban runoff may actually increase riparian productivity and N cycling above historical levels.
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