Testing urban drivers of riparian woody vegetation composition in a precipitation-limited system

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 asked 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.