Legacies of flood reduction on a Dryland river

Abstract

The Bill Williams (Arizona) is a regulated dryland river that is being managed, in part, for biodiversity via flow management. To inform management, we contrasted riparian plant communities between the Bill Williams and an upstream free‐flowing tributary (Santa Maria). Goals of a first study (1996–1997) were to identify environmental controls on herbaceous species richness and compare richness among forest types. Analyses revealed that herbaceous species richness was negatively related to woody stem density, basal area and litter cover and positively related to light levels. Introduced Tamarix spp. was more frequent at the Bill Williams, but all three main forest types (Tamarix, Salix/Populus, Prosopis) had low understory richness, as well as high stem density and low light, on the Bill Williams as compared to the Santa Maria. The few edaphic differences between rivers (higher salinity at Bill Williams) had only weak connections with richness. A second study (2006–2007) focused on floristic richness at larger spatial scales. It revealed that during spring, and for the study cumulatively (spring and fall samplings combined), the riparian zone of the unregulated river had considerably more plant species. Annuals (vs. herbaceous perennials and woody species) showed the largest between‐river difference. Relative richness of exotic (vs. native) species did not differ. We conclude that: (1) The legacy of reduced scouring frequency and extent at the Bill Williams has reduced the open space available for colonization by annuals; and (2) Change in forest biomass structure, more so than change in forest composition, is the major driver of changes in plant species richness along this flow‐altered river. Our study informs dryland river management options by revealing trade‐offs that exist between forest biomass structure and plant species richness. Copyright © 2010 John Wiley & Sons, Ltd.

Novel water sources restore plant and animal communities along an urban river

“…Our reference reach was intended as a contrast for the flow-regulated urban Salt River and thus was located in the flow-regulated portion of the river upstream of the city. Reaches below dams often have reduced richness of plant species owing to disruption of longitudinal connectivity and reduction in spatio-temporal heterogeneity (Uowolo et al, 2005;Stromberg et al, 2012) and thus do not necessarily represent regional potential. Although stream flow is perennial, the magnitude and timing of flows have been altered by flow-regulating dams (Fenner et al, 1985).…”

Abandoned Floodplain Plant Communities Along a Regulated Dryland River

“…Species richness was also low on high terraces, which is likely due to the dense canopy cover and dominance of Prosopis in the woody community. The fact that richness was lowest on surfaces with high canopy cover, woody stem density and basal area (riparian and high terraces) is consistent with other studies indicating that herbaceous communities are limited by light, potentially high amounts of leaf litter and, in the case of riparian surfaces, recent flooding (Stromberg, ; Stromberg et al ., ).…”

“…Our results were more consistent with work showing that riparian areas may not necessarily have high richness but contribute unique species to the landscape (Deiller et al ., ; Sabo et al ., ). Further, riparian areas along regulated rivers such as the Bill Williams River can have lower diversity compared with unregulated river reaches (Uowolo et al ., ; Stromberg et al ., ). Because we sampled after a wet winter and subsequent spring flooding, it is likely that diversity in the uplands and ephemeral washes was relatively high because of increased soil moisture from precipitation and relatively low in the riparian areas because of recent flooding disturbance and associated understory vegetation removal.…”

“…In another study along the Bill Williams River, Stromberg et al . () sampled more intensively along the transverse gradient and found on average 14–32 species per riparian site depending on the year, season and location.…”

“…Conversely, on riparian surfaces, frequent flooding and new tree establishment in recent years (Shafroth et al ., ) resulted in stands with high densities of smaller trees. Other studies on the Bill Williams River have reported similar or higher stem densities and basal areas in riparian areas (Shafroth et al ., ; Stromberg et al ., ).…”