Effects of stream flow intermittency on riparian vegetation of a semiarid region river (San Pedro River, Arizona)

Abstract

The San Pedro River in the southwestern United States retains a natural flood regime and has several reaches with perennial stream flow and shallow ground water. However, much of the river flows intermittently. Urbanization-linked declines in regional ground-water levels have raised concerns over the future status of the riverine ecosystem in some parts of the river, while restoration-linked decreases in agricultural ground-water pumping are expected to increase stream flows in other parts. This study describes the response of the streamside herbaceous vegetation to changes in stream flow permanence. During the early summer dry season, streamside herbaceous cover and species richness declined continuously across spatial gradients of flow permanence, and composition shifted from hydric to mesic species at sites with more intermittent flow. Hydrologic threshold values were evident for one plant functional group: Schoenoplectus acutus, Juncus torreyi, and other hydric riparian plants declined sharply in cover with loss of perennial stream flow. In contrast, cover of mesic riparian perennials (including Cynodon dactylon, an introduced species) increased at sites with intermittent flow. Patterns of hydric and mesic riparian annuals varied by season: in the early summer dry season their cover declined continuously as flow became more intermittent, while in the late summer wet season their cover increased as the flow became more intermittent. Periodic drought at the intermittent sites may increase opportunities for establishment of these annuals during the monsoonal flood season. During the late summer flood season, stream flow was present at most sites, and fewer vegetation traits were correlated with flow permanence; cover and richness were correlated with other environmental factors including site elevation and substrate nitrate level and particle size. Although perennial-flow and intermittent-flow sites support different streamside plant communities, all of the plant functional groups are abundant at perennial-flow sites when viewing the ecosystem at broader spatial and temporal scales: mesic riparian perennials are common in the floodplain zone adjacent to the river channel and late-summer hydric and mesic annuals are periodically abundant after large floods.

Effects of hydrologic modifications to riparian plant communities in a large river system in northern China

“…Our results also indicate that riparian perennial species prefer an environment with high soil nutrient levels and permanent streamflow, which are key factors for successful long-term plant growth Stromberg et al 2005). The strong correlations between plant moisture group and hydrologic parameters reveal the long-term response of plants to hydrology, whereas the positive correlations between biomass and TN and hydrologic variables suggest that riparian plant community function was strong in such a moist and nutrientrich environment (Araya et al 2013).…”

“…These groups were used as an indicator to characterize plant response to water resource availability over the long term and reflect the water condition of the wetland habitats Stromberg et al 2005;Minggagud and Yang 2013). Similarly, the plant species were classified into three categories according to lifespan, namely, annuals, biennials and perennials.…”

“…This result is consistent with several studies (Stromberg et al 2009a;Katz et al 2012), suggesting that intermittent flow sites support higher mean richness than perennial and dried-up flow sites. This indicates that the ''disturbance'' of periodic drought at the intermittent (seasonal) flow sites may increase plant diversity (Tabacchi et al 1996;Stromberg et al 2005). This result may be related to the ''Goldilocks effect'' (Berlow et al 2008), which could explain the mechanism by which high diversity occurred at the seasonal flow sites.…”

“…Most annuals (both hydric and mesic species) complete their life cycle during late summer rain at seasonal flow sites, which feature inflowing groundwater or surface flow that moisten disturbed fluvial surfaces in riparian zones . The ''disturbance'' of periodic drought in seasonal flow reaches may provide increased opportunities for establishment of annuals (Lake 2003;Stromberg et al 2005). Researchers of other semi-dry and semi-humid regions have also observed that annual grass species dominate riparian zones influenced by the hydrologic disturbance of flow regime (Capon 2003;Salinas and Casas 2007), suggesting that intermittent flows favor annual riparian plants .…”

“…Hydrologic modifications to river systems caused by anthropogenic activity have major impacts on riparian ecosystems, leading to severe ecological and environmental consequences (Poff et al 1997;Nilsson and Svedmark 2002;Stromberg et al 2005;Steward et al 2012). This is true in northern China, especially for rivers in semi-dry and semi-humid regions, where the flow regimes of many rivers have been modified by anthropogenic activities such as stream diversions, dams, and groundwater pumping for agricultural irrigation (Changming et al 2001;Ren et al 2002;Zhang and Do¨ll 2008).…”