The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

Abstract

1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition. 2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River. 3. Patterns of species richness varied with temporal scale of analysis, that is between single-year and multi-year time frames. At the annual timescale, quadrat species richness (m )2 ) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single-year pattern, the highest long-term richness occurred at intermittent sites. 4. Quadrat species richness, total species richness at a site (per 18 1-m 2 plots) and cover were more variable year to year at non-perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter-annual compositional variance, while the perennial sites had the lowest. 5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types. 6. High long-term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non-perennial sites.

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

“…Among the three hydrologic conditions, the stream section with seasonal flow had the greatest plant diversity. 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).…”

“…Compared to dried-up sites, seasonal flow sites experience more frequent surface flow events, which facilitate spatially heterogeneous moisture conditions in the riparian zone. Compared to perennial sites, seasonal flow sites experience greater ''disturbance,'' i.e., greater variability of surface flow (Katz et al 2012). Overall, environmental variability at intermittent sites, combined with the immigration of species by flood flows, generates high species diversity.…”

“…Lifespan (life history) is one of the most useful characteristics for characterizing community response to disturbance (McIntyre et al 1995), and can be used in riparian areas (Pettit et al 2001). Perennial plants are intolerant of fluctuating flow and are favored in more stable hydrologic envi- SWL surface water level, FR flow rate, FP surface flow permanence, Alt streambed altitude, Dist distance from river to plot, TN total nitrogen content, TC total carbon content, CN carbon-to-nitrogen ratio, Moist soil moisture, pH pH value of soil sample, SOM soil organic matter, TP total phosphorus content, Sand amount of sand in soil *SWL and FR were not measured in RDA analysis but used to categorize streamflow type ronments, which provide consistently saturated soils Katz et al 2012). In contrast, in strongly disturbed environments, annual species are favored because of their fast growth rates and early and prolific seed set (Grime 1979).…”

“…A number of eco-hydrological studies have identified relationships between riparian vegetation and hydrologic parameters, including frequency, timing, duration, rate of change, and magnitude of base flows (Nilsson and Svedmark 2002;Elmore et al 2003). Vegetation can be characterized in many ways, and studies have examined effects of surface water flow regimes on plant species diversity (Katz et al 2009;Stromberg et al 2009a;Katz et al 2012), species composition (Shafroth and Stromberg 2002), plant moisture group Katz et al 2009), and plant growth (Stromberg and Patten 1990). Flow regime adaptations of riparian and aquatic plant communities are dictated by the pattern of temporal river flow variation, which involves plant life histories and morphologies (Lytle and Poff 2004) and compositional differences between streamflow types Leigh et al 2010).…”

“…Flow regime adaptations of riparian and aquatic plant communities are dictated by the pattern of temporal river flow variation, which involves plant life histories and morphologies (Lytle and Poff 2004) and compositional differences between streamflow types Leigh et al 2010). Relationships between species diversity and hydrologic streamflow regime change have been described for a few semiarid rivers (Stromberg et al 2009a;Katz et al 2012), but relationships of riparian plant community metrics to those regimes have garnered less research attention.…”