Response of riparian vegetation to short- and long-term hydrologic variation.

Increasing demand for river water now conflicts with an increasing desire to maintain riparian ecosystems. Efficiently managing river flows for riparian vegetation requires an understanding of the time scale of flow effects, but this information is limited by the absence of long-term studies of vegetation change in response to flow variation. To investigate the influence of short- and long-term flow variability and dam operation on riparian vegetation, we determined the occurrence of 107 plant species in 133 permanent plots of known inundating discharge along the Gunnison River in Colorado on five different occasions between 1990 and 2013. Individual species moved up and down the gradient of inundating discharge coincident with increases and decreases in mean annual flow, and the correlations between flow and species occurrence were strongest when flows were weighted by time before vegetation sampling with a median half-life of 1.5 years. Some tall, rhizomatous, perennial species, however, responded to flows on a longer time scale. Logistic regression of species occurrence showed a significant relation with inundation duration for 70 out of 107 species. Plot species richness and total vegetative cover decreased in association with desiccation at low inundation durations and with fluvial disturbance at high inundation durations. Within-plot similarity in species occurrence between years decreased strongly with increasing inundation duration. Moderate inundation durations were dominated by tall, rhizomatous, perennial herbs, including invasive Phalaris arundinacea (reed canary grass). Over the 23-year study period, species richness declined, and the proportion of rhizomatous perennials increased, consistent with the hypothesis that decreases in flow peaks and increases in low flows caused by flow regulation have decreased establishment opportunities for disturbance-dependent species. In summary, annual-scale changes in vegetation were strongly influenced by flow variation, and decadal-scale changes were influenced by decreases in fluvial disturbance from upstream flow regulation beginning decades prior to the onset of this study.

Designing flows to enhance ecosystem functioning in heavily altered rivers.

More than a century of dam construction and water development in the western United States has led to extensive ecological alteration of rivers. Growing interest in improving river function is compelling practitioners to consider ecological restoration when managing dams and water extraction. We developed an Ecological Response Model (ERM) for the Cache la Poudre River, northern Colorado, USA, to illuminate effects of current and possible future water management and climate change. We used empirical data and modeled interactions among multiple ecosystem components to capture system-wide insights not possible with the unintegrated models commonly used in environmental assessments. The ERM results showed additional flow regime modification would further alter the structure and function of Poudre River aquatic and riparian ecosystems due to multiple and interacting stressors. Model predictions illustrated that specific peak flow magnitudes in spring and early summer are critical for substrate mobilization, dynamic channel morphology, and overbank flows, with strong subsequent effects on instream and riparian biota that varied seasonally and spatially, allowing exploration of nuanced management scenarios. Instream biological indicators benefitted from higher and more stable base flows and high peak flows, but stable base flows with low peak flows were only half as effective to increase indicators. Improving base flows while reducing peak flows, as currently proposed for the Cache la Poudre River, would further reduce ecosystem function. Modeling showed that even presently depleted annual flow volumes can achieve substantially different ecological outcomes in designed flow scenarios, while still supporting social demands. Model predictions demonstrated that implementing designed flows in a natural pattern, with attention to base and peak flows, may be needed to preserve or improve ecosystem function of the Poudre River. Improved regulatory policies would include preservation of ecosystem-level, flow-related processes and adaptive management when water development projects are considered.

Early vegetation development on an exposed reservoir: implications for dam removal.

The 4-year drawdown of Horsetooth Reservoir, Colorado, for dam maintenance, provides a case study analog of vegetation response on sediment that might be exposed from removal of a tall dam. Early vegetation recovery on the exposed reservoir bottom was a combination of (1) vegetation colonization on bare, moist substrates typical of riparian zones and reservoir sediment of shallow dams and (2) a shift in moisture status from mesic to the xeric conditions associated with the pre-impoundment upland position of most of the drawdown zone. Plant communities changed rapidly during the first four years of exposure, but were still substantially different from the background upland plant community. Predictions from the recruitment box model about the locations of Populus deltoides subsp. monilifera (plains cottonwood) seedlings relative to the water surface were qualitatively confirmed with respect to optimum locations. However, the extreme vertical range of water surface elevations produced cottonwood seed regeneration well outside the predicted limits of drawdown rate and height above late summer stage. The establishment and survival of cottonwood at high elevations and the differences between the upland plant community and the community that had developed after four years of exposure suggest that vegetation recovery following tall dam removal will follow a trajectory very different from a simple reversal of the response to dam construction, involving not only long time scales of establishment and growth of upland vegetation, but also possibly decades of persistence of legacy vegetation established during the reservoir to upland transition.

Germination and Establishment of the Native Plains Cottonwood (Populus deltoides Marshall subsp. monilifera) and the Exotic Russian-olive (Elaeagnus angustifolia L.).

Russian-olive (Elaeagnus angustifolia) is a small Eurasian tree that has escaped from cultivation and become naturalized, primarily along watercourses throughout the western United States. We examined germination and establishment of Russian-olive and plains cottonwood (Populus deltoides), the principal native riparian tree of the Great Plains, under a range of experimental moisture and light conditions. The fewest seedings established under the driest conditions; seedling biomass was predictably lower in the shade; root-to-shoot ratios were higher for cottonwood, higher in the sun, and higher under drier conditions. Several interactions were also significant. The timing of germination and mortality varied between plains cottonwood and Russian-olive: cottonwood germinated in mid-June in all treatments in a single pulse with subsequent mortality; the timing and amount of Russian-olive germination differed substantially across treatments with little net mortality. Differences in life-history traits of these species, including seed size, viability, and dispersal, help explain treatment differences. Russian-olive will likely remain an important component of riparian communities along both unregulated and regulated western rivers because it succeeds under conditions optimal for cottonwood establishment and under many conditions unfavorable for cottonwood. Furthermore, many western states still encourage planting of Russian-olive, and control techniques tend to be labor-intensive and expensive. Germinación y establecimiento del álamo nativo de las planicies (Populus deltoides Marshall subsp. monolifera) y el olivo ruso exótico (Elaeagnus angustifolia L.).

Resumen: El olivo ruso (Elaeagnus angustifolia) es un árbol pequeño nativo de Europa y Asia que ha escapado de ser cultivado y se ha naturalizado principalmente a lo largo de cursos de agua en el oeste de los Estados Unidos. Examinamos la germinación y el establecimiento del olivo ruso y del álamo del llano (Populus deltoides), el principal árbol nativo rivereño de las Grandes Planicies, bajo un amplio espectro de condiciones experimentales de humedad y luz. La menor cantidad de retoños se estableció bajo las condiciones más secas; la biomasa de los retoños fue predeciblemente menor en la sombra; la proporcián raiz; vástago fue más alta para el álamo, mayor bajo el sol y más alta bajo condiciones más secas. También fueron significativas varias interacciones. El tiempo de germinación y la mortalidad para el álamo del llano y el olivo ruso fue diferente. El álamo del llano germinó durante un único pulso a mediados de Junio en todos los tratamientos con la mortalidad subsiguiente, mientras que los tiempos y la cantidad de germinación del olivo ruso difirieron substancialmente entre los distintos tratamientos con muy poca mortalidad neta. Diferencias en los caracteres de la historia de vida de estas especies, incluyendo el tamaño de la semilla, la viabilidad y la dispersión ayuda a explicar las diferencias entre tratamientos. El olivo ruso permanecerá probablemente como un componente importante de las comunidades ribereñas a lo largo de ríos regulados y no-regulados del oeste. Esto es posible porque el olivo ruso se desempeña exitosamente bajo condiciones que son óptimas para el establecimiento del álamo del llano y bajo condiciones diversas desfavorables para álamo. Más el aún, varios estados del oeste todavía fomentan la plantación del olivo ruso y las ténicas de control requieren una labor intensiva y costosa.