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The Newsletter of the Colorado Riparian Association

Volume 13, Number 3, Fall 2002

Research Summaries

Friedman, J. M., and V. J. Lee. 2002.

Extreme floods, channel change, and riparian forests along ephemeral streams. Ecological Monographs 72:409-425.

The geomorphic effectiveness of extreme floods increases with aridity and decreasing watershed size. Therefore in small dry watersheds extreme floods should control the age structure and spatial distribution of populations of disturbance dependent riparian trees. We examined the influence of extreme floods on the bottomland morphology and forest of ephemeral streams in a semi-arid region. Along six stream reaches on the Colorado Piedmont, we examined channel changes by analyzing a rectified sequence of aerial photographs spanning 56 years, and we investigated the spatial distribution of different aged patches of forest by aging 189 randomly sampled cottonwood trees. Channel change in these ephemeral sandbed streams is dominated by widening which occurs over a span of hours during infrequent floods and post-flood narrowing which occurs over decade between floods. Narrowing is accelerated where reliable moisture increases the density and growth rate of vegetation on the former bed. Reproduction of cottonwood trees has occurred mostly in former channel beds during periods of channel narrowing beginning after floods in 1935 and 1965 and continuing for as long as two decades. Thus cottonwood establishment is related to low flows at the time scale of a year but also to high flows at the time scale of decades. At sites that have not experienced major floods in the past 80 years, little channel change has occurred, cottonwood reproduction has been limited, tree density has declined and succession to grassland is occurring. Because channel change and tree reproduction in this region are driven by infrequent local events, channel width and tree age distributions vary greatly over time and among sites. For the same reason, riparian forests along these ephemeral streams can be as wide as forests along perennial streams with much higher mean discharge.

Chambers, J. C., and A. R. Linnerooth. 2001.

Restoring riparian meadows currently dominated by Artemisia using alternative steady state concepts - the establishment component. Applied Vegetation Science 4:157-166.

We evaluated the potential for restoring riparian grass and sedge meadows currently dominated by Artemisia tridentata var. tridentata with an experiment in which we burned sites with low, intermediate, and high water tables, i.e., dry, intermediate, and wetter sites.

To define the alternative states and thresholds for these ecosystems, we examined burning and water table effects on both abiotic variables and establishment of grasses adapted to relatively high (Poa secunda ssp juncifolia) intermediate (Leymus triticoides), or low (L. cinereus) water tables. Wet sites had lower soil temperatures and higher soil water content than dry sites. Burning increased soil temperatures on all sites. Under shrub microsites on control plots had the lowest temperatures while former undershrub microsites on burned plots had the highest temperatures. Surface soil water was low on burned plots early in the growing season due to dessication, but higher at deeper depths after active plant growth began. Emergence was generally greater on wet sites but survival was microsite and species specific. Undershrub microsites on control plots facilitated emergence in first year survival, but seedlings that survived initially harsh conditions on burned plots had similar numbers alive at the end. In general, favorable environments and establishment of species adapted to mesic conditions indicate that wet sites represent an alternative state of the naturally occurring dry meadow ecosystem type and can be restored to grass and sedge meadows. Harsh environments and lack of establishment of species adapted to mesic conditions indicate that dry sites have crossed a threshold and may represent a new ecosystem type. Understory vegetation in seed banks on dry sites have been depleted and restoration will require burning and reseeding with species adapted to more xeric conditions.

Munk, L. M ., M., A. L. Hild and T. D. Whitson. 2002.

Rosette recruitment of a rare endemic forb (Gaura neomexicana ssp. coloradensis) with canopy removal of associated species. Restoration Ecology 110:122-128.

Gaura neomexicana ssp. coloradensis (Colorado butterfly plant), a short-lived perennial forb, became listed as a threatened species by the US Fish & Wildlife Service in October 2000 because of its small global range and population size. This endemic subspecies consists of only 18 extant populations within southeastern Wyoming, southwestern Nebraska, and northeastern Colorado. Wyoming occurrences are on private land with the exception of three occurrences on Warren Air Force Base, near Cheyenne. Gaura neomexicana ssp. coloradensis may be displaced by many competitors, including Canada thistle, an invasive noxious weed. In June 1998, three Gaura neomexicana ssp. coloradensis populations were examined for rosette establishment in the presence of and after the removal of associated species. The four study treatments removed 1) Canada thistle, 2) grass and forb canopy and associated litter, 3) grass and forb canopy, litter and Canada thistle, and 4) no removal of associated plant species (control). Mature Gaura neomexicana ssp. coloradensis plants were evaluated for height, number of leaves, and capsule production. The mature plant characteristics monitored in 1998 were not good indicators of subsequent Gaura neomexicana ssp. coloradensis rosette densities in the following spring. Recruitment was assessed by counting new rosettes in the fall of 1999. Although both Gaura neomexicana ssp. coloradensis and the exotic Canada thistle have comparable habitat, removal of Canada thistle did not increase Gaura neomexicana ssp. coloradensis vegetative growth, seed capsule production or rosette density. Removal of other forbs, grass, and litter, however, increased Gaura neomexicana ssp. coloradensis rosette density in the following two years, indicating that canopy removal of associated species can enhance rosette establishment of this rare native. The accumulation of dense vegetative cover and litter associated with the absence of herbivory and fire may contribute to the decline of rare species in rich riparian habitats. Return of herbivory and fire in mesic sites to reduce standing biomass accumulations should be considered in restoring recruitment potential to rare monocarpic species.

McEldowney, R. R., M. Flenniken, G. W. Fraser, M. J. Trlica, and W. C. Leininger. 2002.

Sediment movement and filtration in a riparian meadow following cattle use. Journal of Range Management 55:367-373.

Improper livestock grazing practices in the western US riparian areas may reduce the nutrient and pollutant removal function of riparian communities, resulting in degradation of surface water quality. Short duration, high-intensity cattle use in 3 x 10 meter plots was evaluated in a montane riparian meadow in northern Colorado to quantify livestock effects on sediment movement and filtration under simulated rainfall (=100 mm per hour) plus overland flow (=25 mm per hour) conditions. Four treatments: 1) control, 2) mowed to 10 cm stubble height, 3) trampled by cattle, and 4) cattle grazed plus trampled were evaluated. Sixty kilograms of sediment were introduced to overland flow in each plot. Sediment movement was evaluated using sediment traps positioned in microchannels and on vegetation islands at 5 distances downslope from the upper end of the plots and by sediment front advancement. Most of the sediment deposition occurred within the first meter downslope of application. About 90% of the applied sediment was filtered from runoff within 10 meters in the control and mowed treatments, while about 84% and 77% of the applied sediment was trapped in the trampled and grazed treatment plots, respectively.

The primary variables that influenced sediment filtration were stem density and surface roughness. Stem density was the most influential variable that affected sediment filtration. Cattle grazing reduced the stem density by 40%. Monitoring of stem density should aid managers in regulating cattle use of riparian communities and facilitate the protection of surface water quality from sediment in overland flow.

Kemp, M. J., and W. K. Dodds. 2002.

Comparisons of nitrification and denitrification in prairie and agriculturally influenced streams. Ecological Applications 12:998-1009.

Whole-stream nitrification and denitrification rates were calculated from field incubation studies of representative substrata in Kings Creek and Shane Creek watersheds at the Konza Prairie Biological Station, Kansas, USA. Substrata from a relatively pristine upstream and an agriculturally influenced downstream reach were measured in both watersheds. Rates were scaled to the whole stream by the mass of each substratum in the stream reach. Substrata sampled included epilithon, leaf packs and wood (coarse benthic organic matter - CBOM), filamentous green algae, bryophytes, fine benthic organic matter (FBOM) and suspended particulate organic matter. Upstream sites had significantly lower nitrification and denitrification than downstream sites. Nitrification rates were dominated by nitrification associated with epilithon and filamentous green algae. Downstream sites had a higher proportion of nitrification occurring in the water column relative to upstream sites. Whole stream denitrification was dominated with denitrification associated with FBOM and CBOM and was higher downstream. Water column ammonium and nitrate concentrations were correlated with whole-stream nitrification rates, but not whole stream denitrification rates. Whole stream nitrification rates were positively correlated with whole stream denitrification rates, being coupled at the whole stream level but not at the substratum level. Relative to the amount of nitrate produced through nitrification, the proportion of nitrate denitrified was 1.4 - 4.6 times greater in the downstream sites compared to the upstream sites, but the proportion of the in-stream nitrate load denitrified was about 10 times greater in the upstream sites. These data indicate that changes in stream nitrogen cycling related to increased dissolved organic nitrogen concentrations do not result in a large enough increase in denitrification (the stream's capacity to retain nitrogen) to compensate for the increased nitrogen loading. Data also suggest that whole stream denitrification and nitrification may be de-coupled at the reach level by factors that decrease habitat heterogeneity (the variety of substrata types) in stream channels. This indicates that anthropogenic disturbances of small streams such as channelization and removal of riparian vegetation would be expected to decrease capacity of streams to retain nitrogen.

Merritt, D. M., and E. E. Wohl. 2002.

Processes governing hydrochory along rivers: hydraulics, hydrology, and dispersal phenology. Ecological Applications 12:1071-11087.

Rivers are important corridors for movement, migration, and dispersal of aquatic organisms as well as for dispersal of the seeds and vegetative propagules of riparian plants. In this investigation, the relationships between flow regime, channel morphology, dispersal phenology, and seed deposition patterns were evaluated using experimentation in a flume. A channel with geomorphic features common to a wide range of stream morphologies was constructed in a 1.8 x 20 meter experimental flume through which three hydrologic regimes (one natural and two typical of dam releases) were routed in replicated trials. Relationships between dispersal phenology and hydrologic regime were examined using color-coded Betula fontinalis seeds released over each 10-min trial. Spatial patterns of seed deposition along stream margins were then compared to determine the individual and combined effects of flow regime, fluvial feature, and timing of seed release. Reynolds number, Froude number, Weber number, flow velocity, and a dimensionless recirculation index were then used to develop predictive models of patterns of seed dispersal. Fluvial feature, flow regime, and timing of seed release all described proportions of the variability in concentrations of seeds deposited. Fewer seeds were deposited during the ascending flow releases than under descending and stepped regimes, indicating that ascending flows during seed release may have a significant negative effect on hydrochoric seed deposition. Ascending flows entrain and transport rather than deposit seeds. Retention and recirculation of seeds in fluvial environments with low recirculation indices extend the period over which plants with bouyant seeds are able to disperse, possibly to the advantage of such species. Systematic delivery of seeds to specific types of fluvial environments in repeated trials and in varied regimes suggests that hydrochory may serve as a form of directed dispersal of some riparian plant species. Froude number explained a large proportion of the variation in seed deposition, followed by flow velocity, Reynolds number, and Weber number. The recirculation index, which quantifies the strength of recirculation in the fluvial environment, accounted for the greatest proportion of the variability in seed dispersal patterns. Poisson regression models of seed deposition as a function of dimensionless hydraulic parameters provide an empirical basis for incorporating spatial patterns of seed dispersal into riparian vegetation models. In addition, through incorporating dispersal phenology and information regarding the regeneration niche of species of special concern, specific hydrograph characteristics may be managed below dams to accommodate or inhibit species using hydrochory.