Research Summaries

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Editor’s Note: Research summaries are generously provided by Dr. Alan T. Carpenter. Alan is a Senior Ecologist with Biohabits, Inc. of Boulder, Colorado and may be reached by email at atcarpen@infionline.net.

Ladenberger, C. G., A. L. Hild, D. J. Kazmer, and L. C. Munn.
2006. Soil salinity patterns in Tamarix invasions in the Bighorn Basin, Wyoming, USA. Journal of Arid Environments 65:111-128.

Saltcedar (Tamarix spp.) is an exotic, invasive shrub of riparian corridors in the western United States that can promote soil salinization via leaf exudates as Tamarix litter accumulates on the soil surface. Tamarix stands occur in association with big sagebrush (Artemisia tridentata), greasewood (Sarcobatus vermiculatus), and cottonwood (Populus deltoides) in northern Wyoming, depending on topographic position. Revegetation of Tamarix invaded sites can be limited by altered soil conditions. Tamarix stands in northcentral Wyoming were selected to determine the relationship of Tamarix shrubs and associated vegetation to soil salinity, pH, and nutrients. In general, salinity of surface soils (0-5 cm) was greater and pH was lower than in deeper soils. Surface soils (0-5 cm) beneath Tamarix have greater salinity and lower pH than soil in interspaces. Because soil salinity in the Bighorn Basin is lower than levels documented in most Tamarix stands of the southwestern United States, many species used for revegetation should tolerate soil conditions here following Tamarix control.

Friedman, J. M., G. T. Auble, E. D. Andrews, G. Kittel, R. F. Madole, E. R. Griffin, and T. M. Allred.
2006. Transverse and longitudinal variation in woody riparian vegetation along a montane river. Western North American Naturalist 66:78-91.

This study explores how the relationship between flow and riparian vegetation varies along a montane river. Occurrences of woody riparian plant communities were mapped along 58 km of the San Miguel River in southwestern Colorado. The recurrence of inundation was determined for each plant community by combining step-backwater hydraulic modeling at 4 representative reaches with Log-Pearson analysis of stream gauging stations. Finally, bottomland surficial geology was mapped and a Geographic Information System was used to overlay the coverages of geology and vegetation. Plant communities were distinctively arrayed along the hydrologic gradient. The Salix exigua Nuttall (sandbar willow) community occurred mostly on surfaces with a recurrence interval of inundation shorter than 2.2 years; the Betula occidentalisHooker (river birch) community peaked on sites with recurrence intervals of inundation between 2.2 and 4.6 years. The hydrologic position occupied by communities dominated by Populus angustifolia James (narrowleaf cottonwood) was strongly related to age of trees and species composition of understory shrubs. The fraction of riparian vegetation on surfaces historically inundated by the river decreased in the upstream direction from almost 100% near Uravan to < 50% along the South Fork of the San Miguel River. In upstream reaches much of the physical disturbance necessary to maintain riparian vegetation is provided by valley-side processes including debris flows, floods from minor tributaries, landslides, and beaver activity. Where valley-side processes are important, prediction of riparian vegetation change based on alteration of river flow will be incomplete.

Sexton, J. P., A. Sala, and K. Murray.
2006. Occurrence, persistence, and expansion of saltcedar (Tamarix spp.) populations in the Great Plains of Montana. Western North American Naturalist 66:1-11.

Saltcedar (Tamarix spp.), a shrub native to Eurasia, is associated with major alterations to wetland and riparian systems in the southwestern US. Since the 1960s, saltcedar has naturalized in the northern states in the US where its growth potential and impacts are not well known. Here, the occurrence, age, size, and relative cover of saltcedar populations in several river basins in central eastern Montana are described to identify potential patterns of spread across the region and changes in individual populations as they age. Stands were aged according to the oldest saltcecar individuals and were sampled for dominant plant cover and soil properties. Multiple introductions appear to have occurred in Montana, with the oldest stands occurring on the Bighorn River in southern Montana. Saltcedar absolute and relative cover and stand area increased significantly with stand age, while native tree and shrub relative cover remained low across all stand ages, These results suggest that saltcedar stands establish where woody natives are not abundant and that they persist and expand over time. Although soil salinity remained constant, soil pH decreased with saltcedar stand age, indicating a possible effect on organic matter inputs. An analysis of annual wood increment of saltcedar and sandbar willow (a native with analogous growth form) stems along a latitudinal gradient showed that stem growth of both species did not differ significantly along regions. Stem growth decreased inversely with elevation for both species while growth responses to elevation did not differ between species. Study results show an increase in number of populations and continued viability of these populations. Mechanisms of saltcedar increases in this region are yet to be determined. Anthropogenic influences, such as saltcedar plantings, watershed alterations (e.g., river flow control), and habitat disturbances (e.g., cattle grazing or habitat clearing) may facilitate its spread in similar climates of the Great Plains.

Auble, G. T. M. L. Scott, and J. M. Friedman.
2005. Use of individualistic streamflow-vegetation relationships along the Fremont River, Utah, USA to assess impacts of flow alteration on wetland and riparian areas. Wetlands 25:143-154.

The transverse pattern of vegetation along a reach of the Fremont River in Capitol Reef National Park, Utah, USA was analyzed using models that support both delineation of wetland extent and projection of the changes in wetland area resulting from upstream hydrologic alteration. Stage-discharge relations developed by a hydraulic model were linked to a flow-duration curve derived from the flow history in order to calculate the inundation duration of 361 plots (0.5 x 2 m). Logistic regression was used to relate plant species occurrence in plots to inundation duration. A weighted average of the wetland indicator values of species was used to characterize plots as Aquatic, Wetland, Transitional, or Upland. Finally, how alterations in the flow duration curve would change the relative widths of these four zones was assessed. The wetland indicator values of species and the wetland prevalence index scores of plots were strongly correlated with inundation duration. The results support the concept that plants classified as wetland species typically occur on sites inundated at least two weeks every two years. The portion of the riparian zone along the high-gradient study reach of the Fremont River that satisfied the vegetation criterion for a regulatory wetland was narrow (2 m wide). Both the unvegetated Aquatic zone (7.8 m) and the Transitional zone (8 m) were substantially wider. The Transitional zone included the maxima of several species and was, therefore, not merely a combination of elements of the Wetland and Upland zones. Multiplicative increases or decreases in streamflow regime produced a wetter, or drier, bottomland vegetation, respectively. Systematic reductions in flow variability reduced the width of both the Wetland and Transitional zones and increased the width of the Upland zone. This approach is widely applicable to inform water management decisions involving changes in flow regime.

Birken, A. S. and D. J. Cooper.
2006. Process of Tamarix invasion and floodplain development along the lower Green River, Utah. Ecological Applications 16:1103-1120.

Significant ecological, hydrologic, and geomorphic changes have occurred during the 20th century along many large floodplain rivers in the American Southwest. NativePopulus forests have declined, while the exotic Eurasian shrub, Tamarix, has proliferated and now dominates most floodplain ecosystems. Photographs from the late 19th and early 20th centuries illustrate wide river channels with largely bare in-channel landforms and shrubby higher channel margin floodplains. However, by the mid-20th century, floodplains supporting dense Tamarix stands had expanded, and river channels had narrowed. Along the lower Green River in eastern Utah, the causal mechanism of channel and floodplain changes remains ambiguous due to the confounding effects of climatically driven reductions in flood magnitude, river regulation by Flaming Gorge Dam, and Tamarix invasion. This study addressed whether Tamarix establishment and spread followed climate- or dam-induced reductions in annual peak flows or whether Tamarix was potentially a driver of floodplain changes. 235 Tamarix and 57 Populus individuals were aged, the hydrologic and geomorphic processes that controlled recruitment were determined, the spatial relationships of germination sites within floodplain stratigraphic transects were identified, and woody riparian vegetation cohorts along three segments of the lower Green River were mapped. The oldest Tamarix established along several sampling reaches in 1938, and 1.50-2.25 m of alluvium has accreted above their germination surfaces. Nearly 90% of the Tamarix and Populus samples established during flood years that exceeded the 2.5-year recurrence interval. Recruitment was most common when large floods were followed by years with smaller peak flows. The majority of Tamarix establishment and Green River channel narrowing occurred long before river regulation by Flaming Gorge Dam. Tamarix initially colonized bare instream sand deposits (e.g., islands and bars), and most channel and floodplain changes followed the establishment of Tamarix. Results suggest that Tamarix recruitment was triggered by large annual floods that were followed by years with lower peak flows, not by periods of low flow alone. Tamarix appears to have actively invaded floodplains, while Populus colonization has been limited. Thus, Tamarix invasion may have greatly influenced floodplain development and riparian vegetation composition along the lower Green River since the early 20th century.