Research Summaries

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Compiled by Alan Carpenter

Sher, A. A., S. Gieck, C. S. Brown, S. J. Nissen.
2008. First-year responses of cheatgrass following Tamarix spp. control and restoration-related disturbances. Restoration Ecology 16:129-135.

Current invasion ecology theory predicts that disturbance will stimulate invasion by exotic plant species. Cheatgrass or Downy brome (Bromus tectorum) was surveyed in three sites near Florence, Colorado, U.S.A., immediately following Tamarisk or Saltcedar (Tamarix spp.) control and restoration activities that caused disturbance. Despite predictions to the contrary, neither mowing with heavy machinery nor tilling for seedbed preparation stimulated invasion, with a trend for the opposite pattern such that highest percent cover of B. tectorum was observed in the least disturbed transects. Aerial application of imazapyr for Tamarix spp. control caused mortality of nearly all B. tectorum and other understory plant species in all sites. Mechanical control of Tamarix spp. will not necessarily result in increased abundance of invasive species already present, possibly due to the effects of mulch usually left on-site. Imazapyr will control B. tectorum and other herbaceous understory species when applied aerially for Tamarix spp. control. These results are encouraging for managers of riparian systems who may fear that control of woody invasives will stimulate herbaceous invasions.

Nagler, P. L., E. P. Glenn, K. Didan, J. Osterberg, F. Jordan, J. Cunningham.
2008. Wide-area estimates of stand structure and water use of Tamarix spp. on the Lower Colorado River: implications for restoration and water management projects. Restoration Ecology 16:136-145.

Tamarix spp. removal has been proposed to salvage water and allow native vegetation to recolonize western U.S. riparian corridors. We conducted wide-area studies on the Lower Colorado River to answer some of the scientific questions about Tamarix water use and the consequences of removal, combining ground surveys with remote sensing methods. Tamarix stands had moderate rates of evapotranspiration (ET), based on remote sensing estimates, averaging 1.1 m/yr, similar to rates determined for other locations on the river and other rivers. Leaf area index values were also moderate, and stands were relatively open, with areas of bare soil interspersed within stands. At three Tamarix sites in the Cibola National Wildlife Refuge, groundwater salinity at the site nearest to the river (200 m) was relatively low (circa 2,250 mg/L) and was within 3 m of the surface. However, 750 and 1,500 m from the river, the groundwater salinity was 5,000-10,000 mg/L due to removal of water by the Tamarix stands. Despite the high groundwater salinity, the sites away from the river did not have saline surface soils. Only 1% of the mean annual river flow is lost to Tamarix ET on the Lower Colorado River in the United States, and the opportunities for water salvage through Tamarix removal are constrained by its modest ET rates. A possible alternative to Tamarix removal is to intersperse native plants among the stands to improve the habitat value of the riparian zone.

Sogge, M. K., S. J. Sferra, and E. H. Paxton.
2008. Tamarix as habitat for birds: implications for riparian restoration in the southwestern United States. Restoration Ecology 16:146-154.

Exotic vegetation has become a major habitat component in many ecosystems around the world, sometimes dramatically changing the vegetation community structure and composition. In the southwestern United States, riparian ecosystems are undergoing major changes in part due to the establishment and spread of the exotic Tamarix(saltcedar, tamarisk). There are concerns about the suitability of Tamarix as habitat for birds. Although Tamarix habitats tend to support fewer species and individuals than native habitats, Arizona Breeding Bird Atlas data and Birds of North America accounts show that 49 species use Tamarix as breeding habitat. Importantly, the relative use of Tamarix and its quality as habitat vary substantially by geographic location and bird species. Few studies have examined how breeding in Tamarix actually affects bird survivorship and productivity; recent research on Southwestern Willow Flycatchers has found no negative effects from breeding in Tamarix habitats. Therefore, the ecological benefits and costs of Tamarix control are difficult to predict and are likely to be species specific and site specific. Given the likelihood that high-quality native riparian vegetation will not develop at all Tamarix control sites, restoration projects that remove Tamarix but do not assure replacement by high-quality native habitat have the potential to reduce the net riparian habitat value for some local or regional bird populations. Therefore, an assessment of potential negative impacts is important in deciding if exotic control should be conducted. In addition, measurable project objectives, appropriate control and restoration techniques, and robust monitoring are all critical to effective restoration planning and execution.

C. van Riper III, K. L. Paxton, C. O’Brien, P. B. Shafroth, L. J. McGrath.
2008. Rethinking avian response to Tamarix on the Lower Colorado River: a threshold hypothesis. Restoration Ecology 16:155-167.

Many of the world’s large river systems have been greatly altered in the past century due to river regulation, agriculture, and invasion of introduced Tamarix spp. (saltcedar, tamarisk). These riverine ecosystems are known to provide important habitat for avian communities, but information on responses of birds to differing levels ofTamarix is not known. Past research on birds along the Colorado River has shown that avian abundance in general is greater in native than in non-native habitat. In this article, we address habitat restoration on the lower Colorado River by comparing abundance and diversity of avian communities at a matrix of different amounts of native and non-native habitats at National Wildlife Refuges in Arizona. Two major patterns emerged from this study: (1) Not all bird species responded to Tamarix in a similar fashion, and for many bird species, abundance was highest at intermediate Tamarix levels (40–60%), suggesting a response threshold. In Tamarix-dominated habitats, the greatest increase in bird abundance occurred when small amounts of native vegetation were present as a component of that habitat. In fact, Tamarix was the best vegetation predictor of avian abundance when compared to vegetation density and canopy cover. Our results suggest that to positively benefit avian abundance and diversity, one cost-effective way to rehabilitate larger monoculture Tamarix stands would be to add relatively low levels of native vegetation (20-40%) within homogenousTamarix habitat. In addition, this could be much more cost effective and feasible than attempting to replace all Tamarix with native vegetation.

J. Bhattacharjee, J. P. Taylor, Jr., L. M. Smith, L. E. Spence.
2008. The importance of soil characteristics in determining survival of first-year cottonwood seedlings in altered riparian habitats. Restoration Ecology 16:563-571.

Most major rivers in the southwestern United States have been hydrologically altered to meet human needs. Altered hydrological regimes have been associated with declines in native riparian forests. Today, many riparian areas have little or no regeneration of native riparian species and are now dominated by exotic saltcedar (Tamarix chinensis Lour.). Success of riparian restoration efforts at least partially depends on the number of seedlings surviving the first growing season. Seedling survival is influenced by many abiotic and biotic factors including competition from other plants and available soil moisture, which is partially dependent on soil texture. In this study, we evaluated the relative importance of four soil categories (sandy loam, loam, silt, and clay), rate of soil moisture decline, salinity, beginning- and end-season saltcedar density, initial cottonwood (Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.) seedling density, percent vegetation cover by potential dominant competitors Pigweed (Amaranthus L.) and Barnyard grass (Echinochloa crusgalli L., Beauv.), and average total vegetation height to cottonwood seedling survival. Factors influencing seedling survival differed among the four soil types. Rate of moisture decline was important in sandy soils, whereas vegetation height influenced seedling survival in loamy soils. Overall, models of seedling survival in all the four soil types indicated rate of moisture decline as the single most important variable influencing cottonwood survival. High initial densities of saltcedar were correlated to higher survival in cottonwood seedlings. Therefore, it is important to identify soil texture and understand soil moisture decline rates when proposing riparian cottonwood restoration.

L. S. Craig, M. A. Palmer, D. C. Richardson, S. Filoso, E. S. Bernhardt, B. P. Bledsoe, M. W. Doyle, P. M Groffman, B. A. Hassett, S. S Kaushal, P. M. Mayer, S. M. Smith, and P. R. Wilcock.
2008. Stream restoration strategies for reducing river nitrogen loads. Frontiers in Ecology and the Environment 10:529-538.

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N-removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st-3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in-stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitude of N reduction possible, potential approaches should be tested in various landscape contexts; until more is known, stream restoration alone is not appropriate for compensatory mitigation and should be seen as complementary to land-based best management practices.

J. A. Boudell and J. C. Stromberg.
2008. Propagule banks: potential contributions to restoration of an impounded dewatered riparian ecosystem. Wetlands 28:

The Agua Fria River in Arizona’s Sonoran Desert was impounded and diverted more than 70 years ago. Immediately below New Waddell dam there are semi-permanent pools, but water has been released into the channel only during rare wet years. To determine whether a propagule bank exists below the dam, and whether it could contribute to the revegetation of the Agua Fria riparian ecosystem should flow be restored to the dewatered reach, we collected 45 soil cores from four plant associations. We examined species in the samples in a growth chamber using the seedling emergence method. A total of 74 species (mostly herbaceous) and an abundance of individuals were present in propagule banks. The propagule banks were similar to those of a free-flowing reference river despite considerable differences in extant vegetation. Riparian species were present in propagule banks of all four associations and were the dominant type in three (Tamarix forests, Tamarix-Salix forests, andBaccharis-Bebbia shrublands). Propagule distribution varied with soil depth in three of the associations (Tamarix forests and the two xerophytic shrublands) with riparian species more prevalent in deep sediment and upland species more prevalent in surface soil and litter. Collectively these patterns suggest that a riparian legacy is present in Agua Fria propagule banks. However, riparian propagule density was low in the Hymenoclea-Bebbia shrublands, reflecting xerification of the riparian corridor. Given the physical barrier of the dam, continued diversion of stream flow, and rare flood releases, local inputs from xerophytes will dominate propagule bank dynamics in the future. Although propagule banks could contribute to redevelopment of the herbaceous component of the vegetation should stream flows be restored to this river reach, the riparian legacy likely will decline over time as riparian propagules reach the end of their lifespan while propagules of xerophytes continue to be replenished.