Research Summaries

Gaskin, J. F. and D. J. Kazmer.
2006. Comparison of ornamental and wild saltcedar (Tamarix spp.) along eastern Montana, USA, riverways using chloroplast and nuclear DNA sequence markers. Wetlands 26: 339-350.

Saltcedars (Tamarix ramosissimaT. chinensis, and their hybrids) have invaded riverways and lakeshores across the western USA and northern Mexico. In Montana, ornamental plantings of saltcedar have been hypothesized, to varying degrees, to be the origin of nearby wild populations. To examine this hypothesis, we compared chloroplast and nuclear DNA sequences from 36 ornamental and 182 wild saltcedars from Montana, North Dakota, and Wyoming, USA. We found that ornamental and wild population genotype frequencies were highly dissimilar. Also, genotype frequencies of hypothetical propagule populations under scenarios of random mating, self-fertilization, and clonal reproduction in the ornamental population were highly dissimilar to the genotype frequencies of the wild populations. Assignment tests indicated that the majority of wild genotypes originated from other wild plants, not from ornamental plants. However, ornamental plants could not be excluded as contributors to wild populations because all chloroplast and nuclear haplotypes found in the ornamental plants were found at some frequency in the wild. These findings suggest that while ornamental saltcedars are not the sole source of wild saltcedar, they do have potential to contribute genetic material to an invasion or re-establish a population after existing wild saltcedars are removed.

Taylor, J. P., L. M. Smith, and D. A. Haukos.
2006. Evaluation of woody plant restoration in the middle Rio Grande ten years after. Wetlands 26:1151-1160.

Native riparian cottonwood (Populus deltoides) dominated habitats are declining throughout the southwestern United States and being replaced by the exotic salt cedar (Tamarix ramosissimaT. chinensis) dominated systems. Restoration efforts have often involved removing salt cedar and simulating or restoring overbank flooding timed with seed rain of cottonwood and willow (Salix spp.). Success is then often assessed by examining seedling densities of the native species and their known competitive superiority over salt cedar at this stage. We measured plots in the Middle Rio Grande Valley, New Mexico, USA, that had been cleared of salt cedar a decade earlier, compared the initial seedling community to the established community, and determined variables important in predicting establishment of cottonwood. During the decade since establishment, the proportion of cottonwood and seep willow (Baccharis glutinosa) increased, while salt cedar and coyote willow (S. exigua) decreased. Proportion of community foliar volume changed little for salt cedar (17.0 to 17.6%), while it increased for cottonwood (25.5 to 63.3%). Cottonwood dominated (>70%) the overstory. Initially established woody stem density (all species) was the only predictor of the percent of cottonwood canopy cover a decade later as declining coyote willow allowed for greater cottonwood growth. No abiotic variables were important in predictions of subsequent cottonwood community establishment, probably because these variables were similar among sites in this active floodplain. High initial cottonwood and salt cedar seedling densities were related to lower cottonwood foliar volume and smaller individual tree diameters a decade later. Low initial densities of cottonwood allowed individuals to grow rapidly to a greater height, dominating the canopy. Restoration biologists should therefore consider areas that have low initial cottonwood seedling densities as potentially very successful if their goal is to produce a stand of large cottonwoods in a relatively short time.

Bhattacharjee, J., J. P. Taylor Jr., L. M. Smith.
2006. Controlled flooding and staged drawdown for restoration of native cottonwoods in the Middle Rio Grande Valley, New Mexico, USA. Wetlands 26:691-702.

Alteration of natural hydrologic regimes of most rivers in the southwestern United States has led to degradation of riparian habitats. Most areas historically covered by Rio Grande cottonwood [Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.] have been replaced by exotic saltcedar (Tamarix chinensis Lour.). Following an earlier study in the Middle Rio Grande Valley, New Mexico, that evaluated faster staged water drawdowns to restore riparian habitat, we evaluated slower rates, 2 cm/day and 5 cm/day (starting depth = 30 cm) to determine if cottonwood seedling density could be increased. During the period of spring flood recession of the Rio Grande, we placed seed-bearing branches of cottonwood in experimental basins and applied the drawdown treatments. Following the end of drawdowns, we conducted the first vegetation sampling to determine cottonwood and saltcedar seedling densities in the area. We also conducted a mid-season, an end-season, and an over-winter vegetation sampling to observe changes in seedling densities over time. Saltcedar and cottonwood seedling densities did not differ statistically between the drawdown treatments. However, survival of cottonwood seedlings during the first growing season in the 2 cm/day drawdown was greater than in the 5 cm/day drawdown. Greater seedling survival in the slower drawdown was likely due to increased soil moisture levels in that treatment, corresponding to a more gradual descending limb of the historical hydrograph. Use of a slow water drawdown (2 cm/day; about 20 days duration) synchronized with natural seed rain of cottonwoods will result in high first season densities of this important riparian species.

Loo, S. F., R. Mac Nally, and P. S. Lake.
2007. Forecasting New Zealand mud snail invasion range: model comparisons using native and invaded ranges. Ecological Applications 17:181-189.

Evaluations of the potential distribution of invasive species can increase the efficiency of their management by focusing prevention measures. Generally, ecological models are built using occurrence data from a species’ native range to predict the distribution in areas that the species may invade. However, historical and geographical constraints can limit a species’ native distribution. Genetic Algorithm for Rule-set Production (GARP), an ecological niche modeling program, was used to predict the potential distribution of the invasive, freshwater New Zealand mudsnail, Potamopyrgus antipodarum, in Australia and North America. We compared the strength of the predictions made by models built with data from the snail’s native range in New Zealand to models built with data from the locations invaded by the species. A time-series analysis of the Australian models demonstrated that range-of-invasion data can make better predictions about the potential distribution of invasive species than models built with native range data. Large differences among the model forecasts indicate that uncritical choice of the data set used in training the GARP models can result in misleading predictions. The models predict a large expansion in the range of P. antipodarum in both Australia and North America unless prevention measures are implemented rapidly.