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Invasion of Siberian Elm (Ulmus pumila) Along the South Platte River: the Roles of Seed Source, Human Influence, and River Geomorphology

  • Riparian wetlands and floodplains
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Abstract

Riparian ecosystems in the western USA have been invaded by non-native woody species deliberately introduced for stream bank stabilization, agricultural windbreaks, and urban shade. Recent work suggests that the non-native tree Ulmus pumila (Siberian elm) is capable of significant spread in western riparian ecosystems, that range infilling is still incomplete, and that the invasion is dispersal-limited. Our objective was to understand the interacting roles of propagule pressure from upland U. pumila, human influences, and river geomorphology in promoting riparian U. pumila invasion along the South Platte River, Colorado, USA. We used linear regression and information-theoretic model selection to evaluate the relative importance of these factors to riparian U. pumila stem density. U. pumila stem density increased with increasing channel and floodplain restriction and increasing human influence from both urban and rural development. Model selection indicated that local upland U. pumila seed sources were relatively unimportant to riparian U. pumila stem density, suggesting that upland propagule pressure is currently contributing less than other human influences to U. pumila spread along the South Platte River. In particular, higher road density was the most important predictor for the proportional abundance of smaller U. pumila individuals (DBH<5-cm and 5-15-cm), suggesting that human influence in densely populated areas has been the primary driver of recent U. pumila population expansion. U. pumila stem density was only weakly associated with abundance of other common riparian tree species. Land managers and other entities concerned with non-native tree invasion into important riparian habitat may be able to reduce U. pumila spread most effectively by focusing U. pumila control efforts where human influences are greatest.

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Data Availability

The datasets used and/or analyzed in this study are available as a USGS data release https://www.sciencebase.gov/catalog/item/614b937ad34e0df5fb97c6b7 (Reynolds et al. 2022).

Code Availability

The R code used to analyze data during the current study are available from the corresponding author upon request.

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Acknowledgements

The authors would like to thank Colorado State Wildlife area managers for assistance with access to several state wildlife areas along the South Platte River and information on land-use history. Many thanks to Kelsey Dean, Lindsey Elgen, Nicholas Kainrath, Mary-Carolyn Weaver, Lindsey Young, and Aaron Schoelkopf for excellent field data collection and logistical support. Thanks to Anna Sher, Nicole Cappuccio, and the anonymous reviewers who provided helpful comments on the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Funding

Funding for this research was provided by the USGS Invasive Species Program (LR, LP, and PS) and from the Colorado Water Conservation Board (AN and GK).

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Contributions

LR, LP, PB, GK, and AN conceived of the study, hypotheses and acquired the funding to conduct the study in the field. LR and GK coordinated data collection in the field. LR and LP analyzed the data and summarized results in figures and tables. LR and LP wrote the initial draft of the full manuscript. All authors participated in revisions of early drafts, and then read, edited, and approved the final manuscript.

Corresponding author

Correspondence to Lindsay V. Reynolds.

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This article belongs to the Topical Collection: Riparian Wetlands and Floodplains.

Appendices

Appendix 1 Transect Dimensions (length and Width) for Sampling Ulmus Pumila and Other Woody Riparian Species Stems Along the South Platte River, Colorado

Table 4 Transect dimensions (length and width) for sampling Ulmus pumila and other woody riparian species stems along the South Platte River, Colorado

Appendix 2 Predictor Variable Correlation Matrix

Fig. 6
figure 6

Correlations between all predictor variables from top left to bottom right: Average active channel width (AC_Ave_width_m), average floodplain width (Fldpln_Ave_width_m), active channel-to-floodplain width (ac_fldpln), cumulative upstream drainage area (CUMDRAINAGkm2), mean annual flow (MAFLOWU_cms), number of river kilometers downstream from south Denver (RiverKMs), density of upland elms per hectare (NonRipTotal_dens_ha), density of farmsteads per hectare (Farmstd_dens_ha), log of population density (LogPop), number of bridges within 2 river-km (Total.bridges), density of roads within 1 km (Rds_m_ha). Red dots and asterisks indicate strength of correlations. Full descriptions of all predictor variables are in Table 1 and the Methods

Appendix 3 Full Model Selection and Regression Model Results Using Selected Geomorphic/watershed, Upland U. Pumila Population, and Human Influence Predictor Variables

Table 5 Full model selection results using information criterion for all unique combinations of four predictor variables: active channel-to-floodplain width ratio (ac:f width), farmstead density (farms), road density (roads), and upland/non-riparian elm density (upland elms)
Table 6 Results of regression models for total U. pumila stem densities, and proportional abundance of saplings (DBH>5-cm), medium (DBH 5-15-cm), and large (DBH>15-cm) U. pumila, testing whether total densities or proportions are related to predictor variables including geomorphic (channel-to-floodplain width ratio), propagule pressure (upland U. pumila density), or human influence (farmstead density and road density). Bolded text indicate significant (P <0.05) predictor variables

Appendix 4 Regression Model Results for U. pumila Total Stem Density (stems/ha) and Stem Densities of Other Common Riparian Woody Species

Table 7 Results of regression models testing whether total U. pumila stem density, and proportions of size classes (sapling (DBH<5-cm and height>1-m), medium (DBH 5-15-cm), and large (DBH>15-cm)), were related to stem densities of other common riparian plant species
Fig. 7
figure 7

Relationships between log-transformed U. pumila total stem density (stems/ha) and log-transformed stem densities of other common riparian woody species: P. deltoides (top left), S. amygdaloides (top right), F. pennsylvanica (bottom left), and E. angustifolia (bottom right). Fitted univariate regression models are shown (blue line) for models with P<0.05 and include 95% confidence intervals shown in shaded gray around the regression line, and R2 values in a low corner of each panel (see Table 5 for full regression model statistics)

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Reynolds, L.V., Perry, L.G., Shafroth, P.B. et al. Invasion of Siberian Elm (Ulmus pumila) Along the South Platte River: the Roles of Seed Source, Human Influence, and River Geomorphology. Wetlands 42, 10 (2022). https://doi.org/10.1007/s13157-021-01516-4

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