Ecology of invasive Melilotus alba on Alaskan glacial river floodplains

Authors: CONN, JEFFERY; WERDIN PFISTERER, NANCY; BEATTIE, KATHERINE - ALASKA BIOLOGICAL RESEARC; DINSMORE, ROSEANN - U.S. GEOLOGICAL SURVEY (USGS)

Submitted to: Arctic, Antarctic, and Alpine Research (AAAR)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2011
Publication Date: 8/20/2011
Citation: Conn, J.S., Werdin Pfisterer, N.R., Beattie, K., Dinsmore, R.V. 2011. Ecology of invasive Melilotus alba on Alaskan glacial river floodplains. Arctic, Antarctic, and Alpine Research (AAAR). 43(3):343-354; doi: 10.1657/1938-4246-43.3.343.

Interpretive Summary: White sweetclover has invaded three previously pristine glacial floodplains in Alaska: the Stikine, Matanuska and Nenana Rivers. Because many more Alaska river floodplains could potentially be invaded, it is important to understand the effects of white sweetclover on these ecosystems. Vegetation was sampled along transects perpendicular to the rivers in July-August 2004. Percent ground cover and number of individuals were determined for each plant species occurring in each of six one-meter-square quadrats placed as belts every 5 M along each transect. At each belt environmental data was also collected: height above river, average age of woody vegetation (by counting tree rings), percent of soil surface covered by rock, and rock size. A soil sample was also collected at each belt for physical and chemical analysis. Nonmetric multidimensional scaling (NMDS) was used to analyze the relationships between plant species composition in the belts and environmental conditions. The rivers differed in species composition. White sweetclover appeared to be a pioneer species in the primary succession sequence that occurred on the disturbed floodplains. Correlation analysis was used to determine if there were positive or negative relationships between white sweetclover density or cover and belt species richness and density and cover of other species. Relationships between white sweetclover and other species were different between rivers. At the Nenana River white sweetclover was negatively correlated with silverberry, but this was not the case at the Matanuska River. Positive correlations between white sweetclover and the exotic species narrowleaf hawksbeard, and common dandelion were found at the Matanuska River and between white sweetclover and common dandelion at Stikine River Transect 1. However, at Stikine River Transect 3, where white sweetclover densities were the highest in this study, the correlation was strongly negative between white sweetclover and common dandelion and several native species including beach pea and feltleaf willow. Glacial river floodplains in Alaska are highly disturbed and are corridors for exotic plant species movement. Symbiotic nitrogen fixing white sweetclover, when growing at moderate to low density may facilitate exotic species. At high densities this species can reduce cover and density of both exotic and native species.

Technical Abstract: White sweetclover has recently invaded glacial river floodplains in Alaska. We sampled vegetation and measured environmental variables along transects located along the Nenana, Matanuska, and Stikine Rivers to describe plant communities and to determine the effects of white sweetclover on other plant species. Transects were perpendicular to the river and belts consisting of six contiguous 1 M2 quadrats were placed 5-M apart along and perpendicular to each transect. Percent cover and density of each plant species was determined in each quadrat. Environmental data collected at each belt were: height above river level, percent of surface covered by rock, rock size, and organic matter depth. A soil sample (0-15 cm depth) was analyzed for textural and chemical properties. Stem cross sections from the 3 largest woody plants were collected, and annual rings were counted to determine site-specific time since the last flood. Nonmetric multidimensional scaling (NMDS) was used to analyze the relationships between plant species composition in the belts and environmental conditions. Correlation analysis was used to determine if there were positive or negative relationships between white sweetclover density or cover and belt species richness and density and cover of other species. The rivers differed in species composition. Separate NMDS analyses were necessary for the Nenana and Matanuska Rivers and for each transect on the Stikine River. Environmental variables that were important in explaining floodplain vegetation included site specific time from flooding (measured by woody plant age), height above river, % rock, size of rock, soil N, P and percent carbon. A primary secession gradient associated with site-specific time since last flooding occurred at most sites. Soil %C, total N, and P and percent vegetation cover increased with time since flooding. White sweetclover was a pioneer species and occurred on recently disturbed sites and did not persist into closed canopy forest. The relationships between white sweetclover and other species were site-specific. At the Nenana River white sweetclover was negatively correlated with silverberry, but this was not the case at the Matanuska River. Positive correlations between white sweetclover and the exotic species narrowleaf hawksbeard, and common dandelion were found at the Matanuska River and between white sweetclover and common dandelion at Stikine River Transect 1. However, at Stikine River Transect 3, where white sweetclover densities were the highest in this study, the correlation was strongly negative between white sweetclover and common dandelion and several native species including beach pea and feltleaf willow. Glacial river floodplains in Alaska are highly disturbed and are corridors for exotic plant species movement. Symbiotic nitrogen fixing white sweetclover, when growing at moderate to low density may facilitate exotic species. At high densities this species can reduce cover and density of both exotic and native species.