Temporal and nonlinear dispersal patterns of Ludwigia hexapetala in a regulated river

Authors: Skaer Thomason, Meghan; Grewell, Brenda; NETHERLAND, MICHAEL - Us Army Engineer Research And Dvelopment Center

Submitted to: Wetlands Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2018
Publication Date: 4/16/2018
Citation: Skaer Thomason, M.J., Grewell, B.J., Netherland, M.D. 2018. Temporal and nonlinear dispersal patterns of Ludwigia hexapetala in a regulated river. Wetlands Ecology and Management. 1-14. https://doi.org/10.1007/s13157-018-1053-2.
DOI: https://doi.org/10.1007/s13157-018-1053-2

Interpretive Summary: Riverine wetlands are vulnerable to biological invasion due to hydrologic connectivity within watersheds, which facilitates post-entry movement of aquatic plant propagules by water currents. The dispersal of plant propagules with water currents is influenced by many ecological and watershed factors that results in the spread of the invasion. Generally, the number of propagules moving in a watershed is assumed to increase downstream, and this assumption influences management decisions, such as the prioritization of management activities from upstream to downstream reaches of a river to reduce reinvasion. Uruguayan water primrose (L. hexapetala), an emergent aquatic macrophyte from South America, is a clonal plant that has invaded many riverine wetland ecosystems in the United States and Europe. This invasive plant species can reproduce asexually from shoot fragments that break off from a mother plant, and these buoyant fragments are readily transported in flowing rivers. We investigated morphology of shoot fragments and their dispersal in the Russian River watershed of California, capturing shoot fragments of L. hexapetala during repeated summer surveys at five locations in the river and quantifying their morphological traits that predict establishment success. Highly variable capture counts suggest the importance of pulse disturbance events in local dispersal of L. hexapetala. Unexpectedly, dispersing propagule pressure was nonlinear, with more shoot fragments captured in the middle rather than the lower or upper river reaches . Captured fragments in the middle river were twice the length of fragments captured in the lower river and bore 83% more stem nodes, characteristics associated with greater establishment success. Our results support development of spatially targeted management, outreach, and prevention efforts within watersheds that could lead to decreased propagule pressure and improve management success.

Technical Abstract: Rivers are vulnerable to biological invasion due to hydrologic connectivity, which facilitates post-entry movement of aquatic plant propagules by water currents. Ecological and watershed factors may influence spatial and temporal dispersal patterns. Field-based data on dispersal could improve risk assessment models and management responses. Ludwigia hexapetala, an invasive emergent macrophyte, provides a case study for understanding dispersal patterns throughout a watershed. The species spreads via hydrochory and is increasingly imposing detrimental ecological and economic impacts within watersheds of the United States and Europe. We investigated morphology of shoot fragments and their dispersal in the Russian River watershed of California, capturing shoot fragments of L. hexapetala during repeated summer surveys at five locations in the river and quantifying their morphological traits that predict establishment success. Highly variable capture counts suggest the importance of pulse disturbance events in local dispersal of L. hexapetala. Unexpectedly, dispersing propagule pressure was nonlinear, with more shoot fragments captured in the middle rather than lower river. Captured fragments in the middle river were twice the length of fragments captured in the lower river and bore 83% more stem nodes, characteristics associated with greater establishment success. Our results support development of spatially targeted management, outreach, and prevention efforts that could lead to decreased propagule pressure in the watershed.