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

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.