Dynamics of Ludwigia hexapetala invasion at three spatial scales in a regulated river

Authors: Skaer Thomason, Meghan; Grewell, Brenda; NETHERLAND, MICHAEL - Us Army Corp Of Engineers (USACE)

Submitted to: Wetlands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2018
Publication Date: 7/16/2018
Citation: Skaer Thomason, M.J., Grewell, B.J., Netherland, M.D. 2018. Dynamics of Ludwigia hexapetala invasion at three spatial scales in a regulated river. Wetlands. 38:1285-1298. https://doi.org/10.1007/s13157-018-1053-2.
DOI: https://doi.org/10.1007/s13157-018-1053-2

Interpretive Summary: Plant invasions are a major threat to the health of freshwater ecosystems and can exacerbate negative anthropogenic impacts to these sensitive ecosystems. Because of this threat, it is critically important to perform research that leads to a better understanding of the biology and ecology of the most noxious invaders with the goal of informing land manager decision-making. Exotic invasive water primroses (Ludwigia spp.) are considered a priority for research and management in the United States. L. hexapetala, an emergent aquatic macrophyte, is of particular concern in Western US watersheds due to its rapid expansion, but little is understood about what drives its invasion and spread. In a four-year field study conducted in the Russian River, California, we evaluated environmental factors at three spatial scales associated with growth and abundance of L. hexapetala. Hydrologic factors including water depth, velocity, and flow correlated with changes in spatial extent of L. hexapetala. Surprisingly, we observed contraction of L. hexapetala patches, which was associated with greater reduction in water velocity at the patch scale, as well as lower flow commonly found behind seasonal instream inpoundments. These results indicate hydrology is a major driving factor for L. hexapetala spread, and give natural resource managers knowledge that may help them manage L. hexapetala invasion in US watersheds.

Technical Abstract: Environmental changes to watersheds, including alien plant invasions, threaten riverine ecosystems. Because invasive plant species are often heterogeneously distributed, understanding how watershed environmental variation influences their distribution, abundance and spread is needed to guide management and restoration strategies. Several regulated rivers in the United States and Europe are heavily invaded by Ludwigia hexapetala, a floating-leaved emergent aquatic plant from South America. Hydrochorous dispersal of clonal, vegetative fragments may support rapid spread of L. hexapetala, but mechanisms driving expansion following establishment are under studied. We evaluated environmental variables correlated with local patch, reach- and watershed-scale spread of L. hexapetala in a four-year field study in the Russian River, California. Hydrology was the most influential factor associated with L. hexapetala distribution and abundance at all scales. Velocity reductions were substantial within invaded patches, and greatest in contracting patches. Invaded patches do not expand unabated, and degree of change varies spatially and temporally. Reaches with higher channel velocity had taller patches, but greater lateral spread over water occurred with increasing channel depth. Watershed-wide, high total occupied area of L. hexapetala was associated with high relative variation in flow and greater frequency of high flow events. While hydrology is one of several drivers of riparian plant distribution and abundance, we discovered it is the major, multiscale driver of an emergent aquatic invasive species in a complex watershed where recurring drought, reservoir releases, and seasonal instream impoundments influence hydrology and colonizable habitat.