Environmental changes to watersheds, including alien plant invasions, threaten riverine ecosystems. Invasive plant species are often heterogeneously distributed across a range of spatial scales within watersheds. Understanding how environmental variation within a watershed influences their distribution, abundance and spread is needed to guide management and restoration strategies. Several regulated rivers in the United State and Europe are heavily invaded by Ludwigia hexapetala, a floating-leaved emergent aquatic plant from South America. Hydrochorous dispersal of clonal, vegetative fragments is thought to support rapid spread of L. hexapetala in rivers, but mechanisms driving population expansion following establishment are poorly understood. In a four year field study in the Russian River, California, we investigated the distribution and abundance dynamics of L. hexapetala at watershed, river reach, and local patch scales to understand invasive spread in a managed river. Watershed-scale distribution and abundance of L. hexapetala relative to hydrologic and river morphology variables were analyzed in a Principal Components Analysis. L. hexapetala growth response to hydrologic variation within discrete study reaches of the river was evaluated using a multivariate regression. Multiple environmental characteristics, including light availability, water and sediment nutrients, and hydrology were evaluated as drivers of local patch expansion.
We found hydrology was the single most influential factor associated with L. hexapetala distribution and abundance at watershed, river reach, and local population patch scales. At the watershed-scale, high total occupied area of L. hexapetala was associated with high relative variation in flow and greater frequency of high flow events (> 3X median). At the reach-scale, channel velocity influenced patch structure, but channel depth explained spread into the channel. Velocity reductions were substantial within invader patches, and differed depending on expansion dynamics. In fact, an important result of our study is that invaded patches do not expand unabated, and degree of expansion or contraction varies spatially and temporally. While hydrology is one of many important 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. We showed that increasing variability in flow has led to greater spread of this invader. This information will be critical for developing integrated water and invasive plant management strategies.