The ecohydrology of a Japanese knotweed invasion

<p>Polygonum cuspidatum (Japanese knotweed) is an herbaceous, clonal invasive plant that can create a monodominant stand that outcompetes native species, particularly throughout riparian zones. To better understand how this species is so successful at invading, and why particular locations in riparian settings are more at risk, we investigated a variably invaded riparian valley site in the northeastern United States. We measured spatial variations in hydrologic and soil attributes (soil texture, depth profiles, volumetric water content) in relation to spatial variations in Japanese knotweed abundance (percent coverage). We also investigated how the arrival of this species alters the hydrologic properties of the soil (e.g., infiltration rate) at a fine spatial scale (< 1 m). Across all variables observed, the strongest linear correlation (<em>p</em> < 0.01, <em>r</em> = 0.70) was found to be a positive relationship between mean percent coverage of Japanese knotweed and soils with a higher clay proportion. To understand causation in this relationship, and to better know how knotweed alters the physical environment it invades, we measured soil infiltration rates in the presence and absence of stems (<em>n</em> = 40). We found that knotweed occurrence can create a significant (<em>p</em> = 0.05) decrease in infiltration rate, but only within finer soils. These results may elucidate new possible ecohydrologic feedback mechanisms between Japanese knotweed and soil properties that could promote a higher competitive advantage in finer soils. This new insight, along with other known mechanisms promoting knotweed growth, can help to better identify and prioritize conservation efforts around locations more at risk of a successful invasion.</p>