Displacement of Elodea canadensis Michx by Elodea nuttallii (Planch.) H. St John in the British Isles.

Abstract

Elodea nuttallii (Planch.) H. St John has spread rapidly since its introduction into the British Isles in 1966. A feature of its spread has been its displacement of E. canadensis Michx in places where the latter was well established. One possible reason is that E. nuttallii is able to achieve more rapid stem elongation, as well as produce a greater number of axillary stems over a given period of time, resulting in E. canadensis eventually being shaded out of a locality. Cultivation experiments suggested that stem elongation and axillary stem production was significantly (P<O·OOl) greater in E. nuttallii under nutrient-poor and-rich conditions and a range of light intensities. Stem elongation in E. nuttallii appeared to be most rapid immediately after establishment, while in E. canadensis there was a gradual elongation as time progressed. These characteristics may lead to more rapid formation of a canopy of stems and leaves by E. nuttallii at or near the water surface, a factor which may help to bring about the eventual displacement of E. canadensis in many places.