Vegetation characteristics and butterfly use of unlined and PVC-lined reaches of an irrigation delivery canal, Government Highline Canal, Colorado, USA

Vegetation growing in irrigation canals has been postulated to furnish an important mesic habitat within arid landscapes. Some canal designs now incorporate plastic (PVC) liners to reduce seepage, and vegetation could be incidentally affected. We tested the hypothesis that PVC lining has no long-term effect on bank vegetation by comparing vegetation on interior banks of a reach of an 88-km long irrigation canal (47 m(3) s(-1) capacity) lined 6-8 years earlier, with vegetation along unlined reaches of the same canal. The lined reach generally had lower values for cover, biomass, and richness than unlined reaches to the east. However, an east-west gradient in biomass noted within a 32-km long unlined reach suggested that the lower values of the lined-reach were at least partially due to factors unrelated to lining. Butterfly communities using the sampled reaches were surveyed to assess habitat quality. No significant differences were detected among the reaches, although the butterfly assemblage in the westmost lined reach tended to differ from those in unlined reaches. Pre- and post-lining surveys conducted on the same reach, as well as natural history data, indicated that canal butterflies were derived from surrounding habitats, rather than the canal itself. Vegetation management practices (e.g. periodic burning) limit the study canal's value as a habitat for butterflies and probably other species.