In the southwestern United States, agricultural lands are interwoven with irrigation canals bringing diverted river water to crops and pastures. Settlers along the Verde River began diverting water into earthen irrigation ditches over 100 years ago, sustaining not only agriculture but extending the riparian plant community beyond the river’s bank. The objectives of this study were to determine if plant communities along the ditches are similar to those along the river in species diversity, abundance, and composition, and to evaluate the soil conditions that influence community structure in both systems. We characterized plant communities along three irrigation ditches and the adjacent Verde River. Herbaceous cover by species was sampled at nine ditch sites and nine adjacent river sites three times during 2009. Soil moisture, texture, phosphate, nitrogen, and nitrate were analyzed at the same sites. Woody vegetation was assessed at the eighteen sites by characterizing vegetation zones and measuring cover, stem density, and basal area within randomly selected plots. In addition to sampling along the active channel, vegetation was also sampled within the Verde River floodplain and terrace zones. Herbaceous cover and corresponding environmental data were analyzed using Nonmetric Multidimensional Scaling. Species richness and abundance were compared using Student’s T-tests.

Results/Conclusions

Plant communities lining the ditches are similar to those adjacent to the river with respect to species richness and abundance (woody basal area and herbaceous cover), but differ in types of species present. Composition of woody and herbaceous plants differs in that rivers are defined by species requiring more water, and ditches are lined predominately with mesic to xeric species. In addition, more mid to late successional species prevail in the ditches, while pioneer species dominate river bank communities. Higher soil moisture at river sites and elevated soil phosphate levels at ditch sites explain differences in species between the site types, with herbaceous community structure along the river related to soil texture and moisture, and ditch community structure related to soil nutrient content. Although the river bank and ditches share the same surface water source, irrigation ditches sustain a different type of plant community that is dominated by a subset of the riparian flora and by upland species. The ditch communities are more similar to the river’s high floodplains and terraces than to the active channel zone. To assess other factors contributing to differences between the plant communities, a companion study addresses the influence of hydrochorous seed dispersal.