Plant diversity in riparian forests in northwest Colorado: Effects of time and river regulation

During the 20th Century the flow of most rivers in the United States was regulated by diversions and dams, with major impacts on riparian forests. Few unregulated rivers remain to provide baseline information for assessing these impacts. We characterized patterns in riparian plant communities along chronosequences on the unregulated Yampa River and the regulated Green River in northwestern Colorado, examining patterns in plant species diversity in relation to the ages of floodplain terraces. On both rivers, mean plant species richness in cottonwood (Populus deltoides Marshall subsp. wislizenzii (Watson) Eckenwalder) dominated riparian forests declined by more than 50% from young sites (<20 years) to old upland terraces (>250 years). Mean species richness (number of plant species/1000 m2) was 40% higher on the unregulated Yampa River than on the regulated Green River, and 84% of the variance in total species richness related to age class, river, and tree cover. One-third of the plant species in all plots were exotic and 63% of the variance in exotic species richness was explained by a positive correlation with the richness of native species. On the Yampa and Green Rivers, ecosystem development and river regulation both reduce species richness, and these effects appear to be additive. The methods and insights from this study can be applied to other river systems to investigate the processes that change species diversity over time, and vegetation responses to the major changes in environmental conditions that follow river regulation.