Projecting avian responses to landscape management along the Middle Rio Grande, New Mexico

Most lowland rivers in the southwestern United States have been impounded, diverted, or dewatered. Lack of flooding due to river impoundments on the Middle Rio Grande has contributed to the spread of exotic vegetation such as Russian olive (Elaeagnus angustifolia) and saltcedar (Tamarix) associated with fuel loads of dense understory. Management has largely focused on thinning of understory vegetation to remove nonnative species and reduce fire risk, but it is unclear how these actions impact avian populations. Using distance-sampling methods, we quantified densities of five groups of birds (birds nesting in canopy, midstory, and understory; water-obligates; and spring migrants) across 12 types of vegetation spanning managed and nonmanaged stands. We used a space-for-time substitution model to estimate changes in abundance of birds from scenarios that applied four possible options for management at the landscape scale. One option, mechanical clearing of cottonwood understory, had severe detrimental impacts for abundances of the three nesting guilds and spring migrants when applied across the study area. A hand-thinning method to remove most exotics but retain native shrubs and the ground layer also negatively impacted birds nesting in understory but had positive or no effect on the other four groups of birds. Over the short term (5-10 years), not clearing would increase the proportion of native and nonnative understory and generally increase abundances of birds. With application of "no management" over a longer period (50-75 years), we assumed transition of most cottonwood (Populus deltoides var. wislizeii) stands to shrublands of Russian olive and projected that canopy-nesting birds would decrease but other groups would increase. A scenario of wetland restoration that converted 25% of open habitat to wetland increased abundances of understory-nesting birds slightly and water-obligate birds substantially. Our projections of changes in avian populations will help managers evaluate biological impacts of management being considered for the Middle Rio Grande.