Streamflow and Nutrient Constraints on the Productivity and Habitat Quality of Desert Riparian Ecosystems in the West

Project ID: 15
Principal Investigator: Mark Nelson
Research Topic: Sediment Management and River Restoration
Priority Area Assignments: 2012 (Climate Adaptation)
Funded Fiscal Years: 2012, 2013 and 2014
Keywords: nitrogen, riparian, river restoration

Research Question

The Bureau of Reclamation (Reclamation) has interests in riparian restoration projects. These projects are typically related to water delivery, water salvage, or avoiding impacts to endangered species. Reclamation is supporting restoration projects on several rivers in the West. A large amount of money has been directed towards these restorations and $1 billion dollars has been spent just in the Southwest since the 1980s. Restoration is largely used to ensure that Reclamation can continue to supply users with water and power, often via trade of restoration for water.

There is little information on riparian zone nutrient dynamics in the semiarid and arid western U.S. despite its great importance to ecosystem structure and functioning. This gap in understanding of desert riverine ecosystems constrains water and land management options. We will examine the hypothesis, generated from an array of empirical studies, that desert riparian vegetation is generally co-limited by both soil nitrogen (N) and water. We will couple manipulative (fertilization) landscape-scale experiments in two contrasting river basins with an assessment of N in restoration site riparian soils and foliage to 1) address the importance of N in limiting productivity and determining the community composition of riparian vegetation and 2) resolve the interaction between seasonal flooding and N availability and uptake by plants with emphasis on restoration sites. Dryland riparian areas may be characterized by low plant-available N. The low plant-available soil N results from low N inputs and the relatively slow recycling of organic N through decomposition (mineralization), which is itself co-limited by local weather and flow regime. Understanding the consequences of N limitation remains a key knowledge gap on riparian plants and other biota. A greater understanding of N controls on dryland riparian systems will enhance the ability of water resource managers to adaptively manage restoration projects.

Need and Benefit

Riverine corridors provide habitats for an array of regionally unique species, and intact riverine ecosystems provide a variety of services that enhance human welfare by providing economic, social, cultural, and biological values. Environmentally sound water resources management includes maintaining riverine ecosystem integrity to the greatest extent possible. Optimization of operations to meet human needs while sustaining key riverine ecosystem services is a focus of attention. A critical necessity for optimization is an accurate understanding of the linkages among flow events, physical and biogeochemical processes, and the responses of riparian plants and animals. Multiple factors control the types and productivity of riparian plants along dryland streams. Soil water availability is one factor, but in many or most cases, it may not be the sole factor. Limitation of primary productivity by N is common in many ecosystems and likely true of western riparian systems. Cottonwoods, which are likely N-limited, are a key component of western riparian ecosystems, often the only tree of large stature present, and their presence is critical for a vast array of animals, including yellow-billed cuckoos and other neotropical migratory birds. Plant N status not only affects plant growth and development, but also response to stressors and thus ecosystem resiliency. In restoration projects, stresses related to the absences of floodflows could be mitigated, to some degree, by the addition of N. A greater understanding of N controls on dryland riparian ecosystems will enhance the ability of water resource managers to manage restored riparian ecosystems.

Contributing Partners

Contact the Principal Investigator for information about partners.

Research Products

Independent Peer Review

The following documents were reviewed by qualified Bureau of Reclamation employees. The findings were determined to be achieved using valid means.

Nitrogen effects in western riparian areas (final, PDF, 1.0MB)
By Douglas Andersen
Report completed on September 23, 2014

Restoration of riparian productivity lost via flow regulation is a management goal along dryland rivers. Riparian trees often have water needs met in whole or in part by a shallow water table, suggesting productivity limited by N availability. In this study we examined tree and herbaceous vegetation response to nitrogen (N) fertilization. We added 20gN m-2 in a 2-m radius around the base of mature Populus fremontii along a regulated and free-flowing river in northwest Colorado, to test for t

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Last Updated: 6/22/20