| River and riparian restoration activity has exponentially increased throughout the southwestern U.S. since 1990. I analyzed almost 600 restoration project records from Arizona, Colorado, New Mexico, and Utah (Chapter 2). Riparian management was the most common type of project, followed by water quality management, in-stream habitat improvement, and flow modification. Total restoration costs exceeded {dollar}500 million. Monitoring was linked to 28% of projects in the Southwest, as opposed to just 10% nationwide. Restoration efficacy was difficult to ascertain from existing datasets.; The Southwest differed from other regions of the U.S. due to its high proportion flow modification projects. Flow management has reduced the exchange of water, energy, and materials from rivers and floodplains, causing regional declines in native plant populations and the spread of non-natives. Naturalization of flow regime is one form of riparian restoration. I examined the effects of flow regime on soil nitrogen cycling in riparian forests along the middle Rio Grande of New Mexico that were dominated by native Populus deltoides ssp. wislizenii and non-native Tamarix chinensis (Chapter 3). I also compared the abilities of each species to acquire and allocate nitrogen resources (Chapter 4).; The effects of flow regime varied across plant species. Flood sites had a greater number of nitrogen loss pathways relative to sites that were disconnected from flooding. In addition, sites with high quantities of riparian leaf production maintained greater pools of soil nitrogen, irregardless of flood regime. Patterns of nitrogen acquisition and allocation were similar across species, despite differences in mycorrhizal association. Thus, T. chinensis and P. deltoides were functionally similar with regards to their effects on soil nitrogen cycling.; Riparian nitrogen resources are, in part, regulated by the timing and duration of floods. Spring floods mobilize nitrogen resources early in the growing season, allowing for maximal nitrogen uptake by riparian plants. Long-term floods promote anoxia in riparian plant roots, resulting reduced uptake of soil nitrogen and suppressed leaf production. I advocate the continued practice of spring-time naturalized flow within rivers of semi-arid regions, but highlight the need to determine thresholds at which extended flood duration does more harm than good. |
