| Rivers interact with landscapes by exchanging water, materials, and nutrients in longitudinal, lateral, and vertical dimensions. However, regulation of rivers removes pulses of floods and alters exchanges of water and nutrients between rivers and flood plains. My research emphasizes belowground dynamics in riparian ecosystems along the Middle Rio Grande in central New Mexico, USA. This river was regulated during the 20th century and increasingly receives anthropogenic inputs of nutrients. In these studies I investigated how changes in nutrient availability and flooding affect roots of Rio Grande cottonwoods, Populus deltoides ssp. wislizenii. In Chapter 1, I present greenhouse experiments where I altered the availability of nitrogen (N) and phosphorus (P) to cottonwood saplings to determine how limitation by N and P affects whole-plant stoichiometry, as well as colonization of roots by mycorrhizal fungi. Saplings responded to additions of N, but not P, by growing taller, allocating more biomass to leaves, and raising N:P in leaves and roots. Arbuscular mycorrhizal and ectomycorrhizal fungi colonized roots, but no differences in colonization were detected across treatments. In Chapter 2, I present a field study of roots in Rio Grande cottonwood forests. I compared root densities among eight riparian forests that differ in their frequency of flooding by viewing roots seasonally for four years using minirhizotrons. During a three-year drought, root densities were low across all sites, but densities were slightly higher at sites that remain hydrologically connected to the river. During a fourth year where high precipitation and extensive flooding occurred, root densities increased dramatically across all sites, but differences were undetected between flooded and non-flooded sites. In Chapter 3, I describe physical and chemical conditions of surface water, ground water, and soils in these riparian forests during the drought and flood pulse. Collectively, this work demonstrates that cottonwoods may allocate proportionally more carbon belowground when they are limited by nitrogen under greenhouse conditions. Along the river, cottonwoods increase root biomass following pulses of moisture, and floods mobilize organic carbon. I discuss potential contributions of root turnover to the cycling of carbon between terrestrial and aquatic habitats on flood plains. |
