During the past century, the floodplain along the Middle Rio Grande, NM (USA) has been altered by changes to the hydrologic regime, channelization, colonization of non-native species, and anthropogenic nutrient inputs. Isolation of riparian forests from seasonal flooding has recently been coupled with severe drought conditions and water use in the Rio Grande watershed. The native riparian forest is dominated by the Rio Grande cottonwood (Populus deltoids ssp. wislizenii), a keystone species that is showing signs of decline. The ecophysiology of cottonwoods is intricately tied to the hydrologic and biotic processes of the floodplain. Our study measured the response of cottonwoods to natural and anthropogenic disturbances as part of an interdisciplinary study of the role of seasonal flooding in maintaining the ecological integrity of native riparian forests. We measured seasonal foliar and litter chemistry, resorption efficiencies, leaf litter production, and root densities of cottonwoods at eight study sites in normal and dry years.

Results/Conclusions

Cottonwoods at sites with a history of periodic flooding (and greater hydrologic connectivity between surface and ground water) had lower seasonal foliar N:P ratios and annual leaf production, but higher nutrient content and resorption efficiencies. However, these cottonwood stands were more susceptible to fluctuations of ground water levels, especially during drought, that affected leaf production and nutrient content. The physiological responses of cottonwoods in this study served as integrators of ecological processes that are vital in the context of the sustainability of cottonwood forests in this human-dominated system.