Long-term cottonwood establishment along the Green River, Utah, USA

Cottonwood (Populus spp.) riparian forest and woodlands provide valuable ecological goods and services, especially in arid and semiarid regions of North America. Successful establishment and survival for cottonwoods requires bare, moist alluvium that is relatively safe from fluvial disturbance. These are restrictive requirements in dry regions. Cottonwoods can survive for three to four centuries and thus may provide a long-term record of specific fluvial and geomorphic events. We used C-14 analysis and ring counts of cottonwood stems to document a long-term record of cottonwood establishment. We related this record to the century-long gage record at Green River, Utah along with historical photos and paleoflood records. Radiometric carbon dating and ring counts suggest some cottonwood stems established on high alluvial deposits following large floods as early as the late 1600s through the late 1800s. Cottonwoods also established on inset floodplain surfaces following sustained decreases in peak flows resulting from climatic drought and the closing of Flaming Gorge Dam. Finally, establishment occurred on both higher post-dam flood deposits as well as low, active channel surfaces following floods and during a series of low flow years. With continued peak-flow reduction, cottonwood recruitment will likely be restricted to spatially limited, lower elevation surfaces and subjected to competition from an array of herbaceous and woody riparian species.