BACKGROUND

Where have the cottonwoods and willows gone?

Biologists and managers in Yellowstone National Park have noticed changes in the abundance of cottonwood and willows along streams and rivers in the Park, especially in valleys where elk congregate in the winter. Beschta (2003) and Ripple and Beschta (2004) collected data on cottonwood size classes (trunk diameter in cm) and willow height, respectively, and placed the data into time series. Summary figures from these two papers are presented here, but note that cottonwood diameter has been converted to approximate tree age (establishment date) based on linear regression models not included here (see Beschta 2003). Cottonwood data are divided between floodplain sites (i.e. sites in the valley bottom likely to be inundated in years of high spring run-off) and meander sites (i.e. the insides of river bends adjacent to the channel, where sediments are deposited after spring floodwaters recede), because cottonwood seedling establishment and growth rate are influenced strongly by landform/position. Seedlings are most likely to become established on exposed sediments after substantial spring snowmelt floods, and growth rate after establishment differs for meander vs. floodplain sites (Beschta 2003).

The data lead us to ask, what has happened to the woody riparian vegetation in these valleys?