BACKGROUND

What factors could influence suppression and recruitment of woody riparian vegetation?

Two potential causes of cottonwood and willow decline in the greater Yellowstone ecosystem have been proposed and discussed at length among scientists and land managers alike (Beschta 2003; Ripple and Beschta 2004a; Yellowstone NP 2007). The authors of the studies from which these figures are taken collected data to ascertain whether the gap in cottonwood recruitment and the loss of tall willows in Yellowstone could best be explained by:

1. Climatic factors such as drought or low stream flows: Cottonwood seedling establishment occurs most frequently in years with peak flows > 290 m³/second, which is the peak flow for a 5-year return interval on the Lamar River and similar-sized streams in the Yellowstone area (Beschta 2003). Growth of existing riparian vegetation like willows may also be affected by the availability of shallow groundwater, and so it is possible that willow height could be suppressed during drought years or low stream-flow years.

OR

2. Biotic factors such as over-browsing by ungulates (e.g. elk): In their winter range, elk may switch from their preferred food (grass) to more nutritious woody plants like willow and cottonwood seedlings. In the setting of a National Park, where human hunting is not allowed, and in the absence of their natural predators (e.g. wolves), elk browsing might have a major impact on the vegetation.

The graphs in this figure set give historic streamflow data from stream gauges on the Lamar and Clarks Fork Rivers, as well as more recent climatic data to address the first hypothesis.

Fenced exclosures that keep ungulates out were established in the Northern Range of Yellowstone from the 1930's through the 1950's , when Park managers were becoming increasingly alarmed at the loss of riparian vegetation, and were trying to understand the cause of this loss (Beschta 2005). Repeat photographs of these exclosures (showing the contrast in vegetation heights inside vs. outside) and more recent willow height / browsing data help address the second hypothesis.