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:
OR
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.