STUDENT INSTRUCTIONS

Your instructor will divide the class into two groups, one group to address the climate-change hypothesis for lack of growth and recruitment in woody riparian vegetation, and another group to address the browsing-change hypothesis. Which group you and your partner are assigned to will determine which figure you will focus on: Figure 2-1 contains data to help you assess the accuracy of the climate hypothesis (changes in climate account for a reduction in the growth and recruitment of riparian trees and shrubs); Figure 2-2 contains data to help you address the browsing hypothesis (changes in browsing pressure might have affected the growth of woody riparian plants). Work with your partner first to describe and then to interpret the graphs for your assigned hypothesis and group.

Group One Instructions (Climate change hypothesis, Figure 2-1):

Figure 2-1a. First, you should assess the range of variability in streamflow for rivers in Yellowstone's Northern Range during most of the 20th century (1920-2000).

• What were the maximum and minimum flows in each river?
• In general terms, how does the streamflow change over decades and over the whole 80 year period of record?
• Were there periods during this 80 year record when flows were high enough to allow for the recruitment of cottonwood seedlings? Remember that cottonwood seedlings establish best in years of peak flows > 290 m³/second (why might this be?).
• Is there information in this figure that will allow you to either support or refute the climate change hypothesis for lack of growth and recruitment in woody riparian vegetation during much of the 20th century?

Figure 2-1b. Next, you should study data summarizing water availability in northern Yellowstone during more recent years (1996-2002), as indicated by three variables: snowpack depth, peak spring flows, and July flows in the Gallatin Basin (same general region as the rivers shown in Figure 2-1a).

• Based on these data, were some years wetter or drier than others? How often was water availability (as recorded by these three measures) above or below average?
• The shaded area of the graph represents a time of increasing willow height. If willow height is limited by water availability, then what would you expect the trends in the water variables to look like during that period? Do the data fit this expectation?
• Is there information in this figure that will allow you to either support or refute the climate change hypothesis to explain increased growth in willows during the end of the 20th century?

Group Two Instructions (Browsing change hypothesis, Figure 2-2):

Figure 2-2a. First, study the repeat photographs of an ungulate exclosure in the Gallatin River floodplain. The exclosure is a square area surrounded by fencing (fence posts are visible, especially in the top two photos; fence posts are approximately 3 m tall), which keeps ungulate browsers out. The deciduous shrubs visible inside the exclosure in the top photo are willows, as are the shrubs both inside and outside of the exclosure in the lower two (more recent) photos. The exclosure had been in place for over 50 years when this study was completed in 2003 (Ripple and Beschta 2004a).

• In the 1969 photo, are there obvious differences between willows inside and outside the exclosure fence?
• Are there obvious changes in willow height or abundance in and around the exclosure as you move through the progression of repeat photographs (from 1969 to 1999 and 2003)?
• Look back at the data from Figures 1-1 and 1-2. When did a change in riparian vegetation start to be observable in those figures? Can you see a corresponding change in the repeat photographs?
• Is there information in these photographs that will allow you to either support or refute the browsing change hypothesis for lack of growth and recruitment in woody riparian vegetation during much of the 20th century?

Figure 2-2b. Next, you should study the data on browsing evidence and willow height outside of the exclosure, from 1998-2002.

• How does the percentage of new willow growth ("leaders") browsed by ungulates change over this time period?
• Is there a corresponding change in average willow height during this period? If there is a change, what appears to be causing it?
• Is there information in this figure that will allow you to either support or refute the browsing hypothesis to explain increased growth in willows during the end of the 20th century?

Based on the data presented in the figures, your team should decide whether or not the hypothesis you were addressing can be rejected. Be prepared to state your case for the whole class, and to justify it on the basis of the data.

After a group discussion of the figures and data for both hypotheses, and a decision about which (if either) can be rejected, return to your partner and work together to write a new, more refined hypothesis to explain the mystery of the missing cottonwoods and willows. Use all of the data you now have at your disposal to generate the most plausible and specific hypothesis. Your instructor may ask you to turn in this hypothesis, or another written summary of what you've learned so far.