STUDENT INSTRUCTIONS

In 2001 Robert Beschta sampled the cottonwood population in a 9.5 km² study area in the Lamar Valley of Yellowstone National Park. He was interested in measuring first hand a phenomenon that informal observers had noted for several decades: that there had been little to no cottonwood recruitment in that time ("recruitment" means growth of seedlings or suckers into trees > 5 cm in diameter). Dr. Beschta was also very interested in trying to explain this lack of recruitment by looking for links between the cottonwoods and a variety of environmental factors.

In order to address these issues, he identified and measured the diameter of all the cottonwood trees (> 5 cm diameter) within the study area (700 trees!). About half of the cottonwoods were growing on landforms called point bars on the insides of major river bends ("meanders") of the Lamar River. The other half grew in small, open groves spread across the river floodplain. Because cottonwoods are both obligate riparian and early successional species, they rely on the dynamic nature of river systems to insure their survival and reproductive success. Seedlings usually become established in the early summer or late spring after high spring run-off waters have receded, leaving behind freshly scoured surfaces and fresh sediment on meander point bars and in the floodplain where seeds germinate best (Beschta 2003). By measuring the diameter of trees, and correlating size with tree age, Dr. Beschta was able to reconstruct approximate establishment dates for cottonwoods, to address whether there had in fact been a gap in tree recruitment.

If there was a gap in tree recruitment, he needed to know when it was occurring - were seedlings becoming established, or not? So during the 2001 field season, Beschta also measured seedling density in 5 sub-plots (2 x 30 m). Seedlings ranged from 10-60 cm tall, and were 1-5 years old. Density ranged from ~4,000-70,000 seedlings per hectare.

Robert Beschta and his colleague William Ripple have looked for similar trends among other riparian tree and shrub species in the Greater Yellowstone Ecosystem. Willows are the other major streamside woody plant species in this region, and they grow on similar sites and under similar conditions to those preferred by cottonwoods. But unlike cottonwood trees, whose size can be used to infer age, willows are shrubs with multiple side branches and growing shoots ("leaders") that can re-sprout from the root stock over and over again. Because of this difference, willow recruitment could not be measured as it was for the cottonwoods, but willow abundance and relative vigor could be assessed by making density and height measurements in the field. These measurements were then compared with qualitative evidence of willow abundance from historical photographs, records and field measurements. The authors were able to use these estimates to re-construct a general model of the history of willow growth and decline in the Yellowstone region during the 20th century.

Based on the data presented here, what was the pattern of growth and recruitment in woody riparian vegetation for stream valleys in the Greater Yellowstone region during the 20th century? With a partner, use the Step One-Step Two approach, described below, to interpret the results shown from the studies of Beschta (2003) and Ripple and Beschta (2004a)

• Step One: Describe the graphs and what they show. Make sure you understand how the figures are set up, what the axes show, and what information is depicted. Carefully describe the overall patterns in the data, and make comparisons between the two figures.

• Step Two: Try to interpret the data. What do they tell you about the history of woody vegetation establishment in the Yellowstone region? Do both figures tell a similar story? What information is missing from these figures? Why is the expected distribution of cottonwood trees in Fig. 1-1 shown as a swathe as opposed to a single line? How do you suppose the researchers were able to predict the expected number of trees (Fig. 1-1) and tall willows (Fig. 1-2)?

Be prepared to explain the graphs and your conclusions from them to the class.

Once the class has agreed on a general interpretation of the figures, return to your partner and work together to generate (and write down) at least two testable hypotheses that could explain the patterns shown in the figures. Be prepared to share your hypotheses with the rest of the class. You may also be instructed to turn in your hypotheses along with a brief explanation of why each one presents a viable explanation for the observed patterns.