Figure 1-1. The number of narrowleaf cottonwood trees established during 20-year intervals in a 9.5 km 2 area of the Lamar Valley, northern Yellowstone National Park. Ages were derived from size class data collected in 2001. Open bars represent numbers of cottonwoods on floodplain sites; closed bars are cottonwood numbers on meander sites (see “Student Instructions” for more information). The floodplain and meander tree populations were kept separate, because trees grow at different rates in these locations, requiring different age estimates based on tree diameter. The black bar represents an estimate of cottonwood seedling density on the entire study site in 2001, and is on a different scale (thousands vs. 50-60). The shaded area indicates expected numbers of cottonwoods in each age class under conditions of frequent/regular recruitment. From Beschta (2003).
Figure 1-2. Twentieth century time series of the status of riparian willow communities on the Gallatin River, within and adjacent to northern Yellowstone National Park. Willow height and abundance were estimated from historical photographs as well as historical records and field measurements; “tall” willows are those > 100 cm (but note that the shrub willows in this region may reach heights of 3 m or more under good conditions). The shaded region between dashed lines reflects the range of variability/uncertainty in the data, since they are based mainly on qualitative assessments as opposed to absolute measurements. After Ripple and Beschta (2004a).
what has happened to the woody riparian vegetation in these valleys?
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 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.
Figure 2-1. (a) Time series of annual peak flows for the Lamar River and the Clarks Fork River in Yellowstone National Park, for their periods of record. For these streams, 5-year peak flow events average ~ 290 m3/s (305 m3/s for Lamar River, 275 m3/s for Clarks Fork). From Beschta (2003). (b) Annual maximum snowpack depth, annual peak flow, and July streamflow from 1996-2002 in the upper Gallatin Basin of southwestern Montana, adjacent to Yellowstone National Park. The long-term average (from the 1930’s to 2002) for each variable is shown by the horizontal line. The shaded area in the figure represents a period of increasing willow height. From Ripple and Beschta (2004a).
Figure 2-2. (a) Repeat photographs for an ungulate exclosure (inside the fenced area – note fence posts are ~3 m tall) in the Gallatin River basin adjacent to Yellowstone National Park, showing the status of willows within and outside of the exclosure during 1969 (winter), 1999 (spring), and 2003 (summer). (b) Percentage of willow leaders (= new shoots) browsed by elk, and average height (error bars = standard deviation) of willow leaders outside of the grazing exclosure shown above from 1998 to 2002. From Ripple and Beschta (2004a).
Figure 3 <ul><li>"Why was elk browsing on cottonwoods and willows so intense during much of the 20th century in Yellowstone National Park?" </li></ul><ul><li>Were they most likely growing, declining, or relatively constant, and why? </li></ul><ul><li>Write down your predictions. </li></ul>
Figure 3. Twentieth century time series of (a) wolf populations and (b) elk population estimates and trend line for the Upper Gallatin Basin in the Yellowstone area. Shaded portions of a graph reflect uncertainty; elk census data are represented by closed diamonds. From Ripple and Beschta (2004a).
Figure 4-1 . Repeat photo pair showing riparian willow habitat on Blacktail Creek in the Yellowstone Northern Range: from 1996 (left), after 70 years of wolf extirpation; and in 2002 (right) after 7 years of wolf recovery. Notice the larger size and abundance of willows in 2002. From Ripple and Beschta (2004b).
Figure 4-2 . Repeat photo pair of upland willow habitat and browsing exclosure in the Gallatin River Basin, Yellowstone Northern Range: from 1995 ((a), top), after 70 years of wolf extirpation; and in 2003 ((b), bottom) after 8 years of wolf recovery. In this habitat, the difference in willow height inside and outside of the exclosure is the same in both photos; arrows indicate location of willows in and out of the exclosure. From Ripple and Beschta (2004a).
Figure 4-3 . Photo pair of aspen in riparian (A - top) vs. upland (B-bottom) habitat along the Lamar River in 2006. In riparian habitat there has been abundant recent recruitment of young aspen (3-4 m tall), while in an adjacent, more open upland there has been little recruitment (aspen <1 m tall). The dark, furrowed bark comprising approximately the lower 2.5m of aspen boles in (B) represents long-term damage due to bark stripping by elk. From Ripple & Beschta 2007. Ecology of Fear?
Figure S1. Cow (female) elk were killed less often than would be expected if wolf kills were random with respect to sex, while bull elk were killed more often than expected. Numbers within bars are the number of wolf kills observed (black bars) and the number expected on the basis of the population’s composition (shaded bars). N = 124 kills in the Gallatin Canyon population, with similar patterns reported for other populations in the Greater Yellowstone Ecosystem ( S4 ).
Concept Check <ul><li>Consider two rivers. One is spring fed and is constant in water volume and temperature year-round; the other drains a desert landscape and floods and dries out as unpredictable intervals. Which is more likely to support many species of iteroparous animals? Why? </li></ul>