Along the Colorado River in Grand Canyon National Park (see map), Goodding willow is the only large willow found downstream from Glen Canyon Dam. Although this species was a dominant riparian, or streamside, tree prior to the construction of the dam, Goodding willow is currently at risk of becoming endangered due to habitat destruction, competition with introduced species, and river regulation. The germination and permanent establishment of willows are ecologically dependent on periodic flood flows. Willows produce seeds during the time of spring flooding on rivers and streams, and higher flows are needed to provide moisture for seeds to germinate. Goodding willow populations in Grand Canyon are presently made up of older individuals, and show little sign of new growth and establishment. Although rare for the majority of the park, Goodding willows are presently abundant on exposed benches of sediments near Lake Mead (the reservoir formed at the downstream end of Grand Canyon by Hoover Dam -- see map). Yet even this population may be in danger from large lake level fluctuations.
Located 15 miles upstream from Lee's Ferry at the Grand Canyon National Park boundary, Glen Canyon Dam controls the Colorado River's flow, changing it from a flood-prone river with highly variable flow to a regulated river with little seasonal variation and a daily cycle based on electricity demands. Results from this river regulation include lower flood flows, higher average flows, colder and less variable river temperatures, sediment trapping, and beach erosion. In turn, these changes provide a stable habitat for riparian vegetation. Before dam construction, frequent flood events limited the amounts of vegetation along the Colorado River in Grand Canyon National Park. Since the construction of Glen Canyon Dam in 1963, riparian vegetation has expanded and diversified. Vegetation communities have established closer to the river at a new dam-controlled high water line (see photo above), and include tamarisk (an introduced species, pictured below along the Colorado River), mesquite, seep-willows, and coyote willows. Even though most species have increased their distribution since dam construction, Goodding willow appears not to be recruiting (generating young individuals) in most of the national park. Instead, Goodding willows are establishing at downstream sites in Upper Lake Mead on newly established benches, possibly due to an ample supply of bare sediments on beaches and bars.
A study by Dr. Joy Nystrom Mast, a biogeographer at Carthage College in Wisconsin, has recently tried to estimate the health of the Grand Canyon Goodding willow populations using biogeographic and ecologic information. Patterns of tree growth rings, which are viewed by extracting a core from the trunk of the tree (a science called dendrochronology), can be used to illustrate climatic influences and river regulation impacts on tree establishment and growth. Field-based surveys of willow populations are also helpful for analyses of willow responses to flood flows. Using these two techniques, Dr. Mast compared tree ring-based evidence of Goodding willow age, size, and health to Colorado River flow records. Results show that at Lee’s Ferry (the upstream boundary of Grand Canyon National Park), significant tree growth suppression occurs a year after dam completion. At a site 70 miles downstream within the park, increased willow growth is observed starting in 1985, which likely reflects the flood of 1983 and above average river flows in 1984-86. These results indicate that river regulation has had a negative effect on willow growth and establishment in upper Grand Canyon, although there appears to have been a broad scale shift in the species distribution from the upper Grand Canyon (Lee’s Ferry) area to lower Grand Canyon (Pearce Ferry) region. The 1983 flood event, the only significant flood since the completion of Glen Canyon Dam, appears to have enhanced Goodding willow growth. These results are important for understanding how river regulation alters the disturbance regime to which riparian willows are adapted. This study will also help park managers understand Goodding willow ecology, hopefully enabling preservation of this species within the park
Here is the source for the photo at top of page.
Based on what you have learned about ecosystems, ecology, and biogeography in (Chapter 19 in Geosystems and Chapter 16 in Elemental Geosystems), would you say that flooding disturbance leads to ecological succession? Why or why not? If it does lead to succession, describe how this process might work.
To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.
