Editor's note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Cathy Whitlock, Regents Professor with the Department of Earth Science and the Paleoecology Lab at Montana State University.

Want to know the geologic history of a specific area of Yellowstone? Then pay attention to the vegetation!

In Yellowstone, geology and ecology go hand in hand. In fact, the geology of Yellowstone National Park can be mapped by its vegetation! For example, the glacial clays in the Lamar and Hayden valleys are rich in nutrients, like calcium and magnesium, and have high water-holding capacity; these qualities favor growth of grassland and sagebrush steppe. These landscapes are called the “Serengeti of North America”— lush grasslands that support large populations of bison and elk, not to mention coyotes, wolves, and grizzlies.

Thinner soils with intermediate fertility form on the eastern side of Yellowstone in the Absaroka volcanic province, where andesitic compositions are common. They support forests of Engelmann spruce, subalpine fir, and whitebark and lodgepole pine, like those found in much of the Northern Rockies. In contrast, the plateaus of central Yellowstone National Park, created from explosive and lava-flow eruptions with rhyolite compositions, are overlain by thin, infertile soils. Their lack of critical nutrients and low water-holding capacity limit most conifers, with the notable exception of lodgepole pine. Lodgepole’s seeds are released when the cones are heated, enabling rapid establishment after fire and further ensuring the conifer’s dominance in central Yellowstone. 

Modern vegetation on different geological substrates in Yellowstone. Left: steppe/grassland on glacial clay found in places like Lamar and Hayden Valleys. Center: Mixed conifer forest in the Absaroka andesite volcanic field in the eastern part of Yellowstone National Park. Right: Lodgepole pine forest on Central Plateau rhyolite (hydrothermal grassland is present in the geyser basin in the middle of the photo)/Graphic by Cathy Whitlock, Montana State University.

Yellowstone didn’t always look like it does today, though. The vegetation has varied over time along with climate. One way to investigate the vegetation and climate conditions of the past is to collect and analyze sediment cores from lakes in the area. Sediment layers in lakes preserve millions of pollen grains that can be used to study changes in vegetation and climate through time. 

Comparing pollen records from three lakes in different geological substrates highlights the influence of geology and climate on long-term vegetation development. Radiocarbon dating of sediment cores indicates that these lakes formed 14,000–15,000 years ago at the end of the last ice age. A pollen record from Slough Creek Pond, near the confluence of Slough Creek and the Lamar River in the northeast part of Yellowstone National Park, describes the history of the fertile grassland regions. Following ice recession, when the climate was cool and wet, the clay-rich areas were covered by shrub tundra. As temperatures rose, Engelmann spruce moved into northern Yellowstone to form a subalpine parkland. As more trees arrived with further warming, a parkland of spruce, fir, and whitebark pine developed. After 11,000 years ago, during the early-Holocene warm period, Slough Creek supported a forest dominated by lodgepole pine and juniper. The present Douglas-fir parkland was established about 7,000 years ago, when the climate of northern Yellowstone became drier and fire activity increased. 

A pollen record from Cub Creek Pond in the Absaroka region shows that the initial vegetation was similar to that of Slough Creek Pond. Following an early period of shrub tundra, a subalpine parkland of Engelmann spruce formed about 12,500 years ago and became more diverse about 11,700 years ago. On andesitic substrates, the early-Holocene warm period supported abundant lodgepole pine, some Douglas-fir, and aspen. In the last 5,000 years, this region became cooler and wetter, and present-day mixed forest of spruce, fir, and pine developed. 

Compare these histories with that of the rhyolitic Central Plateau. A record from Cygnet Lake shows that shrub tundra or steppe was present before 11,000 years ago. While other parts of the park were colonized by spruce, fir, and whitebark pine, the infertile soils limited their establishment in the Central Plateau. It wasn’t until lodgepole pine arrived 11,000 years ago that rhyolite regions became forested, and despite shifts in climate and fire activity, central Yellowstone has been covered by lodgepole pine forest ever since.

Vegetation history based on pollen records from three small lakes on different geological substrates in Yellowstone National Park. Blue is open vegetation, light green is parkland, dark green is forest. Top plot is from Slough Creek Pond, in a present grassland area dominated by glacial and lake sediment in the northeast part of Yellowstone National Park. Middle plot is from Cub Creek Pond in the Absaroka volcanics on the east side of Yellowstone National Park. Bottom plot is from Cygnet Lake in the Central Plateau rhyolites in the center of Yellowstone National Park/Figure by Cathy Whitlock, Montana State University.

So, what do we learn from this? First is that Yellowstone is a geoecosystem in which the geologic template as well as changes in climate and fire shape vegetation development. Imagine how this geo-connection played out through time. If Yellowstone did not overlie a hotspot, we would not have had caldera and post-caldera rhyolitic eruptions. No rhyolite, no lodgepole pine forest. If the hotspot hadn’t created what Ken Pierce and Lisa Morgan call the “Yellowstone crescent of high terrain,” (which is essentially an area of the Rockies that has been uplifted by the hotspot’s interaction with the North American plate) we wouldn’t have had the high elevations necessary for ice to form during glacial periods. No hotspot, no glaciers, no clay-rich soils, no fertile steppe. In short, Yellowstone would have looked like the rest of the northern Rockies. But to our good fortune, all of these events did transpire and created the Yellowstone geoecosystem that we see today. Geology matters!

Additional reading:

Despain, DG. 1990. Yellowstone vegetation; consequences of environment and history in a natural setting. Roberts Rine- hart, New York, New York, USA.

Iglesias, V, Whitlock, C, Krause, TR, Baker, RG. 2018. Past vegetation dynamics in the Yellowstone region highlight the vulnerability of mountain systems to climate change. Journal of Biogeography 45: 1768-1780.

Pierce, KL, Morgan LA. 1992. The track of the Yellowstone hot spot: Volcanism, faulting and uplift. Geological Society Memoir 179, Chapter 1.

Whitlock, C. 1993. Postglacial vegetation and climate of Grand Teton and southern Yellowstone National Parks. Ecological Monographs 63: 173-198.