Shifting Baselines: How Great Basin Sagebrush Quietly Became Forest

It is easy to think of national parks as unchanging. But the world is always moving, and slow changes often go unnoticed until they are taken as the way things have always been. This is shifting baseline syndrome: from one generation to the next, our sense of what is normal for the environment drifts, our tolerance for its decline grows and our expectations of nature fall.

Bison, for example, are missing from 98 percent of their historic range, yet their near-disappearance is often told as a conservation success. Five billion passenger pigeons once darkened the skies of eastern North America; no one alive can picture those flocks or what they did to the land. The examples may seem distant, but similar changes are happening all around us — in the vast sagebrush ocean of the Great Basin, for one.

The sagebrush ocean

More than any other plant, sagebrush defines the Great Basin and forms one of the largest intact habitats in the country. Some animals, such as sage grouse and pygmy rabbits, live only in sagebrush. It matters for recreation and agriculture too, with open spaces, spring wildflowers, long views and deeply dark night skies. Yet it is one of North America's most threatened ecosystems.

Fire, and the lack of it

Fire in the Great Basin is as natural as wind, sun and rain. Its ecosystems evolved with frequent, low-severity fires — "good" fires — some lit by lightning and others set deliberately by Native Americans, who used fire to manage their land. Colonisation all but ended fire as a natural process: aggressive fire exclusion began in the 1920s, with slow but dramatic effects.

Without fire, singleleaf pinyon pine and Utah juniper overtake sagebrush. Birds and small mammals carry their seeds into sagebrush, where they sprout beneath a "nurse plant" that gives them shade, moisture and fertile soil. In time the young trees outgrow their host and take its light and water. Fires used to reset this every 25 to 100 years. Since fire was excluded, pinyon-juniper woodland has grown tenfold, breaking the sagebrush ocean into lakes, ponds and puddles.

How we know

• Soil. Soil reflects the plants that built it over thousands of years. In some woodlands today, the soil was formed by sagebrush and grasses, not by the acidic needles of conifers.
• Tree rings. Pinyon and juniper can live a very long time — the oldest recorded Utah juniper nearly 2,000 years, the oldest pinyon 900. If fire exclusion caused the spread, trees in former sagebrush should be young, and indeed almost all are under 100 years old, matching the start of colonisation and fire exclusion. Ancient trees grow mainly in patches on rocky slopes with thin soils, sheltered from fire, and their regular fire scars record the frequent, low-intensity fires of the past.
• Records and photographs. Journals, place names, wildlife notes and livestock records describe a more open, largely treeless Great Basin, and old photographs confirm the spread of pinyon and juniper at the expense of sagebrush and grassland.

Why it matters

We have forgotten "good fire" and the open sagebrush it kept, and come to see the young pinyon-juniper forests as permanent. But they change a great deal. Sage grouse, pygmy rabbits and yellow-bellied marmots need open sagebrush; mule deer and migratory birds have declined as conifers have spread. Streams and springs carry less water, there is less native grass and more bare soil, and the land is less productive and less diverse.

Restoring it

Fire might seem the obvious cure, but it is now part of the problem: dense, flammable pinyon-juniper stands feed catastrophic fires that are hard to control and can damage soil, after which cheatgrass, an invasive annual grass now covering more than 17 million acres of the Great Basin, can take over. So conifers are cut instead, with chainsaws or masticators, and sagebrush and other natives, such as bluebunch wheatgrass and penstemon, are often seeded afterwards. Success varies, but is usually better where soils are deeper, native plants remain in the understorey and more rain falls. Treatments are designed to recover quickly after wildfire and resist cheatgrass; keeping sagebrush may mean regularly removing young conifers before they take hold, and projects are monitored to check they meet their goals.

As baselines shift, we may come to see pinyon-juniper forest as a fixture rather than as what replaced the sagebrush. Active restoration can bring back the sagebrush ocean and the wildlife that depends on it.

Sources

• Bryan Hamilton, integrated resource program manager, "Shifting Baseline," The Midden (Great Basin National Park), vol. 22, no. 2, winter 2022, National Park Service. https://www.nps.gov/articles/000/shifting-baseline.htm
• The article cites, among others, Soga and Gaston (2018) on shifting baseline syndrome, Miller and Tausch (2001) on pinyon-juniper expansion, and Welch (2005), USDA Forest Service, on the fragmentation of big sagebrush.