Ages of Aquarius: in an Idaho canyon, temperate rainforest plants found refuge from ancient climate change

Natural History, June, 2003 by Robert H. Mohlenbrock

In 1968, when Robert Steele and Frederic D. Johnson, both forest ecologists at the University of Idaho in Moscow, explored a remote region along Idaho's North Fork Clearwater River, they found warm, south-facing slopes, cool north-facing slopes, perennial springs, and moist river terraces. They also found stands of forest populated with western red cedar and Douglas fir, as well as with several species previously unknown in Idaho, notably red alder. As they surveyed the vegetation, they were reminded of the temperate rainforests along the northern Pacific Coast, some 300 miles away. Stranger still, some of the plants were associated with Eastern deciduous forests.

Unfortunately, from a botanical point of view, soon thereafter a dam was built about fifty miles downstream, and two-thirds of the area was flooded by the upper reaches of the Dworshak Reservoir. The good news is that six square miles of the surviving rainforest fall within Idaho's Clearwater National Forest and were designated the Aquarius Research Natural Area in 1991. The name Aquarius came from an old campground farther upstream, but how that site got its name no one knows.

The fossil record indicates that before 30 million years ago, temperate rainforests grew in what is now northern Idaho. At that time, to the west lay only shallow seas and tidal fiats, making for heavy fogs, ample rainfall, and mild temperatures from the Pacific Ocean that nurtured the habitat. The tree species included dawn redwood, ginkgo, bald cypress, and relatives of present-day sassafras, tulip tree, and magnolia. Tall ferns probably grew in abundance, and mosses cloaked rocky terrain and fallen logs.

Beginning 30 million years ago, however, as a result of plate tectonic events, tumultuous volcanic eruptions uplifted the Cascade Range, blocking much of the Pacific moisture. Temperatures in what is now northern Idaho became more extreme. Some trees, such as red alder, were isolated from members of their species farther west. Others, such as bald cypress and numerous broad-leaved trees, disappeared from the region, though they survived in eastern North America. Still other species, such as the dawn redwood and ginkgo, vanished from North America, though they remain to this day in China.

Nevertheless, some temperate rainforest plants persisted in Idaho, finding refuge in canyon bottoms during the ice ages. One such refugium was the canyon of the North Fork Clearwater River. When the last glaciation ended, however, about 12,000 years ago, temperatures rose and the climate became drier. Conditions became generally unfavorable for rainforest plants.

But at least along one river--as discovered by Steele and Johnson--some rainforest plants did survive. In large part they owe their lives to the 5,000- to 7,000-foot-high mountains that surround the steep-walled canyon where the plants grow. The mountains capture plenty of moisture--as precipitation and fog--during the growing season. In addition, the fairly low elevation of the river (about 1,650 feet) keeps the temperature from swinging to extremes.

HABITATS

Temperate rainforest Western red cedar is usually the dominant tree, but Douglas fir is common on the relatively dry slopes facing south. Grand fir occurs alongside both species. Western red cedar and Douglas fir are prominent on the Pacific coast and in the western Cascades, but are less common in the intervening territory. Other plants that fit with the same disjunct pattern are clustered lady's-slipper, white shooting-star, broadleaf starflower, evergreen violet, Henderson's sedge, and crinkle-awn fescue.

Species of the eastern deciduous forest are northern maidenhair, maidenhair spleenwort, and oak fern. Plants that occur only in or near northern Idaho refugia are Clearwater corydalis, Constance's bitter cress (a lavender-flowered member of the mustard family), and Idaho barren strawberry (whose closest relative is in the eastern United States).

Aquarius Research Natural Area is particularly rich in ferns, including Western polypody, spreading woodfern, oak fern, male fern, bracken fern, and sword fern. Huge lady ferns fill the understory on gentle slopes and on terraces above the river. On moist slopes above the terraces, northern maidenhair replaces the lady fern, and oceanspray populates the shrub layer. One unusual native species is phantom orchid, which is pure white. Devoid of chlorophyll, it derives all its nutrients from fungal associates in the soil. One-flowered Indian pipe, which also grows here, pursues the same strategy.

For visitor information, contact: Clearwater National Forest 12730 Highway 12 Orofino, ID 83544 (208) 476-4541 www.fs.fed.us/rl/clearwater/

Streamside Within the Research Natural Area flow several streams that empty into the North Fork Clearwater River. The adjacent habitat usually includes red alder, black elderberry, Scouler's willow, black cottonwood, red-osier dogwood, and yellow monkey flower.

Robert H. Mohlenbrock is professor emeritus of plant biology at Southern Illinois University in Carbondale.