The term “temperate rainforest” usually elicits imagery of the iconic redwood forests of California’s northern coast, dominated by towering Sequoia trees and soaked in fog, or the Olympic rainforest on Washington’s near-coast, draped with moss and drowned in endless rainfall. Only recently do we broadly recognize the presence of another rainforest in the west, separated from the Pacific temperate rainforests by two-hundred miles: the Inland Temperate Rainforest. This rainforest characterizes fragmented swathes of southeastern British Columbia and northern Idaho, as well as slivers of northeastern Washington and northwestern Montana. With many opportunities to observe and study in this stunning and strange region, I have found that many of its most unique characteristics slip under the radar of typical publications. Here are the most notable things about the Inland Temperate Rainforest, through the lens of my experiences in North Idaho:
Climate and Geography
Where is this rainforest, and how can it exist hundreds of miles from the ocean? Normally, temperate rainforests are restricted to narrow coastal corridors where mountainous terrain forces large masses of warm wet air from the ocean to rapidly cool over land, resulting in precipitation, especially in the winter months with the assistance of the jet stream. On the northwestern coast, these masses are so rich that storm systems retain significant amounts of moisture even as they are driven inland, over the wet Cascade Mountains, and across the rain-shadowed lowland steppe of the Columbia Basin. As the terrain begins to rise again into the Palouse Prairie, and especially once it encounters the elevations of the Columbia Mountains and Bitterroot Range, the remaining water is again cooled and concentrated, causing orographic precipitation over North Idaho. These Pacific storms are the primary source of water for the Inland Temperate Rainforest, only augmented in late spring (May/June) by region-wide convective thunderstorms. Like Oregon and Washington, late summer is characterized by long droughts, heat, and wildfire.
But this lower elevation “rainforest” still only receives 30-40 inches of precipitation per year, which is well below the traditional 55 inch threshold for temperate rainforest conditions. Seattle sees as much rain as most sites out here, so why are we calling this a rainforest at all? The key to this problem is one advantage that the forests of North Idaho have over similar elevations in coastal regions: long-lasting snowpack. Isolation from temperature-moderated coastal air and greater influence from Arctic air masses allows Pacific winter storms to cool enough by the time they reach North Idaho that much of the precipitation falls in the form of snow, which accumulates into snowpack that lasts deep into spring, gradually releasing water into the environment, that, in turn, allows for many coastal rainforest species to grow in much statistically drier sites. The abundance of coastal lichen species once supported populations of mountain caribou through the winter. This unique situation also grants the Inland Temperate Rainforest the unique title as the only snow-dominant rainforest in the world! Yet, on the other hand, the region is still largely shielded from brutal winter temperatures by dense mountain ranges west of the Continental Divide; Large river valleys and major lakes help maintain humidity and moderate temperatures, further prolonging moisture in the forest.
Plants and Forest Ecology
One iconic feature of rainforests is their many-layered forest structure, reflecting excessive water that provides for not only a canopy of trees and also plants on the forest floor, but even an understory of minor tree species and large shrubs. Although large sections of North Idaho are dominated by our wet forest trees, a much smaller portion is actually multilayered, featuring a canopy of Western Redcedar (Thuja plicata) and Western Hemlock (Tsuga heterophylla), an understory of Devil’s Club (Oplopanax horridus) and Pacific Yew (Taxus brevifolia), and lush groundcover. At higher elevations, these are usually replaced by a canopy of Mountain Hemlock (Tsuga mertensiana), and an understory of False Huckleberry (Rhododendron menziesii) and Greene’s Mountain-ash (Sorbus scopulina). It is in these stands, where the understory layer is present, that the title of rainforest is most fitting, supporting a wide variety of animals, from moose and bears to millipedes and salamanders. It is also worth noting here that this land houses an abundance of riparian and wetland habitats, including beaver ponds and bogs with floating mats of Sphagnum moss.
So far, we have reviewed similarities between this Inland Temperate Rainforest and the Pacific temperate rainforests, but what sets them apart? What are the unique characters of this insulated land?
The Inland Temperate Rainforest does contain some unique species, such as Idaho Goldthread (Coptis occidentalis) and Coeur d’Alene Salamander (Plethodon idahoensis), but most of the forest’s prominent species are borrowed from other places, creating an ecological intersection between the Rocky Mountains, the Pacific Northwest, and even the Great Lakes region! The coastal disjuncts are well known, but other species like Spinulose Wood Fern (Dryopteris carthusiana), Dwarf Raspberry (Rubus pubescens), and Creeping Snowberry (Gaultheria hispidula) are disjunct from the northeastern U.S. and Canada, and are absent from coastal rainforests. The result, especially farther north, is a truly dynamic mixture. High-elevations and arid south-facing slopes favor Rocky Mountain species, quickly shifting on north aspects or rainy basins into the Pacific Northwest, then blending into humid wetlands where the Great Lakes sneak in.
Conifer diversity is also high in the region, including two trees with core habitat in the Inland Temperate Rainforest: Western Larch (Larix occidentalis) and Western White Pine (Pinus monticola). These seral species are important to regenerating sites after wildfire or timber harvest, preparing them for the return of redcedar and hemlock. One mixed stand near Priest Lake hosts a mosaic of Pinus ponderosa, Pinus contorta, Pinus monticola, Pseudotsuga menziesii, Larix occidentalis, Tsuga heterophylla, Thuja plicata, Picea engelmannii, Abies grandis, and Abies lasiocarpa in near even co-occurrence!
The combination of blended climates, varied terrain, multi-regional plant ecology, and differing fire frequency creates a landscape that changes rapidly between elevation, slope, and basin arrangement. This causes much of the Inland Temperate Rainforest to be fragmented, most common in patches on north aspect slopes, strips along sheltered canyon streams, or vast swathes surrounding our large lakes. Unlike its coastal equivalents, distance between rainforest old growth stands are often due to habitat limitation, rather than human intervention, although logging has exacerbated fragmentation and greatly reduced acreages. Understandably, effective forest management practices here are also complex and may have extremely site-specific prescriptions.
Closing Thoughts
The Inland Temperate Rainforest remains a hidden gem. If you live nearby, I highly recommend taking the time to experience and enjoy it, from backpacking the wilderness of the Upper Priest River, to driving up Beauty Creek near Coeur d’Alene. Obscurity is both a strength and a weakness: It evades many of the strains and stains from overuse seen in other parts of the scenic U.S.; However, it also leaves many irreplaceable sites unaccounted for and vulnerable to damage or destruction.
We still have so much to learn about this place, so I think it is appropriate to leave on a mystery. Pictured is what I call the ‘ghost fern’ phenomenon, which occurs every October in our North Idaho rainforest. Deciduous ferns that reside in shade turn into blankets of white en masse before they die back for the winter. I hypothesize that deep shade removes the need to produce pigments that would otherwise be exposed in autumn, and our region’s cold weather precedes the excessive rainfall that causes wilting. But that’s just a theory, among a multitude of secrets still guarded by this wild land.