Unique Biodiversity Features of the Northern Okanogan RangeThis Web page is a report detailing some of the unique features of biodiversity of this ecological province.
Prepared by George Wooten, November, 1994
for the Scientific Plant Panel,
Interior Columbia River Basin Ecosystem Management Project,
Eastern North Cascades Province
This report has been prepared to help provide plant geographic information about the region of Okanogan County called Horseshoe Basin, on the Canadian border, in the Okanogan Range, at the eastern edge of the North Cascades.
Continentally glaciated character
The area is characterized by high-elevation topography with gentle slopes. The low relief contrasts with typical areas in the North Cascades which are generally highly dissected by alpine and valley glaciation. Presumably, the final phase of glaciation in Horseshoe Basin was the melting of the terminus of the Cordilleran ice sheet, which rounded most of the summits and cirques produced in previous alpine glacier phases (Waitt, 1972). The region thus lies just north of the margin of continental glaciation that can be traced in moraines across North America. Across the continent, this narrow strip of land is characterized anomalous terrain features; glacial erratics; sandy deposits of outwash gravels and boulders; and ice-marginal channels carved by ice and/or water (E.C. Pielou, 1991).
The extent of the continental glaciation can be analyzed by numerous techniques, for example by mapping the degree of roundedness of summits within a local area. There is a rough correlation between the continentally glaciated areas and the area occupied by broad summits (as opposed to pointed summits).
Nunataks as refugia
Nunataks are peaks that protruded from the ice sheet, and which may have offered refugia during the continental glaciation. Because of the great depth of continental ice sheets compared to alpine glaciers, nunataks are believed to have been ice-free refugia for plants and animals. Although such nunataks today are bleak and barren rocky areas of the far north, there is good evidence that the southern terminus and coastal glaciers lay adjacent to viable and thriving plant and animal communities (Dahl, 1946). Peaks believed to have been nunataks have been mapped in the eastern North Cascades (Waitt, 1972). The ice sheet did not go south of the Chelan valley glacier, and was considerably broken up in the Chelan-Sawtooth Range, but only Remmel and Cathedral Peaks, near Horseshoe Basin and the Canadian border, appear sharp enough to have been nunataks for much of the area northward. Evidence for the occurrence of refugia on nunataks is found in the sensitive plant Polemonium viscosum, an alpine plant common further southward in the Sierras, but which occurs in this area only on taller peaks, e. g., Remmel Peak, Burch Mtn, and McLeod Mtn, which are all sharp, alpine- glaciated peaks in this area. If these nunataks were refugia for hot-dry species (Packer and Vitt, 1974) during the last alpine glacial phase of the North Cascades ca. 18,000 yrs bp, the corollary cold-wet refugia during the current interglacial period would be those areas like Horseshoe Basin that still possess a cold, wet climate, and have not yet been overrun by Sierran species.
Contrasting with the nunataks are the rolling Salix/Carex wetlands occupying the depressions of Horseshoe Basin. These are relict boreal ecosystems that still retain much of the character that they developed in the wake of receding ice. They occupy streams and pockets at high elevation, where perhaps because of the low energy gradients, the soil rarely dries out. Adding to the wetness are severe summer thunderstorms which travel up the Chewuch River, ultimately to fall at Horseshoe Basin, and which have given the Chewuch the nickname "fire alley".
Although similar habitats are commoner to the north and west in the Rockies, the North Cascades Range does not continue extensively north of Cathedral Park (Mackinnon et. al., 1992, map on p. 12-13). The northeastern Pasayten Wilderness and the entire Okanogan Range were completely covered by the continental (Cordilleran) glacier. Because the wet, cold conditions of glaciated regions are relatively intact here, these boreal meadow complexes represent a disjunct ecological province of unmatched biological diversity. The unusual nature of this boreal province is documented in this report. What is enigmatic is how it has managed to remain intact.
The boreal province
Partly to confirm or deny the contention that the extent of continental glaciation can be marked by relict ecosystems, I accompanied geologist and Salix specialist Al Hanners in the summer of 1994 on an east-west vegetation transect between Long Swamp and Remmel Lake. The North Cascades is dominated primarily by the tall willows Salix sitchensis, S. barclayi and S. commutata, while Horseshoe Basin and Long Swamp nearby are known to have the willows S. phyllicifolia, S. tweedyi, S. drummondii, S. boothii and S. farriae, (of these only S. drummondii is a full-sized shrub). It was Hanners' idea that there might be a distinct boundary between the alpine and continental provinces. Although we found the westernmost range of S. farriae, and the easternmost of S. barclayi, it was apparent that we had not entered the core of the North Cascades province at Remmel Lake. Returning to continue the transect later in the year, Hanners apparently found a distinct vegetation boundary in west flowing Drake Creek, which has S. phyllicifolia and S. tweedyi in the headwaters and S. sitchensis, S. commutata and S. barclayi at the mouth along the Lost River Gorge.
Other unique characteristics of the boreal zone of the Okanogan Range are its preponderance of Engelmann spruce an lodgepole and the near total absence of subalpine fir. Over 90% of the trees are either lodgepole or spruce and subalpine fir is only found on sloping areas. Both lodgepole and spruce exist in open meadow parklands and lower in elevation as continuous forest, however the continuous lodgepole is seral to spruce. Also occurring only in this area of the North Cascades, are low mounds spaced evenly in wet meadows. The appear to be related to a layer of volcanic ash, as well as the former presence of permafrost (Scotter and Zoltai, 1992).
In passing from Remmel area towards Billy Goat Pass near the headwaters at Drake Creek, one crosses into the Tyaughton trough of Jeletzky and Tipper (1968), also known as the "Methow Graben", south of the border. Rocks along the contact at Billy Goat Pass are hard andesites, and may be more resistant to glacial action. They weather to different chemical and textural types. In any case, there is a geologic and soils boundary as coincident with the glacial boundary in this area of the North Cascades, and it is logical that plant discontinuities would be even sharper along this margin. What is noteworthy is that we have both the southernmost boreal relict province in the North Cascades adjacent to the northeasternmost Sierran refugium, and overlaid on this is a significant geologic boundary.
Plant migration. Ecological inertia
All this does not tell us whether these two ecosystems can continue to persist so close together. That can only be guessed at by extrapolating from palynological, fossil, geological, and phytogeographical histories of the last 12,500 years. In this respect two facts are pertinent. Recent studies indicate that treeline is reinvading territory it lost at the end of the hypsithermal period from ca 8.5 k b.p. to about 5 k b.p. This is apparently happening at an accelerated rate in the center of treeless, logless areas of Horseshoe Basin as indicated by the recent invasion of many square miles of Salix/Carex bogs by spruce-lodgepole forest younger than 100 years old. Unlike documented invasions by subalpine fir and mountain hemlock (Franklin et. al., 1971), this is occurring in wet meadows with Engelmann spruce. The rapid invasion of these large meadows occurred during the early part of the century, as evidenced by the evenness of the stand ages. This indicates that a disturbance event may have initiated the change, for example, drought or overgrazing.
Although the background rate of change has been slow to occur over the past 500 years, the combination of global warming, drought, wildfire, weed invasion and ungulate grazing and trampling are beginning to unravel the Horseshoe Basin ecosystem. The change of meadows to spruce forest in the Sunny Pass area of the Basin represents a raising of timberline by several hundred feet over its present elevation.
Some degree of the rate of change can be inferred by the degree of influx of alien species. Our studies over five years have developed a thorough list containing approximately 311 plant species. Of these 36 (11%) are introduced European plant species. An additional 7 possible introductions or recent arrivals or non-local natives could raise the total to 43 out of 311 or 14%. This rate of change is extreme, and represents a serious threat to ecosystem integrity. Two of the introduced plants have attained dominant status in some of the communities they have invaded, Poa pratensis and Trifolium repens. The change from one vegetative community to another can be measured by the number of species it takes to change the composition by one-half. The change in vegetative composition exhibited here indicates major community changes have already occurred and will continue to occur, since many of the weeds are being observed at the beginning of their influx, due to increased use of the area by stock using hay mixtures from west of the Cascades. A list of the weedy species is included in this report.
While composing the flora of Horseshoe Basin, it was observed that many common plants of the North Cascades are lacking in Horseshoe Basin, often being replaced by allopatric members of the same genus. For instance, Cassiope tetragona is relatively common at high elevation in Horseshoe Basin, but not elsewhere in the North Cascades; Cassiope mertensiana is common throughout the North Cascades at high elevation, but is absent in Horseshoe Basin, except for one plant on the Iron Gate Trail that appears to be recently introduced. In order to assess the extent of influx of typical North Cascades plants into the area of Horseshoe Basin, another transect was designed to assess the first occurrence of these plants in going from east to east. This represents to establish the extent of absence of certain species, and is difficult to measure without a rather complete flora.
For approximately ten miles, the only floristic change noted was a higher representation by subalpine fir, Abies lasiocarpa, and the occurrence of two plants of Anemone occidentalis and a few Phlox diffusa on the trail southwest of Scheelite Lake. In passing the Tungsten Mine, about ten miles from Horseshoe Basin, the following plants were noted for the first time: Spiraea betulifolia, Pachystima myrsinites. In the next half mile we passed through upper Cathedral Creek, where the fire of the late 1920's burned very hot, as evidenced by the loss of lichen cover on all the bedrock, and the replacement of overstory trees by regenerative stems. In crossing the creek, the aspect changed from west to east and the following species were noted for the first time, all within 50 feet distance of each other: Polemonium pulcherrimum, Cirsium edule, Anemone occidentalis, Vaccinium membranaceum, Veronica cusickii, Lycopodium sitchense, Luetkia pectinata, Cassiope mertensiana. Within the next quarter mile, during which we walked below Cathedral Peak, these additional new occurrences were noted: Phlox diffusa, Festuca brachyphylla (F. ovina var. breviflora), Arnica latifolia. Finally, a mile on the other side of Cathedral Peak, we found the first occurrences of Erythronium grandiflorum, Castilleja parviflora var. oreopola (white form), and a Ligusticum, either L. grayi or L. canbyi. Since we covered over ten miles with essentially no change in the species composition, and then encountered 14 new species (4% of the approximate 311 occurring in Horseshoe Basin) all in the space of a mile, there must be a reason for the simultaneous changes in many species. It is possible that Cathedral Peak served as a refugium for these Sierran species during the last ice age, but that their establishment in the boreal zone is retarded until a soil-disturbing event occurs. Such distrubances could occur, as in this instance, in an area scorched by fire, or alternatively the event could be predisposed by heavy grazing pressures, and/or global climate warming. Further floristic work on the distribution of all of these plants, as well as their fossil history may lead to further knowledge of this theory.
- Dahl, E. 1946. On different types of unglaciated areas during the ice ages and their significance to phytogeography, New Phytologist 45:225-42.
- Franklin, J. F., W. H. Moir, G. W. Douglas, and C. Wiberg, 1971. Invasion of subalpine meadows by trees in the Cascade Range, Washington and Oregon, Arctic and Alpine Res. 3:215- 224.
- Hitchcock, C. L., and A. Cronquist (1973). Flora of the Pacific Northwest. University of Washington Press, Seattle, WA.
- Jeletzky, J. A. and H. W. Tipper, 1968. Upper Jurassic and Cretaceous rocks of Taseko Lakes map-area and their bearing on the geological history of southwestern British Columbia, Geological Survey of Canada Paper 67-54, 219 pp., 3 pl.
- Mackinnon, A., J. Pojar, R. Coupé, 1992. Plants of Northern British Columbia, B. C. Ministry of Forests, Lone Pine Publishing.
- Packer, J. G., and D. H. Vitt, 1974. Mountain Park: a plant refugium in the Canadian Rocky Mountains, Can. J. Bot. 52:1393-1409.
- Pielou, E. C., 1991. After the Ice Age. The Return of Life to Glaciated North America, Univ. of Chicago Press, Chicago.
- Scotter, G. W., and S. C. Zoltai, 1992. Earth hummocks in the Sunshine area of the Rocky Mountains, Alberta and British Columbia, Arctic 35: 411-416.
- Waitt, R. B., 1972. Geomorphology and glacial geology of the Methow drainage basin, Eastern North Cascade Range, Washington, Ph.D. Thesis on file at University of Washington, Seattle, WA.