Ecology of The North Cascades - Subalpine Mountain Hemlock Ecoregion

Subalpine Mountain Hemlock Ecoregion

Moving up in elevation from the Silver Fir ecoregion, between about 4,000 to 5,400 feet (1,200 to 1,600 m) on the west side of the Cascade Range, the ecoregion shifts to the Subalpine-Mountain Hemlock Ecoregion. The Silver Fir ecoregion has a colder annual average temperature of 39 °F (4 °C) with average annual precipitation between 160 to 280 centimetres (60 to 110 in). This ecoregion consists of Mountain Hemlock forests, subalpine meadows, streams, lakes, wetlands and avalanche gullies creating distinct patterns of new succession. In the lower ends of this ecoregion there are continuous closed canopy forests while the higher reaches will see mosaics of meadows with patches of Mountain Hemlock forests. The Mountain Hemlock forests consist of Mountain Hemlock, Subalpine fir, Alaska yellow-cedar, and Silver Fir. Washington’s alpine and subalpine areas account for about 4.4% of its total land area.

Progressing upward from the gradient of Silver Fir and Mountain Hemlock Ecoregions, the mountain hemlock (Tsuga mertensiana) tends to become the dominant conifer, although it may codominate with the Alaska cedar and Pacific silver fir. Mountain hemlock trees live as long as 1000 years: longer than the Pacific silver fir. Trees between 500–700 years may be 100 to 125 feet (30 to 40 m) tall. Hemlock cones are about 2–3 inches (5–8 cm) in length and develop at the ends of branches. These conifers are easy to distinguish amongst the others with their dense grayish-green needles. According to Franklin and Dyrness, the understory where the mountain hemlock and Pacific silver fir co-dominate is dominated by tall mountain huckleberry; where the Alaska cedar dominates, the understory is dominated by dense collage of rhododendron, huckleberry and mountain ash.

In the higher boundaries of subalpine ecosystem, where the abiotic conditions are more stressful, trees are clumped together in patchy islands. Trees in this area can be recognized by its krummholz form. Trees of this upper boundary will take on a flag appearance with branches extending from one side indicating the prevailing wind directions. The skirt height of the trees is indicative of the height of snow cover where branches tend not to grow. There are various reasons as to why these trees take this form. Strong winds combined with ice particles will cause abrasion scouring the waxy cuticle from one side of the tree creating damage that will prevent branch formation and growth. In addition, the wind will cause desiccation and evaporation in the needles causing branches to die on this side of the trees.

Scattered amongst the island patches of trees in the upper boundaries are parkland areas with showy meadows. Events such as fire, avalanches, snow slumping and climate change make the boundaries of these areas and the balance of trees and meadows dynamic. The forest islands typically consist of mountain hemlock, subalpine fir, and Alaska cedar. Often there are invasions of trees into meadow areas and this reached a peak in the 1930s due to considerable warming. Invasions of meadows by trees can also occur with disturbances.

The beauty of meadows is very popular amongst hikers. Wildflowers that are found in this ecoregion are the tiger lily, glacier lily, bead lily, queen's cup, columbine, aster, trillium, pearly everlasting, valerian, skyrocket, shooting star, penstemon, lousewort, mountain bog gentian, monkey flower, monkshood, bluebell, bellflower, bleeding heart, Tweedy's lewisia, balsamroot, wild orchids and more. The wildflowers are at their peak in the meadows and along streams from mid-July to mid-August. The parklands of the Mountain Hemlock Ecoregion draw their distinct characteristics from the climate and topography.

The two dominant vegetation types of this mountain region, forests and meadows, have very distinct differences in their microclimates. The amount of solar radiation and UV exposure can vary substantially in our northern latitude largely based on the time of day, slope, season, cloud cover and vegetation. Temperatures adjust accordingly to this solar radiation and exposure. The parklands of this montane region have a much larger range of temperatures as compared to the hemlock forests. The range can be as much as 50 °F (25 °C) while the range in the forests rarely exceeds 20 °F (10 °C). This is due to the canopy of the trees, creating a much more protected environment as compared to the open meadows. In addition soil temperatures directly impact biological activity affecting soil organisms and root systems. Daily and season temperature changes greatly affect the soils' heat loss and gain. However, the snowpack acts as an insulating buffer against temperature change in soils. The Mountain Hemlock forests are the wettest and coldest of the Cascade forest zones.

In the richness of this region many animal species pass through this zone at least one season a year such as mountain goat, black-tailed deer, American black bear, elk, cougar, and many bird species. Only the whistling hoary marmot is restricted to alpine and subalpine areas. Besides the richness of mammals there is a richness of insects that are integral to the abundance of flowering plant species in this area. Another important pollinator in this area is the hummingbird. There is still much to be researched and discovered to better understand species interactions and reliance in both the alpine and subalpine ecosystems.