ESC 401 Spring

Background: The Olympic and Cascade Mountains rise steeply from near sea level to ca. 2500 m, creating strong gradients in temperature and precipitation over short distances. The form of precipitation changes from predominantly rain below about 1000 m to predominantly snow at higher elevations. Even at the same elevation, environmental conditions are quite different on slopes facing different directions. These environmental gradients have a strong influence on the species composition and physical characteristics of forests which can be seen at scales ranging from landscape to stand to tree. Over the quarter, we will visit three sites with distinct climates, forest types and disturbance histories. First, we will visit a low elevation western hemlock forest typical of the western Cascades (This year: Rattlesnake Ridge; Previous: Lake Tradition). Then, we will continue up to the transition between the low elevation western hemlock forest type and the mid-elevation, montane or Pacific silver fir forest type (Asahel Curtis) and the lower elevation of the mountain hemlock forest type (Snoqualmie Pass). Finally, we will go to the eastside of the Cascades and view forests from mid-elevation to tree line (North Fork of the Teanaway). First we will explore the diversity of environments and then we will examine the different forest types.

Environmental Variation: Two physiographic units dominate the climate of the area: the Pacific Ocean and the Olympic and Cascade Mountain Ranges (Figure 1). The Cascade Mountains are a recent (Miocene) system of moderately high, rugged peaks crossing central Washington in a north-south direction and averaging about 60 miles in width. To the west of this range lies the 60-mile-wide Puget Sound Lowland, a flat, rolling, glaciated plain whose western edge abuts the east-facing ramparts of the Olympic Mountains. These, in turn, continue west for another 65 miles, finally dropping gradually into the Pacific. East of the Cascades, the vast, sprawling Columbia Plateau extends approximately 180 miles to the foothills of the Rockies. Climatic variability presents itself both spatially (see Figure 2 below) and temporally, particularly in the quantity of snow, the depth of the snow pack and the duration of the snow pack (see Temporal Variation)


Figure 1. Important physiographic features of western Washington.	Figure 2. Climate map of Washington.

Climate: Because of its proximity to the Pacific Ocean, western Washington receives prevailing southwest winds that bring large quantities of warm, moist air. This air is pushed inland around and over the Olympics and Cascades. Adiabatic cooling in the rising air masses triggers the release precipitation, which saturates the west-facing valleys of the Cascades and Olympics. Passing over the summits, these air masses are largely depleted of their original moisture and are heated adiabatically as they continue downward and eastward. The resulting warm, dry air masses slip down the northeastern and eastern Olympic and Cascade valleys. Thus, each of these ranges produces a rain shadow on its leeward side.

Were the Olympics to extend farther south, they might intercept most of the moisture sweeping in from the Pacific Ocean. However, they are a relatively low, confined range and offset to the north, permitting the wet oceanic winds to intercept the entire western flank of the Cascades. Thus, the Puget Sound Lowland was moderate amounts of precipitaiton (35 inches or rainfall annually), and the heads of many of the west-facing valleys of the Cascades have more than 100 inches.

In addition to influencing the distribution and quantity of rainfall, the Cascades also significantly modify regional temperatures. Acting as a physical barrier to the western maritime air, the Cascades effectively divide the state into a western maritime and eastern continental region: with cool summers and warm-wet winters in the west and cold winters, and hot-dry summers in the east. Furthermore, the climate of the mountain system reflects the effects of elevation. In the western Cascades particularly, snowpacks increase dramatically with elevation and the boundaries between major forest zones correspond to changes in the length of the snow-free period.

Forest zones and zonation: Few areas in North America contain a system of biomes as diverse as those found in Washington state. A 200-mile transect drawn east from the Pacific Coast traverses temperate zone rain forests, northern boreal forests, subarctic and arctic tundra, dry mixed-confer forests, shrublands, grasslands and desert. The ready accessibility of this array of floristic and climatic regions presents ecologists with a field laboratory of extraordinary quality and diversity. Vegetation is profoundly influenced by climate and soils. Soils, in turn, reflect the climatic conditions and vegetation under which they developed. Thus, one would expect the characteristics of vegetation, soils and climate to be strongly correlated. Indeed, this is the case in Washington as elsewhere. The two broadest vegetational regions are related to large-scale patterns in precipitation (both rain and snow): one region lies in the more mesic climate west of the Cascades and the other in more xeric climate east of the Cascades.

Figure 3. Vegetation zonation of the state of Washington

Within these very broad regions are several forest zones, which primarily reflect elevational (thus, temperature, moisture) gradients. The most useful scheme of classifying and describing this vegetational complex is described in Natural Vegetation of Oregon and Washington by J.F. Franklin and C.T. Dyrness, Oregon State University Press. In this scheme, forest zones are named for the tree species that would dominate forests if no disturbances occurred for long periods of time. Thus, the names usually correspond to the most shade-tolerant species in the forest, which is often not the most abundant species in current stands. The zones begin in the Puget Sound, go over the Cascades (mountain hemlock on the westside and subalpine fir on the eastside) and down towards the Columbia River.

Western Hemlock Zone. This zone extends from sea level to about 2,500 feet on the west slopes of the Cascades. Climates are mild (wet winters and dry summers), allowing this zone to support some of the largest conifer forests in the world. Early successional forests are predominantly even-aged Douglas-fir and red alder, with Sitka spruce sometimes in wet valley bottoms. Later stages of forest development (old growth) are dominated by shade-tolerant western hemlock and western redcedar. Douglas-fir trees are prominent members of these stands, however, often reaching 6 to 8 feet in diameter and 400-600 years in age. These forests are well known for their structural diversity: a wide range of tree diameters and heights, patchy vertical distribution of living foliage (ground level to nearly 300 feet), abundant dead tree snags and downed logs. The unique habitat that these stands provide for wildlife is due largely to their structural diversity. Hardwoods such as bigleaf maple, red alder, and black cottonwood are common in bottom lands, along water courses and following logging. Lodgepole pine, ponderosa pine, and Oregon white oak occur sporadically in the Puget Sound Lowland on prairie and glacial outwash soils. Occasional grand fir and western white pine are found on poorer sites. Pacific madrone is a frequent inhabitant of coarse, poor soils throughout the lower portion of the zone. Common forest understory species are salal, Oregon grape, vine maple, sword fern blackberry, and blueberry	Pacific Silver Fir Zone. This is a mountain forest zone dominated by true fir and hemlock. It is the largest zone of uncut forests in the state, extending from approximately 2,500 to 4,500 feet in elevation. The climate is more severe than in the Western Hemlock Zone, but quite mild compared to other boreal areas in the northern hemisphere. Winters are long, wet, snowy and fairly warm, with temperatures seldom dropping below 10°F. Summers begin late and are relatively warm and dry, with snow often returning by late October. The typical young forest is a mixture of Douglas-fir, noble fir, western hemlock and western white pine, with noble fir or western hemlock often dominating the canopy. Lower elevation, old-growth forests are commonly mixtures of Pacific silver fir and western hemlock. Western hemlock is replaced by mountain hemlock at higher elevations. Western white pine is scattered throughout the zone but never occurs in pure stands. Alaska yellow cedar, subalpine fir, and Engelmann spruce are often found on the colder, drier sites. Common shrub and herb species are mountain ash, beargrass, bracken, bear-berry, lupine, vine maple, Sitka alder, and several blueberries.
Mountain Hemlock Zone. This zone extends from the upper limits of the Pacific Silver Fir Zone to timberline. It is dominated by subalpine meadows with intermittent clumps of trees. The clumps are composed of subalpine fir, whitebark pine, mountain hemlock, Pacific silver fir, and occasionally Alaska yellow cedar (now Xanthocyparis nootkatensis, see below). The long, severe winters with bitter cold, high winds and heavy snowpacks in concert with short, but hot and droughty summers undoubtedly have a profound effect on germination and survival of seeds. These climatic factors and their interactions with the diverse mountain topography are responsible for the distinctive vegetation patterns of this zone. Recent ecological studies have suggested the following successional pattern for this zone. A nucleus of trees composed of one or more whitebark pine or subalpine fir seedlings begins to grow in a meadow from a bird or rodent seed cache. In addition, tree seeds may establish on elevated surfaces or south-facing slopes where snow melts earliest in the summer or late spring. As the nucleus of trees grows, its presence on the site modifies the microclimate sufficiently to allow its seeds or those of more tolerant species to survive and grow, gradually forming a substantial shelter from wind, snow, and high summer temperatures. As the colony develops, the tolerant mountain hemlock and Pacific silver fir gradually replace the colonizing species. Common shrub and herb species of meadows include avalanche fawnlily, paintedcups, penstemons, blueberries and others.	Subalpine Fir Zone: The Subalpine Fir Zone is the highest forest zone in most parts of the east Cascades. Near timberline (approximately 5500 feet in elevations) subalpine fir is restricted to forest patches or tree clumps on favorable microsites. As in the west Cascades, established trees create favorable microsites that allow the growth of new trees at the periphery of clumps. Winters and summers are cold compared to lower elevation forests, although drought stress can limit tree growth and seedling establishment on south-facing slopes. Engelmann spruce and lodgepole pine are other cold- and drought-tolerant species commonly found in theses forests. Fires are generally rare and stand-replacing events. Common shrubs and herbs include blue berry, beargrass, heather, hardhack, bedstraw.
Grand Fir Zone: This zone is most extensive in central and southern portions of the east Cascade Mountains, where it extends from approximately 3000 to 4500 feet in elevation. Climatic conditions are moderate. Summer droughts do not severely limit tree growth and snow accumulations are not deep enough to affect tree establishment on most sites. Douglas-fir and western larch are components of most stands in this zone, particularly in early stages of stand development. Fires are infrequent, but typically intense and kill most trees. Grand fir has thin bark and easily killed by even low-intensity fires. The area covered by grand fir forests has probably increased greatly during the 20th century as a result of fire exclusion, which favors grand fir (shade tolerant, fire sensitive) over other species such as ponderosa pine, Douglas-fir, and western larch (shade intolerant, fire resistant). Common shrub and herb species include boxwood, rose, blueberry, prince’s pine, lupine and others.	Ponderosa Pine Zone: This is the lowest forest zone in the eastern Cascades, typically forming a narrow band of open stands from approximately1800 to 3000 feet in elevation. The climate of this zone is characterized by a short growing season resulting from hot temperatures and low summer precipitation. Total precipitation in July, August, and September averages less than 1 inch. Natural, old-growth stands of ponderosa pine are characterized by widely spaced trees, often reaching 4 feet in diameter, and sparse understory vegetation dominated by grasses. The openness of these stands is maintained by frequent, low intensity fires that kill young trees. The thick bark of ponderosa pine allows large trees and some young trees to survive most of these fires, but thinner-barked species such as grand fir and Douglas-fir are killed. Recent dendrochronological studies of fire scars on living trees and stumps have shown that the typical fire return intervals for these forests are 6-10 years. Fire-prevention practices since the early-to-mid 20th century have been very effective and have severely altered the composition and structure of these forests. Presently, most old stands of ponderosa pine are densely stocked with young Douglas-fir and grand fir. Black cottonwood and a variety of shrubs such as willows and red osier dogwood are common in riparian habitats. Common understory species in this zone are snowberry, snowbrush, bitterbrush, arrowroot, and grasses

Important Concepts

The individual organism: Each individual is unique, yet it shares many characteristics (e.g., flower morphology, fruit structure, phyllotaxy) with all other members of its species. Variation among trees, as in all organisms, is due primarily to the interaction of the genotype with the environment. Within the lifespan of an individual, the expression of the genotype is modified by the environment. Traits (e.g., leaf size, shape) reflect ways that environment modifies expression of genetic make up. DNA (genetic code) contains information for traits.

Populations: A population is an interbreeding group of individuals of the same species. If populations are separated by geographic barriers that stop gene flow between populations (migration), then populations can continue to diverge genetically. Eventually populations will become so different genetically that they can no longer interbreed; then 2 new species have been formed (or one diverged from central population (Pinus strobus and P. monticola are good examples)

Species: Species are the smallest groups that are structurally distinct and usually reproductively isolated from others. They are groups of individuals that share a large portion of their genetic material. The genetic similarity among individuals results from gene exchange in prior and current generations. New species arise under the following sequence of conditions: 1) the flow of genes between populations is cut off, 2) the separate populations experience different selective pressures, and eventually, 3) individuals cannot interbreed once the populations are rejoined. However, genetic separation is not always complete, and members of different species sometimes produce fertile offspring. All species are classified in a hierarchical scheme that is designed to reflect evolutionary relationships. Species are named according to strict rules.

However, molecular biology and new discoveries are creating some dynamic aspects to speciation and what genus, for example, a species belongs to. A wonderful, very recent example of this is the exciting discovery of a new conifer from northern Vietnam made a little over a year ago. This new species was also suspected to be the closest relative to our native Alaska yellow cedar. The formal scientific description of this new species, under the name Xanthocyparis vietnamensis, was published last summer in the journal Novon (Farjon et al. 2002). Previous work on the phylogeny of Cupressaceae had shown that Chamaecyparis nootkatensis is not closely related to the other species of Chamaecyparis, including C. lawsoniana (Gadek et al. 2002, Am. J. Bot). The Farjon et al. paper made use of the phylogenetic data available in the Gadek et al. publication to show that the new species from Vietnam is sister to C. nootkatensis. Thus, they also made the nomenclatural change transferring our native species to the genus Xanthocyparis. Thus, we should now begin using the taxonomically accepted name Xanthocyparis nootkatensis for Alaska yellow cedar in our teaching

Stands: A group of plants sharing similar history, site, composition and structure. Typical the smallest unit of management or classification.

Communities: Tree species are found in recurring combinations called forest communities. Each community is broadly associated with a set of physical conditions (e.g., climate, soil type) and/or disturbance regimes (e.g., fire, flooding). Even in constant environments, forest communities change gradually over an often predictable sequence of stages. The life-history characteristics of species comprising this sequence also change in a predictable pattern. These characteristics include attributes such as the amount of energy allocated to growth vs. reproduction, the numbers and sizes of seeds, and the time spent in juvenile and adult life stages. Because the amount of energy derived from photosynthesis is finite and less than that necessary to meet all of the possible life-history demands, energy allocated to one process is not available to another. Thus, different strategies of allocating limited resources are selected in early vs. late stages of succession.

Ecosystems: An ecosystem, a contraction of "ecological" and "system", refers to the collection of biotic and abiotic components and processes that comprise and govern the behavior of some defined subset of the biosphere. It is typically larger than a stand, but often has a similar size, space, structural scale. For example, the Wind River Canopy Crane is located in an old-growth Douglas-fir/western hemlock stand/ecosystem. Whereas one would focus heavily on composition and structure to describe a stand, one tends to focus on processes in ecosystems (carbon exchange, nutrient cycling, etc.). Elements of an ecosystem may include flora, fauna, lower life forms, water and soil.

References

Kimmins, J.P. 2004. Forest Ecology. Prentice Hall, New Jersey.
Kruckeberg, A.R. 1991. The Natural History of the Puget Sound Region. University of Washington Press.
Kruckeberg, A.R. 2002. Geology and Plant Life. The Effects of Land Forms and Rock Types on Plants. University of Washington Press. Seattle.
Mathews, Daniel. 1999. Cascade - Olympic Natural History, Raven Editions, Portland (Amazon.com)
Pojar, J. and A. MacKinnon (compiled and edited). 1994. Plants of the Pacific Northwest coast : Washington, Oregon, British Columbia & Alaska, written by Paul Alaback and nine others. Renton, Wash. Lone Pine Publishing, Vancouver, B.C. and Edmonton, Alberta: QK143.P53
Waring, R.H. and S.W. Running. 1998. Forest Ecosystems: Analysis at Multiple Scales. Academic Press, San Diego.

[College of Forest Resources]

[University of Washington]

CFR Template Version 0.2.0 APR-018-2003

ESRM 401, Spring 2010 Spring Comes to the Cascades Instructors: Tom Hinckley & Julie Combs

ESRM 401, Spring 2010
Spring Comes to the Cascades
Instructors: Tom Hinckley & Julie Combs