The Climate of the H J Andrews Experimental Forest

Located at latitude 44.2° N and longitude 122.2° W, the Andrews Forest is situated in the western Cascade Range of Oregon in the 6400-ha (15,800-acre) drainage basin of Lookout Creek, a tributary of the Blue River and the McKenzie River (figure 19.1). Elevation ranges from 410 m (1350 feet) to 1630 m (5340 feet). Broadly representative of the rugged mountainous landscape of the Pacific Northwest (PNW), the Andrews Forest contains excellent examples of the region's conifer forests and associated wildlife and stream ecosystems. Lower elevation forests are dominated by Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga het-erophylla), and western red cedar (Thuja plicata). Upper elevation forests contain noble fir (Abies procera), Pacific silver fir (Abies amabilis), Douglas-fir, and western hemlock. Low- and midelevation forests in this area are among the tallest and most productive in the world. As elevation increases, Douglas-fir and western red cedar decline in importance and western hemlock is gradually replaced by Pacific silver fir.

The climate is controlled by its close midlatitude proximity to the Pacific Ocean and by the perpendicular orientation of the Coast and Cascade mountain ranges to the prevailing westerly flow. The Andrews Forest is located near the border between temperate maritime and temperate continental climates as a result of these mountain barriers to passage of air masses from the west. Temperatures are moderated at almost all times of the year by maritime air, particularly in winter.

Winter precipitation is high, averaging 287 mm (11.3 in.) per month between January and March. Low-pressure areas and associated storms are steered into the area by the polar jet stream. Long-duration but generally low-intensity storms result from the passage of strongly occluded fronts that are slowed by the mountains. Daily precipitation is significantly autocorrelated up to 14 days (Post and Jones 2001). Temperatures associated with these storms are often mild enough that rain falls at lower elevations of the Andrews Forest while snow falls at higher elevations. This usually results in deep (2 to 4 m), long-lasting, snowpacks above approximately 1000 m. Occasional strong storms can have severe ecological consequences such as windthrow—the toppling of trees by the force of the wind. Late summer and early fall wind from the central Oregon desert may also drive large forest fires. Summertime precipitation is usually low to nonexistent, averaging 38 mm (1.5 in.) per month between June and August. The North Pacific anticyclone intensifies and expands to the northeast along the coast. This blocks the passage of cyclonic storms and stabilizes the air.

Summer drought, mild wet winters, a heavy snowpack above 1000 m, and light to nonexistent snowpack below 800 m are factors affecting the flora and fauna. Late summer moisture stress of the forest has an important part in determining the composition and structure of various forest communities. This moisture stress also helps to give rise to the coniferous nature of the Pacific Northwest forest (Waring and Franklin 1979). Snow and lower temperatures at upper elevations play an important role in the formation of a distinctly different forest zone—the Pacific silver fir (Abies amabilis Dougl. ex Forbes)—through mechanical force and modification of temperature and moisture regimes. Large animals, such as elk and deer, are forced to lower elevations by the heavy upper elevation snowpack, whereas smaller animals use it for shelter and cover. At lower elevations, the mildness and wetness of the winters, combined with little snow, produces a nearly stress-free environment for plants and animals. The mild climate also results in a long growing season. Water use by evapotranspiration in the old growth forests is greatest during the spring and fall and is limited by the low precipitation of the summer months.

Superimposed on this general picture is considerable temporal variability in the climate. At a daily scale there can be severe storms. The El Niño-Southern Oscillation (ENSO) operates at a 2-7 year (quasi-quintennial) scale and provides a context for warmer and drier (El Niño) or cooler and wetter (La Niña) conditions. The Pacific Decadal Oscillation (PDO) functions at a multidecadal timescale that is also characterized by warmer and drier or cooler and wetter periods. Evidence exists for similar climate variability at century, subcentury, and millennial timescales, and these signals have varied in strength over time. There is also the possibility of climatic trends at century and longer timescales. Change is one of the few certainties in the dynamic environment of the Pacific Northwest.

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