A Transition Coniferous Forest in the Cascade Mountains of Northern Oregon

Abstract

A 29 km transect was established in northern Oregon from near the summit of the High Cascades, eastward across Green Ridge, to the W rim of Fly Creek canyon. The gradual eastward slopes descend in the Cascades from 1770 to 825 m, andc on Green Ridge from 1370 to 915 m. The area is geologically and physiographically simple. Precipitation averages over 150 cm annually in the W, 50 cm or less in the E. This transition region possesses an unusually rich coniferous flora, with 19 species on or near the transect. The subalpine forest on the upper slopes of the High Cascades includes Tsuga mertensiana, Abtes lasiocarpa, Pinus albicaulis, Pinus contorta, Abies amabilis and Juniperus communis var. saxatilis. The middle elevations of the Cascades and the crest of Green Ridge have forests of Pinus ponderosa, Abies grandis, Pseudotsuga. menziesii, Larix occidentalis, Picea engelmannii and Taxus brevifolia. Found on the Cascades slope, but absent from Green Ridge, are Pinus monticola, Tsuga heterophylla and Abies procera. Pinus ponderosa, Pseudotsuga menzzesii and Libocedrus decurrens are common to the lower altitudes on both the Cascades and Green Ridge slopes. The more xeric Green Ridge lowerslope also has Juniperus occidentalis in quantity. In isolated, well-watered sites at low elevations a few Pinus lambertiana and Thuja plicata grow. High elevation forests appear relatively stable in composition with the slow-growing T. mertensiana, A. lasiocarpa and possibly A. amabilis gradually increasing in proportion. Between 1050 and 1500 m on the Cascades, and between 1200 and 1370 m on Green Ridge, A. grandis is replacing the shade intolerant Pinus ponderosa, Pseudotsuga menziesii and Pinus contorta. L. decurrens is replacing Pinus ponderosa as the predominant species throughout much of the lower elevations. Only on the driest locations on the Cascades slope is regeneration adequate for P. ponderosa replacement. The lower eastern slopes of Green Ridge now appear to have a stable forest composition.