Structure of mature Douglas-fir stands in a western Oregon watershed and implications for interpretation of disturbance history and succession

Graduation date: 1986The structure of a mature Douglas-fir (Pseudotsuga menziesii)\ud forest in a watershed in the western Cascades of Oregon was examined.\ud Two age classes were detected in the stand, the oldest originating\ud about 1855 after an extensive fire and the younger following a second\ud fire about 1895 Although the trees in the older age class had statistically\ud greater diameters and heights, only open grown individuals\ud mixed with the younger age class could be readily distinguished B\ud cause reburns at young ages are common and may not leave firescars,\ud great care is be required to distinguish between slow regeneration and\ud patchy reburns\ud The early stand history varied greatly between the two age\ud classes More than 70% of the trees in the younger portion of the\ud stand were established within a 15 year period while comparable establishment\ud in the older areas required over 35 years The broad range of\ud ages in older age class, combined with significantly lower stocking\ud density and mortality, resulted in a nearly flat diameter distribution\ud compared with a bell-shaped distribution for the younger age class. The stand is heavily dominated by Douglas-fir which accounts for\ud about 90% of the trees in the younger age class and 77% of the trees in\ud the older portions of the stand. The older portion of the drainage has\ud significantly more western hemlock (Tsuga heterophylla) and western\ud dogwood (Cornus nuttalili). The younger portion of the drainage contains\ud more early successional hardwoods including the remnants of a\ud considerable population of bitter cherry (Prunus emarginata) Currently,\ud almost no western redcedar (Thuja plicata) is found in the\ud drainage although old redcedar logs or snags are still present on one\ud quarter of the plots. The abundance of western hemlock and redcedar is\ud much less than similar aged stands in the nearby H. J Andrews Experimental\ud Forest The slow regeneration of the site following the first\ud fire probably reflects a shortage of seed due to a hot burn and dispersal\ud distances four to ten times greater than those reported by Issac\ud (1943) The low abundance of western hemlock and virtual elimination\ud of redcedar are attributed to even greater dispersal distances, low\ud mobility of redcedar seed, and harsh establishment conditions\ud The rapid regeneration following the second fire suggests efficient\ud seed dispersal or storage with young trees and the potential\ud importance of the understory exclusion phase of stand development on\ud regeneration