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

The structure of a mature Douglas-fir (Pseudotsuga menziesii)forest in a watershed in the western Cascades of Oregon was examined.Two age classes were detected in the stand, the oldest originatingabout 1855 after an extensive fire and the younger following a secondfire about 1895. Although the trees in the older age class had statis-tically greater diameters and heights, only open grown individualsmixed with the younger age class could be readily distinguished. Be-cause reburns at young ages are common and may not leave firescars,great care is be required to distinguish between slow regeneration andpatchy reburns.
The early stand history varied greatly between the two ageclasses. More than 70% of the trees in the younger portion of thestand were established within a 15 year period while comparable estab-lishment in the older areas required over 35 years. The broad range ofages in older age class, combined with significantly lower stockingdensity and mortality, resulted in a nearly flat diameter distributioncompared with a bell-shaped distribution for the younger age class.
The stand is heavily dominated by Douglas-fir which accounts forabout 90% of the trees in the younger age class and 77% of the trees inthe older portions of the stand. The older portion of the drainage hassignificantly more western hemlock (Tsuga heterophylla) and westerndogwood (Cornus nuttallii). The younger portion of the drainage con-tains more early successional hardwoods including the remnants of aconsiderable population of bitter cherry (Prunus emarginata). Cur-rently, almost no western redcedar (Thuja plicata) is found in thedrainage although old redcedar logs or snags are still present on onequarter of the plots. The abundance of western hemlock and redcedar ismuch less than similar aged stands in the nearby H. J. Andrews Experi-mental Forest. The slow regeneration of the site following the firstfire probably reflects a shortage of seed due to a hot burn and dis-persal distances four to ten times greater than those reported by Issac(1943). The low abundance of western hemlock and virtual eliminationof redcedar are attributed to even greater dispersal distances, lowmobility of redcedar seed, and harsh establishment conditions.
The rapid regeneration following the second fire suggests effi-cient seed dispersal or storage with young trees and the potentialimportance of the understory exclusion phase of stand development onregeneration.