Abstract:
This study investigated the development of riparian zone vegetation at varying stand ages from one to 29 years following clearcut timber harvesting in western Oregon. Vegetation was classified into three layers for observation including the ground (less than 13 cm), the understory (less than 2.5 m), and the overstory (greater than 2.5 m) layers. Variables observed included cover by vegetation types, vegetation directly overhanging the stream channel, canopy density and angular canopy density (an
effective estimate of stream shade). Five vegetational zones as presented by Franklin and Dyrness (1973) were selected for study. The primary sampling unit for this study was the transect with ten transects at each site located along the stream reach. A site in an unmanaged old growth stand in each zone was measured in the same manner to provide a
comparison with the several harvested sites. A total of 40 sites were sampled during the course of the study. Equations relating the development of angular canopy density (ACD) with the period since harvesting are presented for each of the five vegetation zones. These equations may be used to predict
the percent ACD within a given zone for a proposed time period which may be useful to the resource manager in harvest planning. In three of the vegetational zones, seventyfive percent ACD can be expected in eight to 20 years following harvesting. The maximum ACD value observed in two of the zones averaged approximately 60 percent in the 29 year period since harvesting. Depending upon the vegetation zone, deciduous cover provided approximately 40 percent cover in the understory layer after a period of three to 14 years. Litter cover (organic material greater than 1 mm in diameter) generally
dominated the ground layer at all the harvested sites with approximately 45 percent of the ground surface in that cover
type. Coniferous vegetation was generally less than 20 percent in both the understory and overstory layers during the two and one-half decade period since timber harvesting. The tendency for the riparian zone to develop dense corridors of deciduous vegetation at some sites was noted. In terms of the number of stems per acre, some Alnus rubra corridors in the Coast range zones were ninety times more dense than the unmanaged forested sites in that area. In contrast, the high elevation Cascade range zone only had 40 percent of the unmanaged forested site stand density after a period of 19 to 24 years following harvesting. The large organic debris loading in the harvested streams was generally lower than in the unmanaged forested sites. The LOD load averaged 13.4 pieces per 100 feet (30.5 m) for the five forested sites. The harvested sites had an average of only 10.4 pieces. A few harvested streams had unusually
high loadings while 77 percent of the streams contained less than the forested streams.
