- Red alder (Alnus rubra Bong.) is a fast-growing pioneer species that colonizes disturbed forest sites west of the Cascade Mountains in the Pacific Northwest.
Streambottoms, toe slopes, and mass movement of surface soils have historically provided the scarification and soil moisture necessary for successful regeneration of red
alder. During the past century, however, extensive logging activity has greatly increased the availability of suitable conditions for alder establishment. The species has responded by expanding upslope throughout its range, displacing native conifers over large areas. As a relatively short-lived pioneer species, red alder would be expected, by conventional succession
theory, to relinquish sites to longer-lived conifers such as Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). However, studies conducted on sites dominated by red alder
have found very little tree regeneration, suggesting that shrub species such as salmonberry (Rubus spectabilis Pursh) or vine maple (Acer circinatuin Pursh) may dominate sites after senescence of red alder.
In this study, I examined both the understory and overstory components of 44 stands dominated by red alder in the Alsea River drainage of the Oregon Coast Range.
Stands ranged in age from 7 to 87 years, the entire span of age classes found within the study area. Using a chronosequence approach, I inferred patterns of stand development and of change in the understory vegetation
throughout the lifetime of the canopy dominants. I also evaluated a variety of tree and stand characteristics to relate stand structure and development to site factors and disturbance history. I then employed multivariate procedures to classify and ordinate understory vegetation, culminating in a description of five community-types. Four of the community-types were further differentiated into two variants associated with site factors.
Disturbance was found to be important for successful colonization of sites by red alder, but the type of disturbance was not strongly related to structure or productivity of red alder stands. Logging and fire were the most common types of disturbance opening up sites for colonization by red alder. Evidence of logging and fire were observed on 30 and 19 plots, respectively. Landslides and unstable soils were evident on 16 sites. Site conditions such as physiographic position,
elevation, and slope were more important determinants of alder stand structure than disturbance agents. Toe slopes had the highest site index values, but stands in midslope positions tended to attain greatest basal area and highest relative density index. Dominance and suppression were
exhibited in diameter distribution patterns that expanded and became multimodal as the canopy matured. Self-thinning consistently removed the smallest individuals in young stands, but mortality in larger size classes
gradually became important in older stands.
Five understory communities are described in this thesis. Swordfern was dominant under most young stands of red alder, with occasional appearance of shrubs normally
found on open, disturbed sites. Middle-aged alder stands supported one of three community-types: a mixture of swordfern and several shrub species, a community dominated
strongly by swordfern and salmonberry, or a community characterized by dense thickets of salmonberry. Under many stands of senescing red alder, particularly those at high elevation, vine maple was the single dominant, but salmonberry was well represented in old stands at low elevation. Abundance of many non-woody species was found
to fluctuate during the growing season. However, certain key species were identified that maintained relatively constant cover during the summer, and these species were
used as the characteristic species for classification of communities. Very little tree regeneration was observed under intact red alder canopies or under the discontinuous
canopies of senescing alder. In older stands, abundant understory vegetation apparently inhibited tree regeneration through competition for light and other
resources and through the direct physical effects of litter deposits on the forest floor. On most sites, succession to communities dominated by salmonberry or vine
maple appeared likely, although scattered saplings of western hemlock and western redcedar might eventually form a discontinuous canopy on some sites.