The role of disturbance in seed germination and first-year survival of red alder (Alnus rubra) was studied over two growing seasons at four sites representing a climatic gradient within the central Coast Range of Oregon. Disturbance affected red alder seed
germination and seedling establishment by altering the temperature and moisture properties of the seedbed, improving light conditions, and disrupting the activities of predators and pathogens. Seedling emergence did not differ significantly between recent
clearcuts and adjacent unlogged forests but was higher on disturbed mineral soil seedbeds than on undisturbed organic seedbeds. In clearcuts, mean emergence on disturbed seedbeds ranged from 7 to 65 percent of viable seed sown and was positively correlated (r2 = 0.70) with spring soil moisture conditions across the climatic gradient. On disturbed forest seedbeds, emergence ranged from 23 to 57 percent and did not appear to be limited by soil moisture. Emergence on undisturbed seedbeds averaged below 10 percent except
when seedbeds remained near saturation levels and light was not limiting. In the absence of forest disturbance, light conditions play an important role in controlling seed germination. Laboratory and field experiments demonstrated that germination is inhibited by light conditions in the forest understory. Broad-leaved trees such as red alder inhibit germination more than conifers because they filter out red light and increase the proportion of far-red light. Understory vegetation and litter layers have an additional inhibitory effect.
Alder seed is subject to heavy losses from seed predators and pathogens. Predation by small mammals and birds averaged 54 to 77
percent. Although vertebrate predation was higher on disturbed seedbeds, losses to invertebrates and pathogens averaged more than three times higher on undisturbed seedbeds. A small percentage of seeds remained viable for more than one year, but longer-term seed storage seems unlikely.
Rates of first-year seedling establishment observed in the study reflected natural patterns of red alder abundance in the Coast Range. During the two years of the study, no seedlings survived at the two sites on the Interior slopes of the Coast Range. At the two Coastal sites, survival averaged between 3 and 30 percent on clearcuts but was 2 percent or less in forests. Best survival occurred on a sheltered north-facing clearcut. Survival on undisturbed seedbeds
was equal to or greater than that on disturbed seedbeds
