THE DEVELOPMENT OF OLD-GROWTH DOUGLAS-FIR FORESTS NORTHEAST OF MOUNT ST. HELENS, WASHINGTON, FOLLOWING AN A.D. 1480 ERUPTION

Cross-dated ring counts on Douglas-fir (Pseudotsuga menziesii) stumps were used to reconstruct the early development of old-growth forests northeast of Mount St. Helens, Washington, following catastrophic forest disturbance by tephra fallout from a large eruption in A.D. 1480. In addition to documenting volcanic and other disturbances in forests near the volcano, this investigation tests the previously advanced hypotheses that distant seed sources, repeated disturbances, or competition from shrubs and hardwoods during stand development could have caused Douglas-fir to recolonize many sites in the western Cascade Range over 90-230 yr periods 400-500 yr ago.; Results show that many forest stands in the study area established after an extensive disturbance, probably fire(s), about A.D. 1300. About 180 yr later, tephra fallout from the 1480 eruption killed trees along a corridor near the volcano, and created zones of complete, partial and limited forest mortality. Ages for 78 to 91 Douglas-fir established after the 1480 eruption in 6 to 7 ha plots 0 to 2 km from seed sources show that these trees seeded in during 40 to 60 yr periods (A.D. 1500-60). In contrast, ages for 45 trees established after the 1480 eruption in an 18-ha plot 3 to 4 km from seed sources show that these trees seeded in during a 180-yr period (A.D. 1500-1680). Ninety-seven trees in a 10-ha plot in the limited-mortality zone established during a 70-yr period (A.D. 1300-70), with 94 of the 97 trees seeding in during A.D. 1300-40. Three to four fires, dated from fire scars, occurred in most plots during the first 150 yr of stand development.; These findings confirm that long distances from seed sources after catastrophic disturbances could have caused some old-growth Douglas-fir stands in the western Cascades to develop slowly. They also suggest that repeated disturbances during stand development were probably not causal, because new pulses of tree establishment did not occur after fires in all plots. Competition from shrubs and hardwoods also appears to have been of minor importance, because trees seeded into the stand established before the 1480 eruption, where such species were probably abundant early in stand development, over a relatively short time period.; The documented rates of stand development suggest that slow recolonization of disturbed sites was not universal among old-growth stands in the region. They also suggest a possible range of 40 to 180 yr for the development of new forest stands in the Mount St. Helens National Volcanic Monument.