Long-Term Regeneration Responses to Overstory Retention and Understory Vegetation Treatments in the Northern Rocky Mountains

Classic regeneration cuttings retaining trees at harvest (shelterwood with reserves, group selection) can be analyzed as analogs of variable-retention harvesting. A 1974 silvicultural experiment in the northern Rocky Mountains was analyzed at 38 years to evaluate the long-term effects of retention harvests on stand development, with a focus on both regeneration and retention tree responses. The postharvest understory treatments (understory removed and broadcast burned) effects were also evaluated. Results indicate that overstory retention results in relatively long-term regeneration growth reduction. Compared with the overstory-free condition (clearcut), the shelterwood with reserves and group selection overstories both reduced the regenerated cohort's basal area, 63 and 44%, respectively. Postharvest burning increased regeneration stem density and also decreased mean regenerated tree size; consequently, these treatment effects were somewhat offsetting, as they produced a zero net difference in regenerated cohort basal area. Considerable regeneration growth reduction associated with retained overstory trees in the shelterwood with reserves was partially mitigated by understory vegetation protection measures that conserved advance regeneration. We conclude that both retention treatments somewhat suppressed regenerated cohort development, but that these impacts were lessened when overstory trees were aggregated and cuttings were in groups, rather than regularly dispersed through the cutting unit.