Twelve-year responses of planted and naturally regenerating conifers to variable-retention harvest in the Pacific Northwest, USA

Abstract

We studied patterns of conifer regeneration over 12 years as part of a regional-scale experiment in variable-retention harvest in the Pacific Northwest, the DEMO Study. We compared survival and height growth of planted conifers and density and seral composition of natural regeneration among treatments with differing retention levels (15% versus 40%) and patterns (dispersed versus aggregated) replicated across a range of latitudes and forest zones. We also assessed plot-scale relationships of natural regeneration with overstory density and basal area, competing vegetation, and slash accumulations. Early (1- to 2-year) survival of planted seedlings was greater in dispersed treatments (Pinus monticola Douglas ex D. Don, Abies spp.) or unaffected by retention level or pattern (Pseudotsuga menziesii (Mirb.) Franco). Later (5- to 12-year) survival did not differ (all species), but growth was distinctly reduced in dispersed treatments and (or) at higher levels of retention. Density of natural regeneration was 1.5–2.5 times greater in dispersed treatments than in the cleared areas of aggregated treatments. Low-level dispersed retention promoted Pseudotsuga, the early-seral dominant, presumably by enhancing seed rain within a relatively high-light environment. Dispersed retention favored late-seral conifers. The ability to manipulate retention pattern and level to influence regeneration density and composition provides managers with flexibility in developing structurally complex and compositionally diverse forests.