Variable-density Retention Promotes Spatial Heterogeneity and Structural Complexity in a Douglas-fir/Tanoak Stand

Abstract

1. Abstract After harvesting the merchantable conifers decades ago, many secondary forests in northern California regenerated naturally and are now fully stocked with low value hardwoods intermingled with conifers. Partial harvesting to reduce hardwood densities and release conifers is expected to enhance tree vigor and reduce risk of stand-replacing wildfire. Planting a new cohort of merchantable conifers in the understory would enhance structural complexity and future value. A flexible new forest restoration treatment called variable-density retention (VDR) was designed to achieve these objectives. Desirable trees can be kept regardless of their location while the new cohort of conifers is planted among existing trees or in gaps created by removal of patches of undesirable trees. At our study site in northern California, 20 × 20 m (0.04 ha) squares each received one of five treatments (gap, low-/medium-/high-density retention, and no cut). This created a mosaic of different densities. Planted conifer seedlings exhibited variable growth rates. Different stand variables were associated with growth of seedlings of the native coast Douglas-fir ( Pseudotsugamenziesii var.menziesii ) versus coast redwood ( Sequoia sempervirens ) planted outside its natural range. We found no correlation between point counts of overstory basal area and either leaf area index or understory light at each seedling. The VDR system promoted heterogeneity in the spatial pattern of tree locations, restored conifer dominance, and enhanced structural complexity by introducing a new cohort of trees growing at different rates. 3. Keywords: Forest Restoration; Notholithocarpusdensiflorus ; Pseudotsugamenziesii ; Redwood; Sequoia sempervirens ; Variable-Density Thinning; Variable Retention