Informing mixed conifer gap-based silviculture with growth dynamics of regeneration and gap expansion trials

Where frequent fires are the primary driver of gap dynamics, gap-based silviculture can be deployed to restore fire-maintained structures. We studied the growth of regeneration for six species in a western US mixed conifer forest, following 12 years of growth across a range of gap sizes (0.1–1 ha). We found that severe edge effects limited growth within 3 to 4 m of surrounding canopy trees. Using an edge zone where competition is expected to include more moderate levels of growth suppression, we compared 12-year growth in edge zones versus interior zones. Ponderosa pine was the tallest among edge trees, followed by giant sequoia, Douglas-fir, sugar pine, incense-cedar, and white fir. Following experimental gap expansions (i.e., femelschlag harvest), we observed that increases in light availability were substantial along southern edges, whereas northern edges had no increase. Saplings in edge zones grew substantially in response to expansion. Our results demonstrate that using gap-based silviculture can create coarse-scale heterogeneity while regenerating all species, including shade-intolerants in smaller gaps. Gap expansions can be used to restore some of the structural complexities that fire historically maintained. Colonnades of tree-free space may enhance objectives of heterogeneity and fuel discontinuity that frequent fires used to maintain.