Effects of restoration thinning on spatial heterogeneity in mixed-conifer forest

Spatial pattern is an essential attribute of forest ecosystems and influences many ecological processes and functions. We hypothesized that restoration thinning conducted in fire-excluded ponderosa pine ( Pinus ponderosa Douglas ex P. Lawson & C. Lawson) – western larch ( Larix occidentalis Nutt.) – mixed-conifer forest would restore spatial patterns characteristic of active fire regime old-growth. We evaluated effects of thinning on spatial patterns and also compared post-treatment patterns with reconstructions of pre-suppression patterns at nearby old-growth reference sites that developed in the historical mixed-severity fire regime. Restoration thinning reduced spatial aggregation and resulted in globally random tree patterns comprised of local tree clumps, openings, and widely spaced single trees, similar to reference conditions. Post-treatment spatial patterns in the replicate treatment units spanned the range of variability bounded by the reference sites. Our analyses indicate that, under certain circumstances, restoration of spatial heterogeneity in unlogged, fire-excluded forests can be achieved by retaining live legacy pre-suppression trees during thinning treatments. However, success is not assured. Restoration of spatial heterogeneity in forests where few live pre-suppression trees remain due to past mortality or harvest, a common condition of candidate restoration sites, presents a greater silvicultural challenge. Thus, we recommend that, as a general rule, managers deliberately address spatial pattern when crafting forest restoration treatment objectives and prescriptions.