Initial tree regeneration responses to fire and thinning treatments in a Sierra Nevada mixed-conifer forest, USA

Abstract

Fire is a driver of ecosystem patterns and processes in forests globally, but natural fire regimes have often been altered by decades of active fire management. Following almost a century of fire suppression, many western U.S. forests have greater fuel levels, higher tree densities, and are now dominated by fire-sensitive, shade-tolerant species. These fuel-loaded conditions can often result in high-intensity crown fires replacing historic low- to moderate-intensity fire regimes. In the mixed-conifer forests of the California Sierra Nevada, thinning and prescribed fire are widely used to reduce fuels and shift future stand composition from shade-tolerant species to more fire-resistant pines ( Pinus lambertiana and Pinus jeffreyi) that were historically more abundant. The impacts of these treatments, however, on forest regeneration composition and abundance are unclear. We examined the effects of prescribed fire and common thinning treatments (understory and overstory thinning) on microsite conditions, seed rain, and tree regeneration in an old-growth, mixed-conifer forest in the Sierra Nevada, California, USA. Treatments significantly altered environmental conditions, but there was substantial variation and overlap in conditions among treatments. Seed rain of shade-tolerant Abies concolor and Calocedrus decurrens was 5–26 times greater than P. jeffreyi and P. lambertiana, creating inertia in efforts to shift stands towards increased pine abundance. Survival of Pinus germinants was greatest in burned microsites. The burn-overstory thin treatment had both the highest mortality of advanced A. concolor and C. decurrens regeneration and the greatest increase in pine regeneration. Species occupied microsites gradating from low light/high moisture to high light/low moisture in the order: C. decurrens, A. concolor, P. lambertiana, and P. jeffreyi. Results suggest prescriptions may need to thin mature A. concolor and C. decurrens to significantly reduce their seed rain, create an abundance of burned open microsites, or plant Pinus seedlings to shift regeneration composition in treated stands.