Natural regeneration responses to thinning and burning treatments in ponderosa pine forests and implications for restoration

Understanding naturally occurring pine regeneration dynamics in response to thinning and burning treatments is necessary not only to measure the longevity of the restoration or fuels treatment, but also to assess how well regeneration meets forest sustainability guidelines and whether natural regeneration is sufficient for maintaining a sustainable forest structure and composition. A synthesis review was carried out on the effects of mechanical thinning and prescribed burn treatments on natural pine regeneration response in frequent-fire ponderosa pine forests across the western United States. The focus was on site-specific variability in pine regeneration dynamics, temporal trends in regeneration presence and abundance, and response to treatment as described in the current literature using 29 studies that met our evidence-based review protocols. Data showed that the effects of thinning and burning treatments on regeneration depended on time since treatment. Mechanical thinning, prescribed burning, and thinning plus burn treatments all increased seedling density, but there was high variability among sites and studies. There were mixed results in the short-term (< 10 years) with both increasing and decreasing regeneration, and a general increase in regeneration 11 - 20 years post-treatment. Some long-term studies (> 20 years) concluded that stands can return to pre-treatment densities in terms of total trees per hectare and forest floor duff levels when there are no maintenance treatments applied. Several studies showed the average ponderosa pine seedling presence, survival and growth found in today's forests to be at a high density; this combined with missed fire cycles could contribute to future fire risk and reduce the efficacy of maintaining fuel reduction goals.

Automated summary

The study on natural pine regeneration dynamics in response to thinning and burning treatments highlights the variability across different sites and studies. The effects of mechanical thinning, prescribed burning, and thinning plus burn treatments on seedling density differ depending on the time since treatment. Short-term results show both increasing and decreasing regeneration, while a general increase is observed 11-20 years post-treatment. Long-term studies suggest stands can return to pre-treatment densities without maintenance treatments, but there are concerns about future fire risk due to high seedling density and missed fire cycles.