Fire-exclusion has acted as a major perturbation on dry conifer forests in the western U.S., increasing tree density and, in mixed-conifer forests, the dominance of shade-tolerant species. Restoration efforts aim to reverse these effects by reducing stand density, restoring relative proportions of tree species, and reintroducing recurrent fire, but there are limited long-term data on the effects of repeated burning on tree regeneration. We analyzed two decades of seedling and overstory data from the Teakettle Experimental Forest in the southern Sierra Nevada, California, USA to determine how thinning and repeated burning affect seedling establishment and overstory recruitment. The treatments included three levels of thinning (no thin, understory thin, overstory thin) crossed with two levels of burning (burn, no burn). Treatments were replicated three times in 4 ha treatment units. Seedlings were surveyed at gridpoints within the treatment units for the pre-treatment period (1999-2000), following the first-entry burn (2004-2006), at 10 years post-treatment (2011-2012), and before (2016-2017) and after (2018-2020) the second-entry burn. We analyzed seedling response using mixed models. Across treatments, pine seedling densities remained much lower than shade-tolerant seedling densities. We found that repeated burns led to modest increases in sugar pine (Pinus lambertiana) and substantial increases in incense-cedar (Calocedrus decurrens) seedling densities four years post-burn. No significant differences in seedling densities among repeated burning treatments were detected for Jeffrey pine (P. jeffreyi) or white fir (Abies concolor). We used overstory stem map data and seedling data to estimate recruitment into the mid-story and found that estimates of natural mid-story recruitment were much higher among white fir and incense-cedar than pines, even following treatments. 

These data were collected using a gridded treatment unit system where seedling data within treatments were collected using circular plots. Thinning treatments were implemented in 2000 and 2001 and the original burn treatment in 2001. The second-entry burn was implemented in 2017.