Data for: Sierra Nevada mixed-conifer regeneration response to repeated burning varies by species

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.