Repeated burns fail to restore pine regeneration to the natural range of variability in a Sierra Nevada mixed-conifer forest, USA

Fire-exclusion has acted as a major perturbation on dry conifer forests in the western United States, 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, United States to determine how thinning and repeated burning affect seedling establishment and overstory recruitment. Across treatments, pine seedling densities remained much lower than shade-tolerant seedling densities. We found repeated burns led to modest increases in sugar pine (Pinus lambertiana) and substantial increases in incense-cedar (Calocedrus decurrens) seedling densities 4 years postburn. No significant differences in seedling densities among repeated burning treatments were detected for Jeffrey pine (P. jeffreyi) or white fir (Abies concolor). Estimates of natural midstory recruitment were much higher among white fir and incense-cedar than pines, even following treatments. However, postharvest planting increased rates of pine midstory recruitment in overstory thinned treatments. Our results suggest that fire-exclusion may have shifted the ecosystem out of its initial domain of attraction, creating a forest dominated by shade-tolerant species that exhibits hysteresis by resisting a return to a natural range of variability even after restoring structure and process. Planting pine species may be effective at overcoming this resistance to restore the forest to a pine-dominated state.