Repeated burns fail to restore pine regeneration in sierra nevada mixed-conifer

by carolina may

Over a century of fire suppression has changed the structure and composition of forests across the western U.S.  Increased tree density and a build-up of surface fuels, along with compositional changes in some forests have changed the fire regimes in low- and mid-elevation forests that used to burn regularly with surface fire to infrequent fire with a greater chance of stand-replacing fire.  In the Sierra Nevada, mixed-conifer forests that were previously dominated by fire-resistant, shade-intolerant pines, now are dominated by shade-tolerant, less fire-resistant firs and incense-cedar.  In light of these changes, managers implement thinning treatments and prescribed burns with a variety of management goals: reducing risk of high-severity wildfire, increasing resilience to drought and disease, and restoring historic species compositions.

In our study, we assessed mixed-conifer regeneration following repeated burns in combination with thinning treatments at the Teakettle Experimental Forest.  Thinning treatments were completed in 2000-2001 and included an understory thin, which removed small diameter trees, and an overstory thin, which removed larger diameter trees. Prescribed fires were completed in 2001 and 2017. Six total treatment combinations were evaluated, including an untreated control. Following thinning and burning, overstory-thinned plots were planted with Jeffrey pine, sugar pine, and white fir proportional to their original overstory tree dominance.  Our analysis focused on the regeneration of the four most common species across the experimental area: Jeffrey pine, sugar pine, incense-cedar, and white fir.

We estimated seedling densities for each species on plots that had experienced no burns, one burn, or two burns, in addition to initial thinning treatments.  We found that repeated burns led to modest increases in sugar pine regeneration and substantial increases in incense-cedar regeneration, with no significant effects detected for Jeffrey pine or white fir.  Estimated seedling densities are shown in the figure below, with letters indicating when treatments are different from one another during a given year. 

Estimated mean seedling density by species for each burn and thinning treatment combination from 2017 (pre-second burn) and 2021 (four years post-second burn). The letters are for comparison between treatments within a given year. Different letters mean that two treatments are statistically different.

​We also estimated recruitment of seedlings to the midstory over a 16-year period, by calculating how many seedlings grew to exceed 5 cm in diameter for each treatment combination. We scaled this rate over 100 years to provide an estimate of long-term recruitment rates, shown in the figure below.  

Species-specific rates of midstory recruitment per hectare scaled over 100 years. Mean recruitment rates are calculated from records of new trees tagged and mapped plot-wide at each measurement period from 2002-2003 to 2018- 2019. Overstory Thin and Burn/Overstory Thin plots include recruitment from both planted seedlings and natural seedlings.

Estimated recruitment rates were much higher for incense-cedar and white fir than for the two pine species, which remained low even following burning and understory thinning treatments.  Historically, this forest supported around 18 mature sugar pine, 15 Jeffrey pine, 10 incense-cedar, and 23 white fir per hectare.  At our current estimated rates of pine recruitment for unplanted treatments, it would be unlikely to impossible to achieve these pine densities.  However, overstory thinned plots that were planted post-thinning had much higher pine regeneration rates.  In fact, the majority of pines recruited to the midstory on overstory thinned plots were originally planted. 

​Our results indicate that repeated prescribed burns, even when combined with thinning treatments, may fail to achieve pine regeneration at a rate sufficient to restore a pine-dominated overstory.  Regeneration of shade-tolerant white fir and incense-cedar remains high, and additional thinning of these species may be necessary to reduce seed production.  Although uncommon after forest thinning and burning treatments, planting pine may be an effective tool for managers to increase otherwise low pine recruitment in these forests.