Repeated burns fail to restore pine regeneration in sierra nevada mixed-coniferRead Now
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.
The Earth Systems Ecology Lab (www.hurteaulab.org) at the University of New Mexico is recruiting a PhD student for a tree regeneration project. Post-fire reforestation in the southwestern US typically has low survival rates because of heat and drought, a challenge that is likely to worsen with ongoing climate change. We have been working to develop predictive models to increase the probability of planted seedling survival. If you are interested in post-fire vegetation dynamics, reforestation, microclimate, or some combination therein, consider applying to join us. Chris Marsh and I have just initiated a USDA funded project that includes; planting 10,000 seedlings , deploying 400 microclimate sensors and collecting conventional and thermal imagery using drones, aiming to better characterize the thermal environment which affects seedling survival and growth.
If you are interested in undertaking research to help post-fire reforestation efforts, this is the project for you. Student support will include some combination of teaching and research assistantships, including summer salary support and research project support. If the adventure of obtaining your PhD is going to be a good and rewarding one, it is important that you find the right advisor for you. I wrote this so potential students can get a sense of my approach. If you read that and are intrigued, I may be a good fit for you. If you read it and are put off, I’m not going to be a good fit for you.
If after reading the potential students page you are still interested, send me (email@example.com) or Chris Marsh (firstname.lastname@example.org) a single pdf with your CV, one-page statement of research interests, and your unofficial transcripts. UNM Biology doesn’t require GREs and the only thing I think they measure is how well you did on your GREs, so I don’t require them. We use this unofficial application process so you don’t have to spend $60 to figure out if we think you are a good fit for the position. I didn’t have an extra $60 when I was applying to grad school and I figure you probably don’t either.
We will begin reviewing application materials on 1 October. We will ensure that any finalists have the opportunity to speak with everyone in the lab. We are a hardworking and supportive group that likes to work on research projects that will hopefully leave the world in a little better shape than before we started.
Hiring PostdocRead Now
We are recruiting a postdoctoral researcher with a strong background in spatial data analysis and programming to contribute to a project on quantifying forest carbon risk across the western US. The objectives of this position include developing spatial layers of forest condition and disturbance and climate risk at the watershed scale using a process that assimilates multiple data sources at varying grain sizes and extents. The position will involve engagement with conservation organizations and collaboration with project members in the Earth Systems Ecology Lab.
This is a 24 month position, beginning fall 2022. Salary is $55,000 per year, plus benefits. Required qualifications include a PhD in ecology, ecosystem science, earth/environmental sciences, or statistics, and programming experience with R. Familiarity or expertise with satellite-based remote sensing is desirable.
Applicants should submit a cover letter detailing research interests and goals, a complete CV, and names and contact information for three references to Matthew Hurteau (email@example.com). Review of applications will begin on 1 SEPT 2022.
The University of New Mexico is committed to hiring and retaining a diverse workforce. We are an Equal Opportunity Employer, making decisions without regard to race, color, religion, sex, sexual orientation, gender identity, national origin, age, veteran status, disability, or any other protected class.