SNAMP Pub #34: Beyond reducing fire hazard: fuel treatment impacts on overstory tree survival
Article Title: Beyond reducing fire hazard: fuel treatment impacts on overstory tree survival
Authors: Brandon M. Collins, Adrian J. Das, John J. Battles, Danny L. Fry, Kevin D. Kkasnow, and Scott L. Stephens
- Observed overstory tree mortality from fuel treatments in the midterm (5-10 years) and projected future vulnerability suggest impacts of fuel treatments on overstory tree vigor vary by treatment type, as well as by tree species.
- Across species, observed mortality and future projected vulnerability were low in mechanical treatments, while in the two fire treatments rates were relatively high.
- The combination of anticipated higher mortality and greater fire hazard in the ‘‘no action’’ management strategy leads to a decrease in resilience.
Fuel reduction treatments have been implemented across millions of acres of forestland throughout the western United States, and are being proposed for millions more. These treatments include the use of fire, mechanical manipulation (e.g., thinning, mastication, chipping), or a combination of the two to reduce surface and ladder fuels, and to increase tree crown spacing. The primary goal is to modify wildland fire behavior and thereby reduce the probability of uncharacteristically severe fire effects.
Longer term effects, of 5 years or more, of fuel reduction treatments has not been well studied and even less is known about the impacts on tree growth and mortality. This study aimed to help bridge that gap by examining mid-term tree mortality and growth responses following the most commonly used fuel reduction and restoration treatments in the mixed-conifer forests of the Sierra Nevada.
This study was primarily performed at the University of California Blodgett Forest Research Station, which is located in the mixed-conifer zone of the north-central Sierra Nevada, California, USA. Common tree species include sugar pine, ponderosa pine, white fir, incense-cedar and Douglas-fir. Tree ring data collected at the Last Chance cite as part of the Sierra Nevada Adaptive Management Project were also used in this study. This consisted of tree cores from 839 paired live and dead trees collected across the entire l3,800 ha Last Chance site.
Three different fuel treatments were tested: mechanical-only, mechanical-plus-fire, and prescribed-fire-only, compared to an untreated control, which were each randomly applied to three of 12 experimental units that varied in size from 35 to 72 acres.
Overstory and understory vegetation was measured in 20, 0.1 acre plots, installed in each of the 12 experimental units (240 plots total) in 2001 (pre-treatment), 2003 (post-1-year), and 2009 (post-7-year). In 2010, plots were revisited to extract increment cores from a random subset of trees in each of the 12 experimental units.
Plot-level tree measurements were used to calculate live tree density and biomass. Repeated-measures analysis was performed to test for differences in live tree density and biomass among time periods (pre-treatment, post-1-year, post -7-year) and among the different treatments. We calculated annual mortality by species for each experimental unit. Then we compared current stand health (as of 2009) among treatments by calculating vulnerability indices for trees within each treatment area.
Seven years after treatment, mortality and vulnerability rates in conifers >20 inches dbh (diameter at breast height) were significantly higher in the treatments with prescribed fires; highest in the mechanical and fire treatments and lowest in the mechanical only treatments. Results suggest that treatments involving prescribed fire, which are most effective at reducing hazardous fire potential, may not be improving overstory tree vigor, relative to the mechanical treatment alone.
The greater vulnerability of trees in the combined treatment (mechanical and fire) is likely related to the additional surface fuel inputs that were generated from the commercial tree harvest (i.e., tree tops and limbs left in the forest), as well as the mastication of sub-canopy trees and shrubs. These additional fuel inputs increased total fuel depth, which likely resulted in greater fire residence time when burned, thus longer potential exposure of tree cambiums to lethal temperatures. Seven years after the treatments, hazardous fire potential in mechanical treatments had decreased.
One potential improvement to the mechanical and fire treatment is to use whole-tree removal systems, which generally do not result in so much augmented surface fuels following treatment. Conducting prescribed burning several years (e.g., 5–10) after mechanical treatment may also help by allowing for decomposition of excess fuels. This might further serve to potentially extend treatment longevity.
Mechanical-only treatment may be the best option if improving overstory tree vigor is the sole objective. The fact that hazardous fire potential initially was only slightly reduced, but then improved over time, suggests that if forest managers can tolerate the lack of fire hazard reduction in the short term for potential tree vigor gain over the longer term, mechanical thinning-from-below might achieve both objectives.
Collins, B.M., A.J. Das, J. J. Battles, D. L. Fry, K. D. Krasnow, and S. L. Stephens. 2014. "Beyond reducing fire hazard: fuel treatment impacts on overstory tree survival". Ecological Applications. 24(8): 1879–1886.
For more information about the SNAMP project and the Fire and Forest Health team, please see the: Fire and Forest Health Team Website.