Forest structure and fuels dynamics following ponderosa pine restoration treatments, White Mountains, Arizona, USA

Southwestern ponderosa pine forest ecosystems have become uncharacteristically dense as a result of intensive livestock grazing, logging, and fire exclusion, which have contributed to a buildup of fuels and increased vulnerability to high-severity, landscape-scale crown fires. In 2002, we implemented a replicated ecological restoration experiment to (1) quantify site-specific reference conditions, (2) analyze effects of elevation on forest structure, surface fuels, and canopy fuels, (3) test responses to three treatments: control, full treatment, and burn-only, and (4) evaluate effectiveness of treatments for restoring attributes to near historical reference conditions. Reconstructed reference conditions showed that basal area (BA) averaged 9.2 m2 ha−1 and tree density averaged 86.2 trees ha−1 across the site in 1880 prior to widespread fire exclusion, with the highest tree density values occurring at the two upper elevation sites. In 2002, prior to treatment, BA averaged 28.9 m2 ha−1 and density averaged 927.9 trees ha−1, representing three- and ten-fold increases, respectively, compared to 1880 values with a higher proportion of the increase coming from mesic species at higher elevations. By 2013 (5-years post-treatment), the full treatment showed BA was reduced by 52% compared to pre-treatment values from 31 m2 ha−1 to 15 m2 ha−1 and density was reduced by 85% from 998 trees ha−1 to 153 trees ha−1. In the burn-only treatment, BA increased by 6% from 27.5 m2 ha−1 to 29.1 m2 ha−1 although density was reduced by 25% from 837 trees ha−1 to 625 trees ha−1 between 2002 and 2013. Canopy fuels dynamics were similar to forest structure responses: low values prior to fire exclusion, a marked increase by the pre-treatment measurement, substantial reductions following thinning plus burning, and minor reductions following burn-only treatments. Post-treatment diameter distributions in the full treatment closely resembled reference conditions; minor reductions occurred in the lower diameter classes in the burn-only. Mortality of trees established before European settlement circa 1880 varied by treatment: 25% of presettlement trees alive in 2002 died by 2013 in the full treatment, 31% died in the burn-only treatment, and 15% died in the control. Of the treatments tested, the full treatment was the only one that rapidly shifted forest structure, diameter distributions, and canopy fuels values to levels near or within the historical reference conditions. Results from this study showed that managers will have to consider tradeoffs between alternative treatments in the context of climate-induced, landscape-scale forest fires.