Fire and Ecosystem Effects Interactions
How do fire behavior, type, and intensity relate to species composition, non-natives, and structure?
The authors found that thinning treatments followed by prescribed fire resulted in stands that maintained forest structure and composition within reference conditions and were the most resilient to climate change based on the Climate-Forest Vegetation Simulator (FVS) simulations.
The authors found that treated sites prior to the 2011 Wallow Fire resulted in lower tree mortality, smaller patches of high severity, and significantly higher understory herbaceous cover post-fire suggesting that fuel treatments imbue resiliency to uncharacteristically severe fire in mixed conifer ecosystems.
In the Rodeo-Chediski Fire, pre-fire treatments resulted in reduced fire severity. Although fuel loads were high at both areas, pre-fire treated areas had lower levels of surface fuel loads than untreated sites with the difference between the two becoming greater over time. In treated areas mean live basal area increased over the past decade as regeneration occurred in the open spaces left by fire-killed trees. Conversely, basal area in untreated areas has decreased due to residual tree death and limited regeneration.
The authors found that prescribed fire can affect forest structure for five or more years post-treatment. Delayed tree mortality was observed up to eight years post-fire causing a 67% reduction in stem density within the time period of the analysis. The authors suggest that long-term, prescribed fire can alter stand structure in old-growth mixed conifer forests.
The author found that eighty percent of the resampled mesic ponderosa pine plots had evidence of surface fire in Grand Canyon National Park; however, they did not find that surface fire related to changes in forest structure and composition from 1935–2004 except for the increase in the smallest diameter classes of white fire in which larger size classes were likely reduced by surface fire.
The authors found evidence of large patches (>100 acres) of stand-replacing fire in upper elevation mixed-conifer forests prior to European settlement in the region via aspen and conifer recruitment pulses, corresponding fire scar and mortality dates, and lack of surviving trees prior to large fire dates. This suggests that recent large patches of high severity fire are within the historical range of variability for upper elevation forests.
Thinning and burning significantly reduced the overall density (>2.5 cm) of ponderosa pine stems by 66%, although this did not meet the objective. Large snags and logs >50cm were retained within objectives. Canopy fuel loads were substantially reduced, allowing for the reintroduction of surface fires.
The authors found that treatments that increased shrub and herbaceous understory resulted in increased fire intensity. Increases in fire intensity then reduced shrub biomass which the authors suggest is more in line with historic conditions of frequent fire.