The authors found that overall species composition was similar between 1913 and 1999; however, the sites showed an increase in basal area of the more shade-tolerant species Douglas-fir and white fir. This suggests a future forest communities may potentially undergo type conversion from forest communities dominated by fire-resistant trees to those supporting denser populations of shade- and fire-intolerant species.

The authors found that during historically cooler periods, forests burned frequently at low severity, which they suggest was driven by increases in understory vegetation growth. Historically warm periods were linked to severe drought and an increase in high severity fires that caused large debris-flow events and fire-related erosion.

The frequency of fire necessary to maintain populations of Q. gambelii is highly uncertain, however, the authors suggest that fire may play an important role in these communities by affecting the rate of succession. Because Q. gambelii is fairly shade-intolerant, long fire return intervals may reduce the regeneration capacity of this species within dense canopies.

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 author found a decrease in aspen density from 1935 to 2004 which is attributable to fire exclusion and herbivory. For mixed-conifer, spruce-fir and ponderosa pine in Grand Canyon National Park, densities have increased due to fire exclusion.

Fire was relatively rare at both sites between approximately 12,000-10,600 cal yr B.P. Most of this period was cool and relatively wet, which likely suppressed fire occurrence. The establishment of ponderosa pine and Picea and Abies spp. at the lower and higher elevation sites, respectively, increased fire occurrence greatly after approximately 10,600 cal yr B.P. The authors suggest that the increased fire frequency likely gave ponderosa pine a competitive advantage at this time over Picea forest.

The author found that the stand of ponderosa pine they studied was fairly heterogeneous and did not regenerate in large, even-aged groups following large patches of mortality. They hypothesize that patches of additional fuel surrounding single or small groups of trees likely led to smaller patches of mortality during low-severity surface fires, which would create openings and seedbeds for new seedlings.

During the 2011 fire season, which was characterized by record-breaking area burned in the southwest, temperature was not anomalously warm, however low precipitation led to exceptionally low atmospheric moisture content and subsequent record-breaking VPD. The authors’ model projections show that the climate conditions like those exhibited in 2011 are likely to occur more frequently in coming decades as temperatures are predicted to increase. The authors suggest increasing trends in VPD could lead to increasingly common catastrophic wildfires if fuels are not limiting.