Fire and Ecosystem Effects Interactions
How does fire severity relate to species composition, non-natives, and structure?
Models did predict fundamental and persistent shifts in forest structure and composition. Under the hot-arid climate scenario, ponderosa pine forests were type converted to shrublands under altered fire regimes (increased fire frequency and severity) along with regeneration failure. Mixed-conifer forests were affected to a lesser degree.
Management at the varying intensities was unable to prevent the structural and compositional changes to either ecosystem or changes to the fire regime suggesting that novel management approaches will be necessary to counteract the effects of climate change.
The authors found that a single entry, low severity fire was not effective in reducing tree densities in ponderosa pine trees with white fir encroachment, although a second entry fire did reduce white fir populations. Low severity fire, however, reduced fuel load density in the dry mixed-conifer stands. They also found that aspen regeneration after high severity fire was abundant. For ponderosa pine, seedlings did increase after fire, however, due to the dense overstory canopy, survival into adulthood was uncertain for both stand types.
The authors found that current tree establishment patterns do not suggest that high-severity fire was widespread historically at this site with only small patches of young trees up to 25 ha in size that may have established after stand-replacing fire.
Post-fire resistance to change in community composition decreased with increasing fire severity over a 10-year period. Furthermore, sites that burned at low or moderate severity returned to pre-fire conditions over time, while high severity sites continued to diverge from pre-fire conditions throughout the study period. Conversely, species richness persisted after the fire regardless of fire severity and exceeded pre-fire levels five years after the fire. However, the colonizing species were often different species than existed on the site pre-fire, but communities remained composed of 89% native species. Non-native species also increased with increasing burn severity, but did not dominate any site in the study area.
The authors suggest that although these mega fires are often catastrophic for overstory vegetation, which may not always be the case for the understory in dry conifer forests. If managers can ameliorate fire severity through forest management, thereby allowing areas to burn at low or moderate severities, ponderosa pine ecosystems may mimic historical fire regimes, increasing the resistance and resilience to community composition change post-fire while stimulating understory growth and species richness.
Fire perimeters that had not been burned recently prior to the Las Conchas fire exhibited the larges changes in community composition whereas those fire perimeters that reburned showed reduced impacts of fire on the vegetation communities. Prior to the Las Conchas fire, the vegetation in the recently burned communities had reduced canopy cover, and higher cover of shrubs and grasses.
In high-frequency/low-severity fire-adapted ecosystems, increases in the frequency of high severity fire may lead to changes in vegetation of forested sites to alternative communities.
Those areas that burned at high severity in the Rodeo-Chediski fire that had received treatment prior to the wildfire had consistently higher future basal area than untreated areas in all simulations, suggesting that the spatial arrangement of treatments highly influences forest structure and recovery.
The authors suggest that although the majority of the landscape on the Coconino Plateau was dominated by low severity fire, there is also evidence of a large amount of mixed severity fire and large patches of high severity were not uncommon. The mixed-severity fire may explain the greater variability in forest structure and composition observed at fine scales across the Coconino Plateau.
The authors found that in areas burned at high severity understory plant cover was greater than in low-severity burned areas, likely due to the increases in resources made available after the death of mature, overstory trees. Exotic plant cover was also greater in areas that burned at high severity, but generally the cover was minimal (< 2%).
The authors also found that after severe fires, over half of the sites lacked any ponderosa pine regeneration and was more likely to be dominated by sprouting species, although one site did have hyperdense pine regeneration. The authors suggest that these sites may go decades as shrublands or grasslands, and it remains to be seen if they return to a forested state.
The authors found that in ponderosa pine forests, low- and moderate- severity resource benefit fires more effectively reduced basal area and crown fire potential more so than prescribed fires. Areas burned multiple times for resource benefit maintained stand structure and fuel loads more consistent with historical forest conditions. Low severity fire in pinyon-juniper woodlands did not have an effect on forest structure and fuels, however, moderate severity fire did have beneficial effects on these ecosystems.
Fire effects on cool, mixed-conifer ecosystems may allow the composition and structure of these ecosystems to be more resilient to the effect of catastrophic wildfire due to climate change.
The authors suggest that repeated higher severity fires may advantage sprouting species such as the Chihuahua pine over non-sprouting species such as ponderosa pine.
High severity fire was a strong predictor of non-native species establishment, and invasion was more likely with increasing fire severity, indicated by depth of char in this study.
The authors propose that variations in fire frequency and timing were more important in shaping forest structure than variations in fire severity.
The authors found that in historically low-severity, frequent-fire forests, stand replacing fire can result in conversion to altered species composition and structure of stands either toward even-aged cohorts of ponderosa pine or to novel vegetation communities through regeneration failure or due to changes in fire regimes which may be exacerbated by climate change and/or extreme weather events.
Despite issues with pseudoreplication and the limited sample size, the authors found that post-fire conditions moved toward range of natural variation conditions after intense burning. Small trees and fire-susceptible understory trees were preferentially killed by fire and forest floor woody debris was within desired conditions. The sites had adequate regeneration of seedlings with no evidence of any landscape conversion to a non-forest vegetation type.
The authors found that currently the forest on the North Rim of Grand Canyon National Park is dominated by dense stands of spruce-fir, mixed-conifer, and aspen. Historically, conditions would have been very open prior to fire exclusion. Fire-initiated tree groups were common at the project site, approximately 60% and likely originated after severe fires in the 1780s.