Fire and Ecosystem Effects Interactions
How do fire frequency, rotation, and return interval 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The authors suggest that manipulating plant assemblages based on traits, instead of historical reference conditions, that infer both drought and fire tolerance may be an effective method to increase forest resilience in an era of rapid climate and environmental change even if those communities have no-analog assemblages currently. For the western U.S., this includes managing for species with traits such as dense wood, tough leaves, and thick bark that can withstand increased fire frequency.
The authors found that fire frequency appeared to be the primary influence on tree age structure in the sky island communities. Fire free periods were related to age peaks. The authors suggest that climate did not limit tree establishment and regeneration, however, climate did influence fire frequency and thus indirectly influenced age structure pattern in these stands. Within the study sites, age peaks occurred synchronously in the early 19th century, coinciding with a period of increased moisture within the ENSO cycle that reduced fire activity.
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.
The authors found that spruce beetle outbreaks expanded coincident with increasing populations of spruce starting 94 years after the last stand-replacing fire and remained relatively small for another century. However, with the onset of fire exclusion, the increased density of spruce-fir has resulted in longer outbreak durations in the present century.
Fire cessation after 1879 has led to an overall increase in density at the site and an increase in southwestern white pine and white fir, now co-dominate with ponderosa pine and Douglas-fir.
The authors found that 10- to 20-year prescribed fire intervals in areas that burned at high severity were too frequent to maintain basal areas of ponderosa pine within the historical range of variation under the most extreme climate scenarios. Prescribed fire was, however, effective at maintaining low-severity sites initially dominated by smaller diameter trees.
An interruption of the fire regime in the 1940’s and 1950’s, likely by the introduction to logging and livestock grazing in the area, resulted in an increase in tree establishment, especially oak and other broadleaf tree species.
The authors found that the density of PJ woodlands did increase as the length of time since fire increased. However, seedling densities were highly variable across the study sites and did not have a significant relationship with time since fire. The authors suggest this may be because the length of time required to return to pre-fire conditions for PJ woodlands likely requires several decades between disturbances to recover. Specifically, the authors observed that juniper species density was positively related to time since fire while pinyon species were not, suggesting that juniper is more likely to establish post-fire as it is typically more drought tolerant. Site structural complexity also increased with time since fire indicating developmental trajectories toward woodland conditions.
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.
In aspen stands, fire suppression over the past 130 years has likely affected the age and stand structure of these stands. Aspen stands shifted from frequently burned stands with dense and multi-aged cohorts of trees toward a monoculture of open, same-aged trees.
Longer freeze-free seasons favor cold-intolerant annual grasses, including cheatgrass (Bromus tectorum) red brome (Bromus rubens) and buffelgrass (Pennisetum ciliare) allowing them to germinate during warmer temperatures and expand their range creating highly flammable ecosystems. In these ecosystems, climate change may result in a fire-invasive feedback loop in deserts where increases in the amount and continuity of annual grasses lead to more extensive and frequent wildfire, thereby increasing the potential for invasive species to proliferate while decreasing the ability of slower-regenerating, native shrub species.
The authors suggest that a shift toward a novel fire-climate-vegetation relationship as the increase in fire frequency becomes incompatible with the persistence of the current vegetation in the region.
The authors found that current and projected conditions in the structure and composition of ponderosa pine forests are approximately 97% departed from reference conditions on the Kaibab Plateau. Conditions in these forests are likely to persist due to the continued fire suppression and lack of management activities to reduce forest density.
The authors found an abrupt cessation of fire after 1868, concurrent with Euro-American settlement, although they also found fire quiescent-periods around 1685 to 1735 and again from 1824 to 1861 related to increased moisture patterns. At the current study site, shade tolerant species, such as white fire and Douglas-fir dominated the understory and were regenerating most abundantly, however, during the historic periods of frequent fire, ponderosa pine dominated the stand until fire ceased in the region.
Gambel oak can survive in forests dominated by frequent fire regimes, although top kill by fire is common. The frequency and intensity of prescribed fire could be used as a tool to alter the species composition of pine-oak forests by maintaining a variety of oak growth forms.
Historically, frequent fire maintained open grasslands across the semi-arid desert. However, fire cessation has allowed for shrub encroachment, and consequently, a reduction in grass and contiguous fuels. The model predicted only 4% of the landscape to burn over the 50 year simulations. The authors suggest that future fire events may have minimal impact on moving the current ecosystem, which is dominated by woody shrubs, back to a grassland state.
Native grassland plant species showed a wide variety of responses to the summer prescribed fire; however, most of the 14 species studies were fire tolerant and either survived the fire or quickly resprouted post-fire within 10-12 years. Exceptions, such as black grama grass, chollas, snakeweed, and fourwing saltbush were highly susceptible to fire and exhibited both high mortality and slow regeneration rates. However, established individuals of the invading shrub creosotebush were highly tolerant of fire, so prescribed fire may not be effective at removing these species. Still, the authors suggest that more frequent fire may reduce creosotebush and other woody shrub recruitment.
The authors found that the interruption of frequent fire in the late 19th century was the main cause of tree invasion on upper mountain slopes and mountain tops within the region.
Subalpine species such as Englemann spruce have encroached into upper elevations of mixed-conifer due to fire suppression since the late 1800’s altering the composition of these stands. At the upper ecotone, no successful conifer establishment occurred during the 1870s and 1880s but aspen cohorts regenerated corresponding to the fire history of synchronized fires during this time.
The author found that historically, the San Francisco Peaks of northern Arizona were dominated by ponderosa pine with an understory of scattered individuals of Douglas-fir, limber pine, and white fire. Currently, however, the species composition has shifted due to fire suppression to dense stands of less fire-tolerant Douglas-fir and white fir.
All forest types were significantly less dense in 1876 compared to 2000; however, the changes were less apparent in the higher altitude forests. The authors observed a shift in composition toward more mesic species in lower altitude forests. They suggest that because fire frequencies are typically longer in higher altitude forests, the impact of fire exclusion was not as great as low altitude forests where frequent fire thinned encroaching fire-susceptible mesic species.
The authors found that along the mixed-conifer/ponderosa pine ecotone there has been a shift toward increasing mixed-conifer forest, especially white fir and other shade-tolerant and fire-intolerant species in historical ponderosa pine stands due to fire suppression. Pole-sized white fir were clumped at all patch sizes due to a lack of thinning by fire. This has caused dense thickets of white fir that do not permit ponderosa pine seedlings from regenerating.
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 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.
The authors found that environmental factors such as topographic features and soils were more important in determining vegetation structure and composition than disturbances such as fire area or fire frequency. Only woodland grasslands were likely maintained by frequent fire historically.
Both climate fluctuations, fire suppression, and land use (i.e. grazing, timber harvest, etc.) have increased the tree densities of forests in Grand Canyon National Park., although the patterns of composition and regeneration varied along fire regime and elevational gradients.
The authors estimated the “natural” fire return interval for petran chaparral and shrubland vegetation in Mesa Verde at approximately 100 years and for piñon-juniper woodlands at approximately 400 years. The authors suggest that these long fire return intervals are unique to Mesa Verde and stands exist within the park that have not burned since 1300 AD and may be some of the oldest in the Four Corners region. They possess unique structural and compositional characteristics in their current condition.
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.
Differing fire survival strategies can lead to differences in abundance post-fire. Pines dominate through resistance to fire until oaks, which grow rapidly, catch up in abundance through resilience to fire. The authors suggest that continued fire exclusion may favor oak species over pine due to prolific sprouting in the understory while disrupting pine seedling establishment.
Fire suppression beginning in the 1800s and improvements in fire suppression techniques after the 1940s led to an increase in numerous ponderosa pine thickets throughout the park.
The cessation of fire beginning around 1883 in these forests has led to an abundance of resprouting oak, a species typically highly susceptible to fire.
Since Euro-American settlement, the density of ponderosa pine stands has increased drastically. Prolific regeneration due to fire exclusion, logging, and climate has changed the age structure and density of the forests over the past century. Without management or the reintroduction of fire to increase tree mortality, they predicted trends in increasing density and changes to forest structure to continue or burn in high severity, crown fire.
The authors found that fire suppression policies in the early 20th century have resulted in increasingly homogenous species composition in mixed-conifer forests with greater horizontal and vertical fuel continuity. These changes make these stands more susceptible to high-severity fires.
Increased tree densities and a shift toward fire-intolerant white fir in the understory due to fire suppression have impacted the timing of and the response of forest stands to infestations to some degree. The authors explain that white fir is more susceptible to mortality from spruce budworm, and increased mortality can lead to increased fire intensity. Regional outbreaks of spruce budworm were directly related to spring precipitation amount.
The authors related the increase in sheep herding and livestock grazing, beginning in approximately 1830, to a significant reduction in fire frequency across the Chuska Mountains. The current forest stand is dense with even-aged cohorts that established in the early 1900s, similar to other stand in the Southwest. The authors suggest, however, fire suppression and land use change alone were not responsible for the drastic changes to the forest structure. Instead, anthropogenic disturbance concurrent with favorable climate conditions for ponderosa pine regeneration likely both influenced the major structural alteration of these forest stands.