Fire and Ecosystem Effects Interactions
How do fire frequency, rotation, and return interval relate to tree dispersal, regeneration, and establishment?
The authors found that fire quiescent periods due to higher moisture availability had the greatest influence on regeneration pulses of ponderosa pine historically.
Climate and topography were more influential on post-fire species composition than time-since-fire in the Sonoran Desert. Precipitation events soon after the fire had the strongest effects on recovery of vegetation height, and vegetation recovery increased with increasing elevation and the associated annual average increase in precipitation. The authors suggest that abiotic climate conditions may more strongly influence post-fire vegetation recovery than time since fire in desert and arid ecosystems.
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 literature generally agrees that stand-replacing fire both reduces encroaching conifer populations and creates favorable conditions for aspen to resprout post-fire. Because fire return intervals in upland, mesic sites are generally longer, it is unclear if fire suppression has affected aspen regeneration.
The authors found that the initial reintroduction of fire reduced tree densities, but resulted in the establishment of a dense stand of lodgepole pine, trending toward an alternative trajectory. However, a second fire killed the regenerating lodgepole pine cohort, as well as the Douglas-fir understory, and returned to a ponderosa pine dominated forest. This suggests that ponderosa pine forests do possess latent resilience to reintroduced fire after extended periods of fire exclusion.
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 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.
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. The authors suggest that more frequent fire may reduce creosotebush and other woody shrub recruitment.
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
Episodes of synchronous tree recruitment and growth were highly associated with periods of increased moisture. These pluvial periods limited fire frequency and likely contributed to the increased establishment and survivorship of seedlings. The authors propose that variations in fire frequency and timing were more important in shaping forest structure than variations in fire severity.
Pulses of regeneration occurred during several periods of increased moisture; however, the authors suggest it was the absence of fire that allowed may trees to survive into adulthood and not due to episodic mortality caused by fire or other disturbance. Climate change likely affected fuel conditions that were less favorable for burning during these fire?quiescent periods allowing fuels to build up until drier years when synchronous regional fires occurred. This pattern has been commonly attributed to the ENSO cycle with wet El Niño years followed by dry La Niña years.
Although the authors did not examine the causal features of tree encroachment into meadows, a disruption in the fire regime beginning in the late 1870s and early 1880s coincided with the establishment of ponderosa pine, spruce, and aspen stands near and within some meadow sites.
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.
In forests where fire has been suppressed long-term, sites are dense and dominated by seed-reproducing conifer species, whereas more frequent fire site regeneration is dominated by sprouting species in the understories of open pine forest.
Historically, frequent fires led to abundant seedling recruitment of both pine and oak species, but low survival into maturity. Fire suppression starting in the early 1900’s led to low levels of seedling recruitment, but allowed young trees to mature without disturbance for long periods of time.
The authors found that the stand age distribution was multimodal with peaks of establishment due to interruptions to the historical frequent-fire regime. Due to fire exclusion, reduced understory competition, and favorable climate, abundant regeneration in the 20th century raised forest density from 60 trees/ha in 1876 to >3000 trees/ha in 1992.
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 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.