Regional Fire Years Research Articles

Climate forcing of regional fire years in the upper Great Lakes Region, USA

Background Drivers of fire regimes vary among spatial scales, and fire history reconstructions are often limited to stand scales, making it difficult to partition effects of regional climate forcing versus individual site histories. Aims To evaluate regional-scale historical fire regimes over 350 years, we analysed an extensive fire-scar network, spanning 240 km across the upper Great Lakes Region in North America. Methods We estimated fire frequency, identified regionally widespread fire years (based on the fraction of fire-scarred tree samples, fire extent index (FEI), and synchronicity of fire years), and evaluated fire seasonality and climate–fire relationships. Key results Historically, fire frequency and seasonality were variable within and among Great Lakes’ ecoregions. Climate forcing at regional scales resulted in synchronised fires, primarily during the late growing season, which were ubiquitous across the upper Great Lakes Region. Regionally significant fire years included 1689, 1752, 1754, 1791, and 1891. Conclusions We found significant climate forcing of region-wide fire regimes in the upper Great Lakes Region. Implications Historically, reoccurring fires in the upper Great Lakes Region were instrumental for shaping and maintaining forest resilience. The climate conditions that helped promote widespread fire years historically may be consistent with anticipated climate–fire interactions due to climate change.

Applying the usual rules to an unusual ecological situation: Fire rotation in Great Lakes Pine Forests

Fire regimes in Eastern North America are often determined from historical data because land-use change and natural resource policy have confounded natural fire processes. It is good practice to combine multiple historical data sources, which can serve to both corroborate findings and fill knowledge gaps that might exist when trying to gain a full picture of historical ecological processes. In the Great Lakes Region (GLR) fire rotation, or the number of years it takes to burn an area equivalent to the area of interest, is an extensively used metric that has been calculated based on the Euro-American settlement era General Land Office Public Land Survey (GLO) records. However, fire rotation methods and GLO records are best suited for understanding high-severity fire, and low- to moderate-severity fires have received less attention in forested ecosystems in this region. We used dendrochronological (tree-ring) data to evaluate GLO data and fire rotation methods in relation to low-severity fires. Tree-ring and GLO data were well-aligned in some ways, with high concurrence of tree species, tree density, and common fire dates. However, GLO data did not identify fires for survey points closest to any of our sites (n = 26), though 71% of sites burned within one year and all sites burned within 8 years of surveys. Mean fire return intervals for our sites ranged from 2 to 9 years for all fires and 6–20 years for fires recorded on ≥25% of samples within sites (1602–2018) with relatively minor effects of filtering on return intervals. Thus, fires were historically frequent and widespread within sites. We estimate that fires burned on average 858 km2 to 2564 km2 per year within five ecological landscapes with rotation intervals ranging between 11 (Northeast Sands) and 34 years (Northern Highlands; µ = 22 years across all five landscapes). We found 25 regional fire years that were synchronous among multiple (2–5) ecological landscapes over a 218-yr period with evidence that drought plays a role in regionally widespread fire years. High-severity fire was likely limited in the GLR; however, low- to moderate-severity fires were abundant, large-scale, widespread, and an important forcing mechanism shaping forests of the GLR over millennia.

Landscape–lake interactions in the Beartooth Mountains, Wyoming: a 350-year fire history reconstruction

Wildfire is the dominant natural disturbance agent in forests of the Greater Yellowstone Ecosystem, and can have long-lasting impacts on local landscapes and limnologic processes. We applied a multi-proxy paleolimnological approach to infer the impact of forest fires on the catchment of Island Lake, Wyoming (USA), over the last ~ 350 years. Specifically, we examined whether fossil diatom assemblages recorded indirect, limnological impacts from fire, including changes in water clarity, nutrient input, and lake mixing. The objective of this study was to determine if post-fire lake changes could be reconstructed effectively using fossil diatom assemblages from Island Lake, Wyoming, by comparing the diatom record to dendrochronological and macroscopic charcoal records. Local and regional fire years AD 1747 and 1900 were determined from tree-ring records, which were corroborated with sediment accumulation rates and magnetic susceptibility in Island Lake sediments. Inferred lacustrine conditions that were associated with fire events, particularly regional, mixed-severity fire year 1900, included increased turbidity, greater water column mixing, and possible augmented influx of total phosphorus to the system. Thus, the fossil diatom assemblage in Island Lake appears to have recorded forest fire activity indirectly, particularly when fire activity was severe or widespread within the catchment. An increase in small colonial Fragilaria (sensu lato) species since 1975, in association with the rise in abundance of Asterionella formosa and Tetracyclus glans, suggests recent changes in thermal stratification patterns, epilimnion temperatures, and available nutrients.

Climate and human influences on historical fire regimes (AD 1400–1900) in the eastern Great Basin (USA)

High fire activity in western North America is associated with drought. Drought and fire prevail under negative El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) phases in the Southwest and with positive phases in the Northwest. Here, I infer climate effects on historic fire patterns in the geographically intermediate, eastern Great Basin and seek out evidence of human influence on reconstructed fire regimes. Surface fire chronologies were constructed for 10 sites using tree-ring-based fire scars. Regional (67) and local (247) fire years and no-fire (187) years were identified from 1400 to 1900 CE. I compared fire chronologies with indices of the Palmer Drought Severity Index (PDSI), ENSO, and PDO. Regionally, fires were significantly more common during drought and were associated with negative ENSO and positive-to-negative PDO transitions while no-fire years were associated with positive ENSO and negative-to-positive PDO transitions. Conditions were significantly wetter 2 years prior to regional fire years and drier 4 years prior to no-fire years, providing evidence that fires were historically fuel limited. Local fire years occurred under a broad range of climate conditions. Most sites showed either persistent late or bimodal (early and late) fire seasonality patterns. These patterns are distinct from the mid-season peak observed for modern lightning-caused fires, suggesting a human influence on historical ignition patterns. Results demonstrate that climate was an important synchronizer of fire at the regional scale and that locally fire regimes were the product of climate-regulated fuels and some combination of human and lightning ignition patterns.

Fuel treatments and landform modify landscape patterns of burn severity in an extreme fire event.

Under a rapidly warming climate, a critical management issue in semiarid forests of western North America is how to increase forest resilience to wildfire. We evaluated relationships between fuel reduction treatments and burn severity in the 2006 Tripod Complex fires, which burned over 70,000 ha of mixed-conifer forests in the North Cascades range of Washington State and involved 387 past harvest and fuel treatment units. A secondary objective was to investigate other drivers of burn severity including landform, weather, vegetation characteristics, and a recent mountain pine beetle outbreak. We used sequential autoregression (SAR) to evaluate drivers of burn severity, represented by the relative differenced Normalized Burn Ratio index, in two study areas that are centered on early progressions of the wildfire complex. Significant predictor variables include treatment type, landform (elevation), fire weather (minimum relative humidity and maximum temperature), and vegetation characteristics, including canopy closure, cover type, and mountain pine beetle attack. Recent mountain pine beetle damage was a statistically significant predictor variable with red and mixed classes of beetle attack associated with higher burn severity. Treatment age and size were only weakly correlated with burn severity and may be partly explained by the lack of treatments older than 30 years and the low rates of fuel succession in these semiarid forests. Even during extreme weather, fuel conditions and landform strongly influenced patterns of burn severity. Fuel treatments that included recent prescribed burning of surface fuels were particularly effective at mitigating burn severity. Although surface and canopy fuel treatments are unlikely to substantially reduce the area burned in regional fire years, recent research, including this study, suggests that they can be an effective management strategy for increasing forest landscape resilience to wildfires.

A 1416-year reconstruction of annual, multidecadal, and centennial variability in area burned for ponderosa pine forests of the southern Colorado Plateau region, Southwest USA

Fire history reconstructions from fire scars in tree rings have been valuable for assessing fire regime changes and their climatic controls. It has been asserted, however, that these two- to four-century long records from the western USA are unrepresentative of longer periods of the Holocene and are of limited use for understanding current or future fire regimes. The Medieval Climate Anomaly (800–1300 ce) is often suggested as a better analog for future Southwestern US climates but is beyond the chronological range of most fire-scar studies in this region. To evaluate fire regime changes over the past millennium, we build on centennial-length fire–climate studies to generate a 1416 year long reconstruction of fire activity in ponderosa pine forests of the Southern Colorado Plateau region of Arizona and New Mexico. We used a split-period calibration and verification protocol to test the reliability of a multiple regression model using annual and antecedent precipitation (reconstructed from tree-ring width chronologies) to predict the percentage of fire-scar localities (i.e. sites, N=45) that recorded extensive fires within those sites (>25% of recorder trees scarred) each year between 1700 and 1899 ce. The model explains approximately 50% of the variation in annual fire activity. Applying the model to the entire precipitation reconstruction provides a proxy for annual area burned since 572 ce. There are no statistically significant differences between the period available for fire-scar study (1600 ce–present) and the Medieval Climate Anomaly (800–1300 ce) in terms of predicted annual area burned or the frequencies of regional fire years. Multidecadal and centennial variation in the frequencies of regional fire years, however, does indicate reduced surface fire frequencies from approximately 700–800 ce and 1360–1455ce. We hypothesize that these were periods when some forests were vulnerable to altered canopy structure, accumulated fuels, and increased fire severity.

MULTI-SEASON CLIMATE SYNCHRONIZED FOREST FIRES THROUGHOUT THE 20TH CENTURY, NORTHERN ROCKIES, USA

We inferred climate drivers of 20th-century years with regionally synchronous forest fires in the U.S. northern Rockies. We derived annual fire extent from an existing fire atlas that includes 5038 fire polygons recorded from 12,070,086 ha, or 71% of the forested land in Idaho and Montana west of the Continental Divide. The 11 regional-fire years, those exceeding the 90th percentile in annual fire extent from 1900 to 2003 (>102,314 ha or approximately 1% of the fire atlas recording area), were concentrated early and late in the century (six from 1900 to 1934 and five from 1988 to 2003). During both periods, regional-fire years were ones when warm springs were followed by warm, dry summers and also when the Pacific Decadal Oscillation (PDO) was positive. Spring snowpack was likely reduced during warm springs and when PDO was positive, resulting in longer fire seasons. Regional-fire years did not vary with El Niño-Southern Oscillation (ENSO) or with climate in antecedent years. The long mid-20th century period lacking regional-fire years (1935-1987) had generally cool springs, generally negative PDO, and a lack of extremely dry summers; also, this was a period of active fire suppression. The climate drivers of regionally synchronous fire that we inferred are congruent with those of previous centuries in this region, suggesting a strong influence of spring and summer climate on fire activity throughout the 20th century despite major land-use change and fire suppression efforts. The relatively cool, moist climate during the mid-century gap in regional-fire years likely contributed to the success of fire suppression during that period. In every regional-fire year, fires burned across a range of vegetation types. Given our results and the projections for warmer springs and continued warm, dry summers, forests of the U.S. northern Rockies are likely to experience synchronous, large fires in the future.

MULTI-SEASON CLIMATE SYNCHRONIZED HISTORICAL FIRES IN DRY FORESTS (1650–1900), NORTHERN ROCKIES, USA

Our objective was to infer the climate drivers of regionally synchronous fire years in dry forests of the U.S. northern Rockies in Idaho and western Montana. During our analysis period (1650-1900), we reconstructed fires from 9245 fire scars on 576 trees (mostly ponderosa pine, Pinus ponderosa P. & C. Lawson) at 21 sites and compared them to existing tree-ring reconstructions of climate (temperature and the Palmer Drought Severity Index [PDSI]) and large-scale climate patterns that affect modern spring climate in this region (El Niño Southern Oscillation [ENSO] and the Pacific Decadal Oscillation [PDO]). We identified 32 regional-fire years as those with five or more sites with fire. Fires were remarkably widespread during such years, including one year (1748) in which fires were recorded at 10 sites across what are today seven national forests plus one site on state land. During regional-fire years, spring-summers were significantly warm and summers were significantly warm-dry whereas the opposite conditions prevailed during the 99 years when no fires were recorded at any of our sites (no-fire years). Climate in prior years was not significantly associated with regional- or no-fire years. Years when fire was recorded at only a few of our sites occurred under a broad range of climate conditions, highlighting the fact that the regional climate drivers of fire are most evident when fires are synchronized across a large area. No-fire years tended to occur during La Niña years, which tend to have anomalously deep snowpacks in this region. However, ENSO was not a significant driver of regional-fire years, consistent with the greater influence of La Niña than El Niño conditions on the spring climate of this region. PDO was not a significant driver of past fire, despite being a strong driver of modern spring climate and modern regional-fire years in the northern Rockies.

Climate effects on historical fires (1630 - 1900) in Utah

We inferred climate effects on fire occurrence from 1630 to 1900 for a new set of crossdated fire-scar chronologies from 18 forested sites in Utah and one site in eastern Nevada. Years with regionally synchronous fires (31 years with fire at ≥20% of sites) occurred during drier than average summers and years with no fires at any site (100 years) were wetter than average. Antecedent wet summers were associated with regional-fire years in mixed-conifer and ponderosa pine forest types, possibly by affecting fine fuel amount and continuity. NINO3 (an index of the El Niño–Southern Oscillation, ENSO) was significantly low during regional-fire years (La Niñas) and significantly high during non-fire years (El Niños). NINO3 also was high during years before regional-fire years. Although regional fire years occurred nearly twice as often as expected when NINO3 and the Pacific Decadal Oscillation were both in their cool (negative) phases, this pattern was not statistically significant. Palmer Drought Severity Index was important for fire occurrence in ponderosa pine and mixed-conifer forests across the study area but ENSO forcing was seen only in south-eastern sites. Results support findings from previous fire and climate studies, including a possible geographic pivot point in Pacific basin teleconnections at ~40°N.

CLIMATE EFFECTS ON FIRE REGIMES AND TREE RECRUITMENT IN BLACK HILLS PONDEROSA PINE FORESTS

Climate influences forest structure through effects on both species demography (recruitment and mortality) and disturbance regimes. Here, I compare multi-century chronologies of regional fire years and tree recruitment from ponderosa pine forests in the Black Hills of southwestern South Dakota and northeastern Wyoming to reconstructions of precipitation and global circulation indices. Regional fire years were affected by droughts and variations in both Pacific and Atlantic sea surface temperatures. Fires were synchronous with La Niñas, cool phases of the Pacific Decadal Oscillation (PDO), and warm phases of the Atlantic Multidecadal Oscillation (AMO). These quasi-periodic circulation features are associated with drought conditions over much of the western United States. The opposite pattern (El Niño, warm PDO, cool AMO) was associated with fewer fires than expected. Regional tree recruitment largely occurred during wet periods in precipitation reconstructions, with the most abundant recruitment coeval with an extended pluvial from the late 1700s to early 1800s. Widespread even-aged cohorts likely were not the result of large crown fires causing overstory mortality, but rather were caused by optimal climate conditions that contributed to synchronous regional recruitment and longer intervals between surface fires. Synchronous recruitment driven by climate is an example of the Moran effect. The presence of abundant fire-scarred trees in multi-aged stands supports a prevailing historical model for ponderosa pine forests in which recurrent surface fires affected heterogenous forest structure, although the Black Hills apparently had a greater range of fire behavior and resulting forest structure over multi-decadal time scales than ponderosa pine forests of the Southwest that burned more often.

Mixed-severity fire regime in a high-elevation forest of Grand Canyon, Arizona, USA

Fire regime characteristics of high-elevation forests on the North Rim of the Grand Canyon, Arizona, were reconstructed from fire scar analysis, remote sensing, tree age, and forest structure measurements, a first attempt at detailed reconstruction of the transition from surface to stand-replacing fire patterns in the Southwest. Tree densities and fire-/non-fire-initiated groups were highly mixed over the landscape, so distinct fire-created stands could not be delineated from satellite imagery or the oldest available aerial photos. Surface fires were common from 1700 to 1879 in the 4,400 ha site, especially on S and W aspects. Fire dates frequently coincided with fire dates measured at study sites at lower elevation, suggesting that pre-1880 fire sizes may have been very large. Large fires, those scarring 25% or more of the sample trees, were relatively infrequent, averaging 31 years between burns. Four of the five major regional fire years occurred in the 1700s, followed by a 94-year gap until 1879. Fires typically occurred in significantly dry years (Palmer Drought Stress Index), with severe drought in major regional fire years. Currently the forest is predominantly spruce-fir, mixed conifer, and aspen. In contrast, dendroecological reconstruction of past forest structure showed that the forest in 1880 was very open, corresponding closely with historical (1910) accounts of severe fires leaving partially denuded landscapes. Age structure and species composition were used to classify sampling points into fire-initiated and non-fire-initiated groups. Tree groups on nearly 60% of the plots were fire-initiated; the oldest such groups appeared to have originated after severe fires in 1782 or 1785. In 1880, all fire-initiated groups were less than 100 years old and nearly 25% of the groups were less than 20 years old. Non-fire-initiated groups were significantly older (oldest 262 years in 1880), dominated by ponderosa pine, Douglas-fir, or white fir, and occurred preferentially on S and W slopes. The mixed-severity fire regime, transitioning from lower-elevation surface fires to mixed surface and stand-replacing fire at higher elevations, appeared not to have been stable over the temporal and spatial scales of this study. Information about historical fire regime and forest structure is valuable for managers but the information is probably less specific and stable for high-elevation forests than for low-elevation ponderosa pine forests.

Fire regimes and forest structure in the Sierra Madre Occidental, Durango, Mexico

El fuego frecuente de baja intensidad es un agente clave de disturbio en los bosques de pinos de hojas largas del occidente de Norteamérica; sin embargo, se conoce poco de la ecología del fuego en los bosques mexicanos que han sido menos afectados por la exclusión del fuego. Comparamos la secuencia de disturbio causado por fuego y la estructura forestal de cuatro sitios, poco o no explotados, que varían en su reciente historial de fuegos. Los frecuentes incendios de baja intensidad, que se presentaron periódicamente cada 4 a 5 años, si se toman en cuenta todos los incendios, y cada 6 a 9 años para el caso de los incendios extensos, caracterizaron todos los sitios hasta el inicio de la exclusión de fuego a mediados del siglo XX en tres de las cuatro parcelas de estudio. Aunque se considera que la mayoría de los incendios en el área son de origen humano, en los sitios de referencia se observaron evidencias de igniciones causadas tanto por las descargas eléctricas, como por el hombre. Una posible conexión entre la incidencia de los incendios y el clima quedó señalada por la correlación encontrada entre los años con incendios regionales y los extremos positivos del índice de oscilación meridional, asociados con condiciones de clima frío y seco. Las estructuras del ecosistema forestal diferían de acuerdo con los efectos del aclareo de árboles y del consumo de material combustible. Dos sitios con una larga exclusión de incendios se caracterizaron por masas relativamente densas de árboles más pequeños y jóvenes, altas cantidades de material combustible, y capas de humus más profundas. En contraste, tanto en la parcela que se quemó después de un período de exclusión de fuego de 29 años, como también en el último sitio donde los incendios frecuentes han continuado hasta el presente, se registraron bosques relativamente abiertos dominados por árboles más grandes y maduros. Las áreas de muestreo recientemente quemadas tenían menos material combustible, especialmente de madera podrida, y capas de humus menos profundas. La gran cantidad de renuevos en combinación con la baja densidad del estrato superior de los sitios recientemente quemados es también consistente con el efecto de aclareo de los incendios de poca intensidad. Las estrategias para la conservación y el manejo a largo plazo de estos bosques deben reconocer el papel histórico del disturbio causado por el fuego, así como el potencial de cambios en la intensidad de los incendios y los efectos ecológicos de una larga exclusión de fuego.