Mean Fire Return Interval Research Articles

A Holocene fire history from Terra Nova National Park, Newfoundland, Canada: vegetation and climate change both influenced the fire regime

Fire is the largest natural disturbance factor in the boreal forest and plays a critical role in the composition, structure, and succession of stands and landscapes. The island of Newfoundland, located in eastern Canada, is subjected to a greater maritime influence, which may result in longer fire return intervals. The limited data on the fire regime does not account for interactions between fire, vegetation, and climate throughout the Holocene. We used sediment cores from Arnold’s Pond, Terra Nova National Park, which covered the last ~11,800 cal. yr BP, to investigate these interactions. We recognize 4 pollen zones and macroscopic charcoal analysis detected 45 local fire events. The 250-year mean fire return interval associated with the current vegetation is longer than a previous estimate for the park, but significantly shorter than other estimates for the island. Our mean fire return interval is within the range of fire estimates from Québec with similar vegetation. Our results suggest that the fire regime was primarily influenced by vegetation and climate. The transition to an open forest from a shrub tundra resulted in increased fire activity and fire frequency, which were likely driven by additional fuel on the landscape, but could have also been influenced by unknown climatic factors. We identified several examples of changes in the fire frequencies and/or charcoal accumulation that coincided with regional climate shifts, but we also identified a non-synchronous change. The non-synchronous shift to drier conditions resulted in a ~500-year time lag between peak Pinus strobus abundance and maximum fire frequency. Synchronous shifts in fire activity and/or fire frequency coincided with the 8200 event, Medieval Climate Anomaly and Little Ice Age. We also noted a decrease in fire frequency between 2600–1500 cal. yr BP that coincided with similar changes in the fire frequency from ~3000–1000 cal. yr BP in Québec. Our study highlights the complex interactions influencing the fire regime in our study area during the Holocene.

Unearthing the legacy of wildfires: post fire pyrogenic carbon and soil carbon persistence across complex Pacific Northwest watersheds

Wildfires have the potential to dramatically alter the carbon (C) storage potential, ecological function, and the fundamental mechanisms that control the C balance of Pacific Northwest (PNW) forested ecosystems. In this study, we explored how wildfire influences processes that control soil C stabilization and the consequent soil C persistence, and the role of previous fire history in determining soil C fire response dynamics. We collected mineral soils at four depth increments from burned (low, moderate, and high soil burn severity classes) and unburned areas and surveyed coarse woody debris (CWD) in sites within the footprint of the 2020 Holiday Farm Fire and in surrounding Willamette National Forest and the H.J. Andrews Experimental Forest. We found few changes in overall soil C pools as a function of fire severity; we instead found that unburned sites contained high levels of pyrogenic C (PyC) that were commensurate with PyC concentrations in the high severity burn sites—pointing to the high background rate of fire in these ecosystems. An analysis of historical fire events lends additional support, where increasing fire count is loosely correlated with increasing PyC concentration. An unexpected finding was that PyC concentration was lower in low soil burn severity sites than in control sites, which we attribute to fundamental ecological differences in regions that repeatedly burn at high severity compared with those that burn at low severity. Our CWD analysis showed that high mean fire return interval (decades between fire events) was strongly correlated with low annual CWD accumulation rate; whereas areas that burn frequently had a high annual CWD accumulation rate. Within the first year postfire, trends in soil density fractions demonstrated no significant response to fire for the mineral-associated organic matter pool but slight increases in the particulate pool with increasing soil burn severity—likely a function of increased charcoal additions. Overall, our results suggest that these PNW forest soils display complex responses to wildfire with feedbacks between CWD pools that provide varying fuel loads and a mosaic fire regime across the landscape. Microclimate and historic fire events are likely important determinants of soil C persistence in these systems.

Top-down control of climate on long-term interactions between fires, tree-cover and soil erosion in a Mediterranean mountain, Corsica

Fire is a potential significant driver of soil erosion in the Mediterranean area, as it results in the partial removal of vegetation and the alteration of organic matter, affecting soil structure and stability. The erosion of particle sizes is influenced by the intensity and duration of rainfall, regulated by disturbance regimes and vegetation cover. We hypothesize that, during the Holocene, high fire frequency and intensity under precipitation control may have contributed to soil erosion, while plant cover and composition could have mitigated it. This plant cover, in turn, might have been influenced by biomass burning. To test this hypothesis, we conducted a detailed analysis of sediments spanning the last 11,500 years from a small mountain lake, Corsica, situated in the black pine forest belt. The high temporal resolution analysis (∼10 years per sample), used granulometry and loss-on-ignition as proxies for erosion and ecosystem productivity in both the lake and watershed, and fire and plant macroremains as fire and tree cover proxies, respectively. The correlation between particle size and the contents of organic or mineral matter with mean fire return intervals (FRI) revealed significant patterns. Long fire intervals were associated with more fine and coarse sands, whereas short mean FRI positively correlated with clay and, fine and coarse silt, along with higher total mineral and organic matter contents. These findings suggest that fires were more frequent when rain duration was sustained but runoff intensity was lower. Conversely, wildfires were less frequent during intense runoff periods (wetter climate). Unlike fire frequency, fire severity did not correlate with erosion, and tree cover and plant richness had minimal to no effect. The 8.2 kyr event was characterized by runoff transporting primarily coarse sands, i.e. a dry period with very intense rains. This suggests that the system is primarily top-down controlled by climate. Multimillennial erosion trends are influenced by fire frequency and precipitation regimes, whereas vegetation does not seem to have a mitigating effect on this process.

Exceptional variability in historical fire regimes across a western Cascades landscape, Oregon, USA

AbstractDetailed information about the historical range of variability in wildfire activity informs adaptation to future climate and disturbance regimes. Here, we describe one of the first annually resolved reconstructions of historical (1500–1900 ce) fire occurrence in coast Douglas‐fir dominated forests of the west slope of the Cascade Range in western Oregon. Mean fire return intervals (MFRIs) across 16 sites within our study area ranged from 6 to 165 years. Variability in MFRIs was strongly associated with average maximum summer vapor pressure deficit. Fire occurred infrequently in Douglas‐fir forest stands seral to mountain hemlock or silver fir, but fire frequency was much shorter than predicted by theory in other forest types. MFRIs within Douglas‐fir stands seral to western hemlock or grand fir ranged from 19 to 45 years, and MFRIs in stands seral to Douglas‐fir ranged from 6 to 11 years. There was little synchrony in fire occurrence or tree establishment across 16 sites separated by 4 km. The lack of synchrony in fire suggests that large, wind‐driven fire events that are often considered to be characteristic of coast Douglas‐fir forests were not an important driver of succession in our study area during the last ~400–500 years. Climate was more arid than normal during fire years in most forest types, but historical fire in stands seral to Douglas‐fir was strongly associated with antecedent moisture and less strongly associated with drought. We interpret the extraordinary tempo of fire we observed in stands seral to Douglas‐fir and the unique climate pattern associated with fire in these stands to be indicative of Indigenous fire stewardship. This study provides evidence of far more frequent historical fire in coast Douglas‐fir forests than assumed by managers or scientists—including some of the most frequent fire return intervals documented in the Pacific Northwest. We recommend additional research across the western Cascades to create a comprehensive account of historical fire in highly productive forests with significant cultural, economic, and ecological importance.

Contemporary wildfires are more severe compared to the historical reference period in western US dry conifer forests

Dry conifer forests in the western US historically experienced frequent fire prior to European American colonization. Mean fire return interval ranged from about 5–35 years, with the majority of fires burning at low-to-moderate severity. The arrival of European Americans initiated notable reductions in fire frequency through the cessation of Indigenous burning, livestock grazing, and fire suppression. These activities effectively excluded fire from most dry conifer forests, thereby allowing live and dead fuels to accumulate. There are concerns that this fuel accumulation is causing contemporary fires (since 1985) to burn more severely compared to pre-colonization time periods, thereby increasing potential for enduring fire-catalyzed conversions to non-forest. Here, we used satellite-derived fire severity data to characterize the prevalence of contemporary (1985–2020) stand-replacing fire in dry conifer forests within four broad ecoregions for all fires, non-wilderness fires, wilderness fires, and prescribed fires. Our results indicate, that as a proportion of area burned, contemporary fires experienced 2.9 to 13.6 times more stand-replacing fire (depending on the ecoregion) compared to the pre-colonization period. Non-wilderness areas exhibit somewhat higher prevalence of stand-replacing fire, relative to the historical fire regime, than wilderness areas (where logging is prohibited). The relatively small difference between non-wilderness and wilderness suggests that fuel accumulation resulting from fire exclusion has played a larger role than historical logging activities on the prevalence of contemporary stand-replacing fire. Prescribed fires do not exhibit a higher prevalence of stand-replacing fire compared to the historical fire regime. We conducted a parallel analysis in the Gila Wilderness in New Mexico, USA, is it is arguably the best example of an unrestrained fire regime (many fires are not suppressed) in western US dry conifer forests. The Gila Wilderness proportionally experienced only 1.8 times more stand-replacing fire compared to the historical reference period; in contrast, the southwest ecoregion experienced 5.9 times more stand-replacing fire. Fire will inevitably burn a given forest no matter how much money and personnel we commit to fire prevention and suppression, and because of the high prevalence of stand-replacing fire, the long-term persistence of dry conifer forests is jeopardized. Unless forest managers are provided the social license and enabling conditions to substantially accelerate efforts to restore conditions resistant to stand-replacing fire, we anticipate that the extent of dry conifer forests will continue to erode as fire-catalyzed conversions increasingly unfold across the western US.

Pastoral Burning and Its Contribution to the Fire Regime of Alto Minho, Portugal

Alto Minho (in northwestern Iberia) is one of the European regions most affected by fires. Many of these fires originate from rangeland management of Atlantic heathlands, and, while being illegal, often are not actively suppressed. In this study, pastoral fires (autumn-to-spring fires unrecorded by authorities), spring wildfires, and summer wildfires were independently mapped and dated from remote sensing. Alto Minho burned at a mean annual rate of 5.0% of the territory between 2001 and 2020. Pastoral burning totalled 40,788 hectares during the period, accounting for 20% of the total burnt area. Rangeland burning occurs mostly from December to April, the rainiest months that guarantee the conditions for pasture renewal and fire self-extinction. The mean fire return interval of pastoral burning is slightly higher than that of wildfires (13 years vs. 11 years), except in part of the inner mountains where it dominates fire activity. Pastoral fires are more frequent and largely prevail over wildfires in the parishes with higher livestock quantities. Conversely, the largest wildfires and higher summer burnt areas correspond with very low livestock and nearly non-existing pastoral fires. Traditional fire knowledge should not be overlooked by fire management, as it contributes to more sustainable fire regimes and ecosystems.

Degradation and restoration of Indigenous California black oak (Quercus kelloggii) stands in the northern Sierra Nevada

BackgroundThe cultural connections of the Maidu to the lands they inhabit are profound with burning being a major component of their culture. California black oak plays an important role in the lifeways of many Indigenous tribes and Native peoples tend black oaks. We used multiple lines of evidence to reconstruct Indigenous fire use in a mixed conifer forest in the northern Sierra Nevada. This includes summarizing oral traditions by the Mountain Maidu, quantifying current and historical forest structure, and characterizing fire effects in an area impacted by the 2021 Dixie Fire.ResultsThe promotion of fire, both lighting- and human-ignited, has always been part of Mountain Maidu land stewardship. The mixed conifer forests in our focal study site had very high tree densities that were dominated by small, shade-tolerant species and the historic mean fire return interval was 8.6 years. Twenty-three fire intervals of 10 years or less and 11 fire intervals of 5 years or less were recorded. Based on forest inventory data collected in 1924, a majority of the forests in the broader region surrounding our focal site (82%) did not experience high levels of conifer competition. This is a stark contrast to contemporary forest conditions which showed that 73% of forests either were experiencing full site occupancy or were within the zone of imminent mortality. Contemporary forests not experiencing high levels of conifer competition had the highest density of large California black oak while forests in the zone of imminent mortality had the lowest density of large California black oak.ConclusionsFire exclusion and suppression led to a slow, albeit catastrophic, shift for California black oaks. The mixed conifer forests sampled in the early 1920s had very low tree densities and were dominated by large ponderosa and sugar pine trees which would have allowed California black oak to thrive. The number of very short fire intervals recorded is difficult to explain by lightning alone, pointing to the management of this area for Indigenous purposes. The 2021 wildfire resulted in 100% mortality of all conifer trees in the area but 61% of the California black oaks resprouted. This provides an opportunity to remove the conifer snags and allow the California black oaks to regain dominance. It would take active stewardship to produce the oak dominance again but it is possible and could be co-led by the Maidu and federal managers.

Holocene fire regimes, fire‐related plant functional types, and climate in south‐coastal British Columbia forests

AbstractPaleoecological records of past fire events and forest composition provide long‐term ecological context for modern changes in fire regimes and forest dynamics. Here, we use pollen and contiguous macroscopic charcoal analyses of lake sediments from Pender Island, British Columbia, Canada to reconstruct changes in fire regimes over the last 10,000 years and investigate how these interact with changes in climate and forest composition with a focus on fire‐related plant functional types. The relatively warm and dry early Holocene was characterized by high charcoal accumulation rates, fire episodes of moderate severity, and a mean fire return interval of 100 ± 27 years. Forests at the time were open‐canopy Pseudotsuga menziesii forests with abundant fire endurer taxa (e.g., Pteridium aquilinum) that have a competitive advantage in regimes of frequent fire. Fire continued to occur every ~100 years, on average, during the establishment of Quercus garryana savanna communities; however, a decrease in charcoal peak magnitudes suggests the fire regime shifted to one characterized by smaller and/or lower intensity surface fires. As temperature and moisture deficits decreased in the mid‐ and late Holocene (i.e., after ~6000 calendar years before present), mean fire return intervals lengthened to 176 ± 54 years and increased variability in charcoal peak magnitudes suggests a mixed fire regime of low‐moderate‐intensity fires combined with infrequent crown or stand‐replacing fires. Relatively stable and moderate climate, longer fire return intervals, and mixed‐severity fires allowed P. menziesii (a fire resister) to dominate closed‐canopy forests and for fire avoiders to gradually become more common forest constituents. Millennial‐scale climate change has acted as the dominant driver of changes in both fire regimes and forest composition over the last 10,000 years; however, changes in fire‐related plant functional types highlight the important role that interactions between vegetation and fire play in long‐term fire regimes and forest dynamics.

Successional Change and Fire History in Montane Longleaf Pine-Dominated Ecosystems of Northwestern Georgia, USA

In the absence of fire, Pinus palustris (Longleaf Pine) ecosystems in the southeastern United States are vulnerable to successional change including mesophication, a process where increases in the importance of mesic and fire-intolerant species reduce biodiversity and thwart efforts to restore these systems. To determine the degree and nature of this successional trajectory at a local scale, we examined changes in species composition of a montane Longleaf Pine ecosystem in northwest Georgia using modern vegetation surveys and historical “witness tree” data. We also determined historical fire frequency using fire scars within Longleaf Pine stumps cross-dated to extant Longleaf Pine trees. Modern forest composition contained significantly more fire-intolerant taxa than the historical forest due to increased abundance of mesophytic species, especially Acer rubrum (Red Maple) and decline of Pinus spp. (pines) and some pyrophytic Quercus spp. (oaks). Counter to expectation, there were few differences in species composition between northeastern- and southwestern-facing slopes in modern or historical data. Fire scars indicated a historical mean fire-return interval of 5.5 years and suggest that without the reintroduction of more frequent prescribed fire, regeneration of montane Longleaf Pine is unlikely. Additional restorative techniques may also help these stands support greater biodiversity over time. For instance, co-dominance of pines and oaks in the historical forest suggests that fire-tolerant oaks should be retained in montane restoration efforts.

Pre-Columbian red pine (Pinus resinosa Ait.) fire regimes of north-central Pennsylvania, USA

BackgroundFire-dependent vegetation communities in the northeastern USA have undergone significant transitions since social and ecological disruptions associated with Euro-American colonization of North America. There is much interest, though little information available, about historical fire regimes of fire-adapted vegetation communities in the northeastern USA in pre-Columbian times (i.e., pre-1492 CE). We intensively investigated a red pine (Pinus resinosa Ait.)—oak (Quercus spp.) forested landscape in north-central Pennsylvania, USA, for exceptionally old red pine remnants (stumps, snags) where previous research had suggested the potential to develop fire-scar and tree-ring chronologies which extend to time periods prior to 1492.ResultsTree-ring and fire-scar chronologies recovered from red pine trees extend back to 1370 and 1402, respectively. Red pine trees were broadly distributed across this landscape (34.4 km2 area) prior to 1492, and frequent fire (i.e., mean fire return intervals of 4.1 years at landscape scale, 9.7–11.7 years at smaller spatial scales (0.5–7.1 km2)) was observed in the time period prior to sustained contact between Indigenous peoples and Euro-American colonizers circa 1609.ConclusionsThis study resulted in the longest fire-scar record in eastern North America and offers unique foundational ecological information regarding pre-Columbian fire regimes of northeastern USA fire-adapted forest communities. Fire scars provide physical evidence that fire was an important ecological component of this landscape before and after 1492 and that the current period of reduced fire activity is unprecedented in the 500 years prior to fire exclusion practices that were widely adopted circa 1915. Beyond fire scars, the spatial and temporal extent of the red pine tree-ring record provides evidence that red pine, a fire-dependent species, was historically a more important component of this landscape than it is today. The conservation of red pine vegetation communities will require incorporating recurring fire into management plans, for the successful regeneration and perpetuation of red pine, and to sustain associated flora and fauna of conservation concern.

The influence of land-use activities and regional drought on historical fire regimes of Buryatia, Siberia

Every year, millions of hectares burn across Siberia, driven by a combination of warming temperatures, regional drought and human-caused ignitions. Dendrochronology provides a long-term context to evaluate recent trends in fire activity and interpret the relative influence of humans and climate drivers on fire regimes. We developed a 400 year record of fire-scarred trees from 17 sites in pine-dominated forests located southeast of Lake Baikal. Site-level mean fire return intervals (MFIs) ranged from 4 to 27 years for all fires and 8 to 35 years for widespread fires within sites. Sites with the lowest MFI values were located within 1 km of agricultural fields in grassland valleys, suggesting that agricultural burning influenced MFIs at nearby sites. Fire frequency varied over the record, with significantly high values around 1790, from 1865 to 1880, 1948 to 1955 and 1995 to 2005. The increased fire activity corresponded with migration waves to the region and major socio-economic change connected with the establishment and breakdown of the Soviet Union. At broader scales, superposed epoch analysis showed that synchronous fire years were associated with regional drought and precipitation deficits. Wet conditions for 2–3 years prior to the event year were also significant, suggesting that increased moisture promoted growth of understory fine fuels to support more extensive fires across the study area. Although fire frequencies increased during the 20th century, fire–climate relationships weakened, suggesting increased human-caused ignitions may override regional climate drivers. Our dataset presents a continuous record of frequent surface fires over the past 400 years, providing a valuable opportunity to compare dendrochronology-based reconstructions with satellite and documentary records.

Resilience and sensitivity of ecosystem carbon stocks to fire-regime change in Alaskan tundra

Fire disturbance has increased in some tundra ecosystems due to anthropogenic climate change, with important ramifications for terrestrial carbon cycling. Assessment of the potential impact of fire-regime change on tundra carbon stocks requires long-term perspectives because tundra fires have been rare historically. Here we integrated the process-based Dynamic Organic Soil version of the Terrestrial Ecosystem Model with paleo-fire records to evaluate the responses of tundra carbon stocks to changes in fire return interval (FRI). Paleorecords reveal that mean FRIs of tundra ecosystems in Alaska ranged from centennial to millennial timescales (200–6000 years) during the late Quaternary, but projected FRIs by 2100 decrease to a few hundred years to several decades (70–660 years). Our simulations indicate threshold effects of changing FRIs on tundra carbon stocks. Shortening FRI from 5000 to 1000 years results in minimal carbon release (<5%) from Alaskan tundra ecosystems. Rapid carbon stock loss occurs when FRI declines below 800 years trigger sustained mobilization of ancient carbon stocks from permafrost soils. However, substantial spatial heterogeneity in the resilience/sensitivity of tundra carbon stocks to FRI change exists, largely attributable to vegetation types. We identified the carbon stocks in shrub tundra as the most vulnerable to decreasing FRI because shrub tundra stores a large share of carbon in combustible biomass and organic soils. Moreover, our results suggest that ecosystems characterized by large carbon stocks and relatively long FRIs (e.g. Brooks Foothills) may transition towards hotspots of permafrost carbon emission as a response to crossing FRI thresholds in the coming decades. These findings combined imply that fire disturbance may play an increasingly important role in future carbon balance of tundra ecosystems, but the net outcome may be strongly modulated by vegetation composition.

Historical fire regimes of North American hemiboreal peatlands

Peatlands contain one-third to one-half of global soil carbon, and disturbances, specifically fire, directly influence these carbon stocks. Despite this, historical variability of peatland fire regimes is largely unknown. This gap in knowledge partly stems from reconstructions of peatland fire regimes with methods limited to evaluating infrequent, severe fire events and not capturing frequent, low-severity events. Furthermore, variability in fire regimes is likely higher in heterogenous landscapes like the hemiboreal subzone, the transition between boreal and temperate biomes, where peatlands are embedded in landscapes including forests with high proportions of fire-dependent species, such as red pine (Pinus resinosa), that are well adapted to frequent low-severity fires. Here, we sought to evaluate the role of low- and moderate-severity fires within hemiboreal peatlands in central North America to better understand historical variability in fire regimes. We reconstructed historical fire regimes using fine-scale (temporal and spatial) dendrochronology methods to estimate frequency of low- and moderate-severity fires, identify synchronous fire events among forested uplands within and surrounding individual peatlands as well as among sites, and assess fire-climate relationships. We collected 220 cross-sections or partial-tree sections within three poor fen peatlands across the Great Lakes Region. Using standard dendrochronological techniques, we crossdated 129 samples, assigning dates to 414 fire scars (128 unique fire years) comprising a 500-year tree-ring record (1520–2019). Prior to the mid-1900s, fire events were frequent and widespread within peatlands we evaluated, with mean fire return intervals (MFRI) ranging from 7 to 31 years. Fire events were also synchronous among forested uplands within and surrounding peatlands. Fires predominantly occurred in the dormant and latewood (growing season) positions and during regionally dry conditions corresponding to mild and moderate drought (Palmer Drought Severity Index ≥ −2.99) but interestingly not during regionally severe drought (Palmer Drought Severity Index ≤ −3.00). While large-scale, high-severity fires are important to the ecology of peatlands and to changing climate-fire interactions, our results suggest that widespread low- to moderate-severity fires were historically frequent in hemiboreal peatlands and likely central to their development and maintenance. Evaluating whether peatlands will continue to be carbon sinks or become carbon sources due to climate change requires an understanding of the inherent variability in fire regimes, especially in hemiboreal systems.

Robust projections of future fire probability for the conterminous United States

Globally increasing wildfires have been attributed to anthropogenic climate change. However, providing decision makers with a clear understanding of how future planetary warming could affect fire regimes is complicated by confounding land use factors that influence wildfire and by uncertainty associated with model simulations of climate change. We use an ensemble of statistically downscaled Global Climate Models in combination with the Physical Chemistry Fire Frequency Model (PC2FM) to project changing potential fire probabilities in the conterminous United States for two scenarios representing lower (RCP 4.5) and higher (RCP 8.5) greenhouse gas emission futures. PC2FM is a physically-based and scale-independent model that predicts mean fire return intervals from both fire reactant and reaction variables, which are largely dependent on a locale's climate. Our results overwhelmingly depict increasing potential fire probabilities across the conterminous US for both climate scenarios. The primary mechanism for the projected increases is rising temperatures, reflecting changes in the chemical reaction environment commensurate with enhanced photosynthetic rates and available thermal molecular energy. Existing high risk areas, such as the Cascade Range and the Coastal California Mountains, are projected to experience greater annual fire occurrence probabilities, with relative increases of 122% and 67%, respectively, under RCP 8.5 compared to increases of 63% and 38% under RCP 4.5. Regions not currently associated with frequently occurring wildfires, such as New England and the Great Lakes, are projected to experience a doubling of occurrence probabilities by 2100 under RCP 8.5. This high resolution, continental-scale modeling study of climate change impacts on potential fire probability accounts for shifting background environmental conditions across regions that will interact with topographic drivers to significantly alter future fire probabilities. The ensemble modeling approach presents a useful planning tool for mitigation and adaptation strategies in regions of increasing wildfire risk.

Fire and the persistence, decline and extirpation of Protea roupelliae subsp. roupelliae trees in a montane grassland

We investigated long-term changes, from 1979 to 2015, in the abundance of Protea roupelliae subsp. roupelliae trees growing in a montane grassland of the uKhahlamba-Drakensberg mountains. The study involved counting the numbers of adults and juveniles (saplings and seedlings) in six groves on three occasions over 36 years, documenting the fire-return intervals and the seasons when fires occurred in each grove, and measuring the fuel loads of the grass swards in five of the groves. Only in one grove did adults persist in their original numbers, but not the juveniles. Adults and juveniles declined in three groves, and in two groves both adults and juveniles died. Neither mean fire-return interval nor its variation, or seasonal distributions of fire could account for the transition from tree persistence to extirpation among the six groves after 36 years. Instead, the only plausible explanation for the persistence of Protea trees in one grove was that its grass sward produced a significantly lower mean, but higher variation in fuel load (biomass), compared with groves where Protea tree abundances either declined or were extirpated. As such, the recruitment bottleneck imposed on seedling establishment by the grass sward is an obvious avenue for future research.

Winter roost selection of Lasiurine tree bats in a pyric landscape.

Day-roost selection by Lasiurine tree bats during winter and their response to dormant season fires is unknown in the southeastern United States where dormant season burning is widely applied. Although fires historically were predominantly growing season, they now occur in the dormant season in this part of the Coastal Plain to support a myriad of stewardship activities, including habitat management for game species. To examine the response of bats to landscape condition and the application of prescribed fire, in the winter of 2019, we mist-netted and affixed radio-transmitters to 16 Lasiurine bats, primarily Seminole bats (Lasiurus seminolus) at Camp Blanding Joint Training Center in northern Florida. We then located day-roost sites to describe roost attributes. For five Seminole bats, one eastern red bat (Lasiurus borealis), and one hoary bat (Lasiurus cinereus), we applied prescribed burns in the roost area to observe bat response in real-time. Generally, Seminole bats selected day-roosts in mesic forest stands with high mean fire return intervals. At the roost tree scale, Seminole day-roosts tended to be larger, taller and in higher canopy dominance classes than surrounding trees. Seminole bats roosted in longleaf (Pinus palustris), slash (Pinus elliotii) and loblolly pine (Pinus taeda) more than expected based on availability, whereas sweetbay (Magnolia virginiana), water oak (Quercus nigra) and turkey oak (Quercus laevis), were roosted in less than expected based on availability. Of the seven roosts subjected to prescribed burns, only one male Seminole bat and one male eastern red bat evacuated during or immediately following burning. In both cases, these bats had day-roosted at heights lower than the majority of other day-roosts observed during our study. Our results suggest Seminole bats choose winter day-roosts that both maximize solar exposure and minimize risks associated with fire. Nonetheless, because selected day-roosts largely were fire-dependent or tolerant tree species, application of fire does need to periodically occur to promote recruitment and retention of suitable roost sites.

FIRE HISTORY OF CONIFER FORESTS OF CERRO EL POTOSÍ, NUEVO LEÓN, MEXICO

Mexican forest ecosystems are exposed to a great variety of natural disturbances such as fire, which is a key disturbance process influencing tree survival and regeneration. Nevertheless, forest fires are one of the primary causes for which a large portion of the forest in Mexico is lost. Thus, the objectives of this research were to determine (1) the pattern of occurrence of forest fires, and (2) their relationship with precipitation and atmospheric circulation patterns. We reconstructed the historical forest fire regime from fire scars of 22 sample trees in a coniferous forest stand on the mountain Cerro El Potosi in southern Nuevo Leon, Mexico. Frequent surface fires characterized the site. The mean fire return interval for all fires was 11 years and for the more-widespread fires (those that scarred ≥25% of all samples) was longer (15.7 years). The Superposed Epoch Analysis indicated as not statistically significant the role of dry years as well for the subsequent wet conditions for the period from 1881 to 2004.

Population Trends in Royal Catchfly (Silene regia) at Wilson's Creek National Battlefield, Missouri

Royal catchfly (Silene regia, Caryophyllaceae) is a rare, tap-rooted, perennial forb known to occupy Midwestern prairies, glades, and savannas, which are increasingly fragmented. Though not federally listed, it is recognized as rare, threatened, or endangered in six states. We resurveyed 15 populations identified in 1980s–1990s monitoring at Wilson's Creek National Battlefield, Republic, MO. Abundance at each site was collected in 6 y (1988–1989, 1998–2000, 2019); measures of plant height and stems browsed were collected in 4 y (1998–2000, 2019). Fire history and precipitation were also explored. Mean royal catchfly abundance per population declined significantly over the study period, and only five of 15 populations supported extant populations in 2019. Year, site location, and the interaction between the two were highly significant factors in explaining variation of height measures and proportion of stems browsed. Plant height was significantly higher in 2019 than any other year, corresponding with overgrowth of competing vegetation and a wet spring. Prescribed fires became less frequent over the study period, and the mean fire return interval was higher than the historical regime of the region (9.7 y vs. 4–8 y). The decline of royal catchfly at Wilson's Creek National Battlefield and similar sites points to the need for fire management and restoration of prairie and savanna ecosystems for the persistence of this rare species.

Population Trends in Royal Catchfly (Silene regia) at Wilson's Creek National Battlefield, Missouri

Royal catchfly (Silene regia, Caryophyllaceae) is a rare, tap-rooted, perennial forb known to occupy Midwestern prairies, glades, and savannas, which are increasingly fragmented. Though not federally listed, it is recognized as rare, threatened, or endangered in six states. We resurveyed 15 populations identified in 1980s–1990s monitoring at Wilson's Creek National Battlefield, Republic, MO. Abundance at each site was collected in 6 y (1988–1989, 1998–2000, 2019); measures of plant height and stems browsed were collected in 4 y (1998–2000, 2019). Fire history and precipitation were also explored. Mean royal catchfly abundance per population declined significantly over the study period, and only five of 15 populations supported extant populations in 2019. Year, site location, and the interaction between the two were highly significant factors in explaining variation of height measures and proportion of stems browsed. Plant height was significantly higher in 2019 than any other year, corresponding with overgrowth of competing vegetation and a wet spring. Prescribed fires became less frequent over the study period, and the mean fire return interval was higher than the historical regime of the region (9.7 y vs. 4–8 y). The decline of royal catchfly at Wilson's Creek National Battlefield and similar sites points to the need for fire management and restoration of prairie and savanna ecosystems for the persistence of this rare species.

Fuel Treatment Longevity in Ponderosa Pine-Dominated Forest 24 Years After Cutting and Prescribed Burning

Fuels reduction treatments to mitigate fire behavior are common in ponderosa pine ecosystems of the western United States. While initial impacts of fuel treatments have been reported, less is known about treatment longevity as live and dead fuels change with time. We analyzed fuel dynamics in ponderosa pine–Douglas-fir forests 21-23 years following experimental fuel reduction designed as two independent studies of cutting combined with burning: one tested a commercial thinning strategy, while a second tested a retention shelterwood strategy to reduce fuels while also restoring ponderosa pine forests. Treated units were harvested in 1992 and half of the units were prescribed burned one-to-two years later. After 22 to 23 years post-treatment, few differences in fuel load persist and all treatments have increased ladder fuels in the form of live saplings and seedlings. Canopy fuel loads were lower in treated units compared to untreated control units; however, no other canopy fuel metric differed between treatments. The only persistent difference in surface fuels was in the retention shelterwood, where 1-hour fuels were lower in the treated units compared to control units. Crown fire hazard varied greatly, but means were similar between treatments. The increased hazard was driven by increases in live surface fuels from seedlings and saplings in the retention shelterwood, which increased canopy bulk density and reduced canopy base height. The overstory was still dominated by ponderosa pine 21-23 years later for all treatments, but the smaller size classes were primarily Douglas-fir, suggesting that without future disturbance, dominance will shift from pine to Douglas-fir dominated forests. The exception to this was the cut+fall burn treatment in the commercial thinning, where ponderosa pine outnumbered Douglas-fir trees across all size classes. The treatments that included a broadcast prescribed burn killed many existing seedlings and saplings. Our findings support other studies showing fuel reduction and restoration treatments are most successful with a combination of cutting and burning strategies, but also show that fuel treatments in low-elevation dry forests will likely not remain effective for much longer than historical mean fire return intervals.