Fire-scarred Trees Research Articles

Using Neutral Models to Identify Constraints on Low-severity Fire Regimes

Climate, topography, fuel loadings, and human activities all affect spatial and temporal patterns of fire occurrence. Because fire is modeled as a stochastic process, for which each fire history is only one realization, a simulation approach is necessary to understand baseline variability, thereby identifying constraints, or forcing functions, that affect fire regimes. With a suitable neutral model, characteristics of natural fire regimes estimated from fire history data can be compared to a “null hypothesis”. We generated random landscapes of fire-scarred trees via a point process with sequential spatial inhibition. Random ignition points, fire sizes, and fire years were drawn from uniform and exponential family probability distributions. We compared two characteristics of neutral fire regimes to those from five watersheds in eastern Washington that have experienced low-severity fire. Composite fire intervals (CFIs) at multiple spatial scales displayed similar monotonic decreases with increasing sample area in neutral vs. real landscapes, although patterns of residuals from statistical models differed. In contrast, parameters of the Weibull distribution associated with temporal trends in fire hazard exhibited different forms of scale dependence in real vs. simulated data. Clear patterns in neutral landscapes suggest that deviations from them in empirical data represent real constraints on fire regimes (e.g., topography, fuels). As with any null model, however, neutral fire-regime models need to be carefully tuned to avoid confounding these constraints with artifacts of modeling. Neutral models show promise for investigating low-severity fire regimes to separate intrinsic properties of stochastic processes from the effects of climate, fuel loadings, topography, and management.

Fire regime in a conservation reserve in Chihuahua, Mexico

Fire regime characteristics were reconstructed from fire-scarred trees in the Tutuaca reserve, a newly designated protected area in the Sierra Madre Occidental of western Chihuahua. The reserve was created to protect thick-billed parrot nesting habitat (large snags) and a relict forest of Chihuahua spruce (Picea chihuahuana Martínez). We collected fire-scarred samples from conifers (Pinus ayacahuite Ehrenb., Pinus durangensis Martínez, and Pseudotsuga menziesii (Mirb.) Franco) in three 25-ha sites arrayed at different watershed positions, from a low site adjacent to the spruce trees up to the watershed divide. Fire analysis periods began in 1702, 1704, or 1761 and continued through the final fire in 1955 (two sites) or 1995. All sites had frequent fire regimes (mean fire interval (MFI) 3.9–5.2 years; MFI for years in which 25% or more of the samples were scarred: 6.9–8.4 years). Almost all fires occurred before cambial growth began or early during the season of cambial growth. Fire years were significantly dry, and the years immediately preceding fire were significantly wet. After 1955, no further fires occurred at two of the three study sites, a pattern similar to that observed elsewhere in northern Mexico. The third site had fires in 1987 and 1995. The extended fire-free period in portions of the Tutuaca landscape may result in fuel accumulation and eventually in severe wildfire. For effective conservation of fire-susceptible habitat features, managers should seek to incorporate surface fire as a management tool.

A fire history from tree rings in a high-elevation forest of Rocky Mountain National Park

Historical fire patterns in a subalpine forest of Rocky Mountain National Park were quantified from an analysis of forest stand ages and fire-scarred trees. A comparatively detailed sample of 3461 tree cores and 212 fire scars was collected from a 9200-ha study area north of Estes Park, Colorado. A total of 41 fire events were identified in the record. Annually precise fire dates, beginning in 1533, include 22 high-severity crown fires, 7 low-severity surface fires, and 8 mixed-severity events with both surface and crown fire components. Fire rotation was estimated for both surface fires (7587 years) and crown fires (346 years). Fire rotation did not appear to vary with fuel characteristics associated with topographical differences in the study area. Fires larger than 300 ha were few, but they determined a large proportion of the area burned since 1700 and were significantly correlated with a reconstructed index of summer drought. Low fire activity in the 20th century was associated with decreased severity and frequency of drought episodes. Long fire rotations preclude definitive conclusions regarding the effects of fire suppression in the 20th century, but relationships between high-severity fires, fuels, and drought suggest that climatic variability remains the primary influence on fire cycles in high-elevation ecosystems of the southern Rocky Mountains.

CLIMATIC AND HUMAN INFLUENCES ON FIRE REGIMES OF THE SOUTHERN SAN JUAN MOUNTAINS, COLORADO, USA

Fire severity, frequency, and extent are expected to change dramatically in coming decades in response to changing climatic conditions, superimposed on the adverse cumulative effects of various human-related disturbances on ecosystems during the past 100 years or more. To better gauge these expected changes, knowledge of climatic and human influences on past fire regimes is essential. We characterized the temporal and spatial properties of fire regimes in ponderosa pine forests of the southern San Juan Mountains of southwestern Colorado by collecting 175 fire-scarred tree samples from nine sites across a wide range of topographic settings. All tree rings and fire scars were dated using standard dendrochronological techniques. Fire-free intervals were statistically modeled using the Weibull distribution to provide quantitative measures that characterized the historical range of variation in pre-EuroAmerican fire regimes. Fires during our reference period were more frequent in the low elevation ponderosa pine forests (6–10 yr) than in the high elevation, mixed conifer forests (18–28 yr). Fires at lower elevations were predominantly low-severity, isolated fires. Fires during some years (e.g., 1748) were spatially extensive throughout the entire mountain range. Intervals that delimited significantly long fire-free periods ranged from 10–19 yr (low elevation) to 27– 50 yr (high elevation). Fire histories were similar between the eastern and western portions of the mountain range, although we found significant evidence of topographic isolation on fire regimes at one site. Pre-1880 fires primarily occurred in the dormant season, and we found no temporal changes in past fire seasonality. We found no compelling evidence that Native Americans influenced fire regimes in our study sites. We found a hiatus in fire occurrence between 1750 and 1770 that we believe was likely related to weakened El Niño-Southern Oscillation activity, an extended series of cool-phase Pacific Decadal Oscillation events, and weakened monsoonal moisture, all possibly entrained in an invasive air mass typical of locations that are more northerly. In addition, pre-1880 fires occurred during years of severe drought, conditioned by above average moisture conditions in preceding years. The 20th century is characterized by a near complete absence of fires (fire-free interval of >100 yr), suggesting future wildfires may be more widespread and ecologically severe compared to pre-1880 fires.

Effects of fire disturbance on the forest structure and succession in the natural broad-leaved/Korean pine forest

Investigations on charcoal in the soil, fire-scarred trees, stand composition, forest structure as well as regeneration status were carried out in the natural broad-leaved/Korean pine (Pinus koraiensis) forest after fire disturbance at Liangshui Nature Reserve on the mid-north of Xiaoxing’an Mountains from 1990 to 1992, and the ecological effects of fire disturbance on the formation and succession of this kind of forest were analyzed according to the survey results. The average depth of charcoal in the soil was related to the timing of the fire. According to the characteristic of fire-scarred trees, the dynamic map of the fire behavior was drawn onto the topographic map. It showed that the dimension and extent of the fire disturbance was closely related with site conditions. Fire disturbance only led to a significant difference in stand composition and diameter, class structure for the stands at different locations, rather than completely destroying the forest. After fire disturbance, the horizontal, community structure was a mosaic of different patches, which were made up of different deciduous species or different sizes of Korean pines, and the succession trend of each patch was also different. In the sites with the heavy fire disturbance, the intolerant hardwood species were dominant, and there were a large number of regenerative Korean pine saplings under the canopy. In the moderate-disturbed sites, the tolerant hardwood species were dominant, and a small number of large size Korean pines still survived. In the light-disturbed sites, large size, Korean pines were dominant.

Assessing fire regimes on Grand Canyon landscapes with fire-scar and fire-record data

Fire regimes were reconstructed from fire-scarred trees on five large forested study sites (135–810 ha) on the North and South Rims at Grand Canyon National Park. Adequacy of sampling was tested with cumulative sample curves, effectiveness of fire recording on individual trees, tree age data, and the occurrence of 20th Century fires which permitted comparison of fire-scar data with fire-record data, a form of modern calibration for the interpretation of fire-scar results. Fire scars identified all 13 recorded fires >8 ha on the study sites since 1924, when record keeping started. Records of fire season and size corresponded well with fire-scar data. We concluded that the sampling and analysis methods were appropriate and accurate for this area, in contrast to the suggestion that these methods are highly uncertain in ponderosa pine forests. Prior to 1880, fires were most frequent on low-elevation ‘islands’ of ponderosa pine forest formed by plateaus or points (Weibull Median Probability Intervals [WMPI] 3.0–3.9 years for all fires, 6.3–8.6 years for ‘large’ fires scarring 25% or more of the sampled trees). Fires were less frequent on a higher-elevation ‘mainland’ site located further to the interior of the North Rim (WMPI 5.1 years all fires, 8.7 years large fires), but fires tended to occur in relatively drier years and individual fires were more likely to burn larger portions of the study site. In contrast to the North Rim pattern of declining fire frequency with elevation, a low-elevation ‘mainland’ site on the South Rim had the longest fire-free intervals prior to European settlement (WMPI 6.5 years all fires, 8.9 years large fires). As in much of western North America, surface fire regimes were interrupted around European settlement, 1879 on the North Rim and 1887 on the South Rim. However, either two or three large surface fires have burned across each of the geographically remote point and plateau study sites of the western North Rim since settlement. To some extent, these sites may be rare representatives of nearly-natural conditions due to the relatively undisrupted fire regimes in a never-harvested forest setting.

AN ASSESSMENT OF FIRE, CLIMATE, AND APACHE HISTORY IN THE SACRAMENTO MOUNTAINS, NEW MEXICO

Fire historians typically attribute the causes of temporal change in past fire regimes to climatic variation, human land use, or some combination of the two. Most long-term historical reconstructions, however, lack time and place-specific chronologies for all three variables of fire, climate, and people. To test the hypothesis that Mescalero Apache of southeastern New Mexico influenced fire regimes of the Sacramento Mountains, we reconstructed and compared chronologies of key variables for the period A.D. 1700 to the present. Fire-scarred trees were used to reconstruct fire frequencies and culturally modified (peeled) trees, and written histories were used to identify places and times of Mescalero presence. Independent precipitation reconstructions from tree rings were compared with fire and human histories. We found that Mescalero frequently visited the western escarpment of the Sacramento Mountains during the late 1700s through the late 1800s, especially the Dog Canyon area. Fire frequency was higher and seasonal timing of fires was different in sites near Dog Canyon compared to relatively distant sites. Inter-annual to decadal-scale drought might explain some temporal variability in fire and peeling activity, but these relationships were not consistent. We conclude that people increased fire occurrence during certain time periods in localized areas, but broad-scale and persistent human impacts did not occur until the end of the 19th century with the rise of livestock grazing by European settlers and fire suppression by government agencies. [Key words: Culturally modified trees, historical fire regimes, fire-climate-human interactions, Apache, New Mexico.]

Fire Regime Changes in La Michilía Biosphere Reserve, Durango, Mexico

Abstract: The ability of reserves to maintain natural ecosystem processes such as fire disturbance regimes is central to long‐term conservation. Fire‐scarred tree samples were used to reconstruct fire regimes at five study sites totaling approximately 230 ha in pine ( Pinus spp.) and oak (Quercus spp.) forests of La Michilía Biosphere Reserve on the dry east slope of the Sierra Madre Occidental, Durango, Mexico. Study sites covered a 20‐km environmental gradient of elevation, topography, and human land uses. Plant communities ranged from oak‐pine to mixed conifer forests. Fires were frequent at all sites prior to 1930, when large‐scale grazing of domestic livestock was initiated. Widespread fires have been excluded from four of the five sites since 1945, with an essentially uninterrupted regime of frequent fires continuing only in the reserve core. Xeric sites had many, smaller fires, whereas mesic sites had fewer but larger fires. On a reserve‐wide scale, a fire burned on at least one site nearly every year, usually in the dry spring or early summer season, but fire years were rarely synchronous among the sites. Fire occurrence was weakly related to the Southern Oscillation climate pattern; major reserve‐wide fire years almost never coincided with wet Southern Oscillation extremes but only occasionally matched dry extremes. Maintenance of the long‐term frequent‐fire regime in the reserve core is one indicator that the biosphere reserve model has been successful in conserving natural processes, but the protected area is small ( 7000 ha). Because of the key role of frequent‐fire regimes in regulating ecosystem structure and function, restoration of the ecological role of fire disturbance is a desirable conservation strategy.

Long-term, landscape patterns of past fire events in a montane ponderosa pine forest of central Colorado

Parameters of fire regimes, including fire frequency, spatial extent of burned areas, fire severity, and season of fire occurrence, influence vegetation patterns over multiple scales. In this study, centuries-long patterns of fire events in a montane ponderosa pine ‐ Douglas-fir forest landscape surrounding Cheesman Lake in central Colorado were reconstructed from fire-scarred trees and inferences from forest stand ages. We crossdated 153 fire-scarred trees from an approximately 4000 ha study area that recorded 77 total fire years from 1197 to the present. Spatial extent of burned areas during fire years varied from the scale of single trees or small clusters of trees to fires that burned across the entire landscape. Intervals between fire years varied from 1 to 29 years across the entire landscape to 3 to 58 years in one stand, to over 100 years in other stands. Large portions of the landscape did not record any fire for a 128 year-long period from 1723 to 1851. Fire severity varied from low-intensity surface fires to large-scale, stand-destroying fires, especially during the 1851 fire year but also possibly during other years. Fires occurred throughout tree growing seasons and both before and after growing seasons. These results suggest that the fire regime has varied considerably across the study area during the past several centuries. Since fires influence plant establishment and mortality on the landscape, these results further suggest that vegetation patterns changed at multiple scales during this period. The fire history from Cheesman Lake documents a greater range in fire behavior in ponderosa pine forests than generally has been found in previous studies.

Dendroecology and species co-existence in an old-growth Quercus– Acer– Tilia talus slope forest in the central Appalachians, USA

Dendroecological techniques were used to examine the disturbance history and patterns of species recruitment in an old-growth Quercus rubra L. (northern red oak)– Acer saccharum Marsh. (sugar maple)– Tilia americana L. (basswood) forest on a steep, talus slope in eastern West Virginia. The forest was uneven-aged as were the populations of red oak. Sugar maple dominated the sapling layer, which comprised little or no basswood and red oak. A compilation of major and moderate releases (indicative of disturbance) in 25 cores revealed single or multiple release events in every decade from 1870–1990. The high elevation of the forest coupled with a fertile sub-soil beneath the talus ameliorated the outwardly harsh conditions of the site, allowing for the domination of typically mesophytic, nutrient demanding tree species. We observed several fire scarred trees as well as extensive small-scale blow-down throughout the forest. Frequent disturbance events were probably crucial to the co-existence and continuous canopy recruitment of the relatively light demanding red oak with highly shade tolerant sugar maple and basswood. The strong successional replacement tendencies of red oak by northern hardwoods noted elsewhere in the eastern US may be less apparent on high elevation, rocky sites in the central Appalachians. Thus, this is a unique case study of long-term red oak domination with later successional species in an old-growth forest.

A cross-dated fire history from coast redwood near Redwood National Park, California

Cross sections from coast redwood trees (Sequoiasempervirens (D.Don.)Endl.) in and near Redwood National Park were dendrochronologically cross-dated and used to develop a fire history from 1714 to 1985. A master chronology for the study area was first developed from old-growth trees and provided dating control for fire-scarred samples. Redwood offers a challenge for dendrochronology owing to partially absent rings (ring wedging) and uniform ring widths (complacency). Cross dating was successful in portions of 12 of 24 fire-scarred trees. Fire events were dated by noting the position of fire scars and other fire-associated ring structures (resin ducts, double latewood, growth releases, and ring separations) in the cross-dated ring series. Using only dates of fire scars, the mean fire interval (MFI) was 9.9 years from the first recorded fire in 1714 to the last in 1962. The MFI was 8.0 years for the best represented (greatest sample depth) presettlement period from 1714 to 1881. Using dates for all fire-associated ring features, the MFI from 1714 to 1962 was 7.0 years and from 1714 to 1881 was 6.0 years. Use of all fire-associated ring characteristics is argued to be a more complete representation of past fire frequency due to possible under-representation of fire-scar records from stump-top samples. Based upon scar positions within annual rings, fires occurred predominately late in the growing season or after growth ceased for the year. The mean fire intervals determined are shorter than those reported in all except one other fire history study from coast redwood and suggest that fire frequency in redwood may have been underestimated in many past studies.

Disturbance and Tree Species Coexistence in Tropical Riparian Forest Fragments

Patterns of past fire incursions and tree falls were documented in a tropical riparian forest. Fire-scarred trees were concentrated on gentler slopes near the savanna:forest boundary, while tree fall sites were of highest frequency on convex upper valley slopes. Most tree falls were in a down-slope direction while the remainder were concentrated in directions that suggested an origin in extreme winds of atypical orientation. Tree species response to these two forms of disturbance was evaluated by assuming that the zones identified were inherently more disturbanceprone, and comparing the tree populations within and beyond the areas identified. Fire incursions had a larger measurable effect in augmenting species richness: 19% of species evaluated were significantly more abundant in burned zones, while only 8% of those evaluated were more abundant in tree fall zones. This difference is tentatively attributed to fire having a larger proximal effect on tree species recruitment as well as a spatially more persistent pattern of occurrence. Observations made at sites of recent fire incursion indicated that this caused little tree death or canopy opening but eliminated seedlings, saplings, litter and root mats: it thus created new seedbed conditions without the high light levels that would promote herbaceous establishment. Both disturbances play an augmentative, rather than exclusive, role in promoting species coexistence in these forests: they provide a varied micro-environment for seedling establishment but their patchy distribution ensures that disturbance-sensitive species can persist elsewhere in the forest patch.

Vegetation and Environment in Adjacent Post Oak (Quercus stellata) Flatwoods and Barrens in Indiana

-To compare adjacent post oak flatwoods and barrens communities in southwestern Indiana, we used vegetational, environmental and fire history data in multivariate analyses. Barrens had greater dominance by post oak and lower tree species richness, but variation in tree species composition was not strongly related to soil moisture, litter depth or other environmental gradients measured. Tree growth has been slow and variable, with little difference between the barrens and flatwoods. Barrens and flatwoods differ in herb species composition, but with considerable overlap. In the barrens, herbaceous vegetation composition was correlated with tree basal area, litter depth and soil moisture; in the flatwoods, it was correlated with soil moisture and microelevation. The barrens and flatwoods differed only slightly but significantly in environment: barrens soils were drier in June 1986 than flatwoods soils. Data from fire-scarred trees show no clear evidence of differences in frequency or extent of fire between the flatwoods and barrens. Multistemmed post oaks in the barrens indicate that past cutting may have contributed to these openings within the closed canopy flatwoods matrix.

THE GENESIS OF MOLLISOLS UNDER DOUGLAS FIR

We sampled mollic epipedons in adjacent forests and grasslands and under and beyond the crowns of three old Douglas fir (Pseudotsuga menziesii) trees in southwestern Montana. Forest fire history was determined by removing cross sections from fire-scarred trees. Grassland soils had three times as many grass opal phytoliths as forest soils, and Douglas fir opal was found only in forest soils. Forest and grassland soil organic carbon contents did not differ significantly, and the old Douglas fir trees had no significant effect on soil organic carbon levels during their lifespan. Mean fire interval was calculated as 58 yr; frequent ground fires swept these forests. Opal data indicate that the forest-grassland ecotone has been stable and that forests have persisted long enough for mollic epipedons to have developed beneath them. Apparently, these forest soils have duplicated grassland Mollisol genesis because of the abundance of sylvan graminoids and the cool, dry climate. Because fire has become rare in modern times, stands are closing and graminoid undergrowth is reduced.

Decay Following Fire in Young Mississippi Delta Hardwoods

During the summer and fall of 1932, 602 fire-scarred trees of 9 species of Delta hardwoods were dissected and analyzed for decay and insect activity. The trees were between 3 and 11 inches in diameter, and were taken from 4 parishes in Louisiana and 3 counties in Mississippi. Forest fires have been of frequent occurrence in the Mississippi Delta area at least for the past 30 years, with the fire seasons 1917-18 and 1924-25 outstanding in severity of damage done during those seasons. Fire scars healed most rapidly in the oaks and red gum, followed by ash, hackberry, and persimmon. The greater the num ber of years since scarring, the greater the proportion of scarred trees decayed. Of the species studied, hackberry was found to be the most susceptible to initial infection, followed by the oaks, ash, red gum, and persimmon. Following scarring, in red gum and persimmon, wound gum is produced just under the scarred surface, which protects the trees against infection. Much of this protective effect is lost, if subsequent fires kill the exposed sapwood. Decay spread upward from the fire scar most rapidly in the oaks (2.3 inches per year), followed in order by ash, red gum, hackberry. And persimmon. A definite relation was found between the rate of decay and each of the following factors: Age of tree, percentage of tree circ~mference scarred, diameter at the time of scarring. Present diameter, and fungus causing the decay. The breaking-over of young trees at the base, because of decay following firescarring, was found to be of infrequent occurrence and chiefly confined to overtopped trees and trees otherwise in poor vigour. A large number of fungi, from several families of the Hymenomycetes, were found to cause decay behind fire scars in the Delta area. Many of these fungi, including Lentinus tigrinus and Polyporus lucidus , can rot dead sapwood, old sapwood of living trees, and the heartwood of living trees. But one fungus in anyone tree was responsible for the major decay. A large variety of insects, chief among which are ants and termites, invade the decayed wood behind fire scars in this area. Only one insect, Parandra brunnea , was found to invade the sound wood beyond the decay column for any distance. Insects appeared to play a minor role in the uitimate damage.