Understory Fires Research Articles

Prescribing Fire in Eastern Oak Forests: Is Time Running Out?

Abstract Before European settlement, vast areas of the eastern US deciduous forest were dominated by oak species. Evidence indicates that periodic understory fire was an important ecological factor in the historical development of oak forests. During European settlement of the late 19th and early 20th century, much of the eastern United States was impacted by land-clearing, extensive timber harvesting, severe fires, the chestnut blight, and then fire suppression and intensive deer browsing. These activities had the greatest negative impact on the once-dominant white oak, while temporarily promoting the expansion of other oaks such as red oak and chestnut oak. More recently, however, recruitment of all the dominant upland oaks waned on all but the most xeric sites. Mixed-mesophytic and later successional hardwood species, such as red maple, sugar maple, black birch, beech, black gum and black cherry, are aggressively replacing oak. The leaf litter of these replacement species is less flammable and more rapidly mineralized than that of the upland oaks, reinforcing the lack of fire. The trend toward increases in nonoak tree species will continue in fire-suppressed forests, rendering them less combustible for forest managers who wish to restore natural fires regimes. This situation greatly differs from the western United States, where fire suppression during the 20th century has made a variety of conifer-dominated forests more prone to stand-replacing fire.North. J. Appl. For. 22(3):190 –196.

DUSKY-FOOTED WOODRAT DEMOGRAPHY AND PRESCRIBED FIRE IN A CALIFORNIA OAK WOODLAND

Abstract We investigated the effects of prescribed understory fire and shrub density on a population of dusky-footed woodrats (Neotoma fuscipes) live-trapped each spring and fall from October 1993 to 2001. We performed model selection with program MARK on capture–recapture data using Pollock's robust design to estimate survival and temporary emigration. Abundance increased from 1993 to a peak in 1997 and then decreased steadily from 1997 to 2001, accompanied by reduced reproduction. Survival was age- and time-dependent. Differential temporary emigration probabilities were explained by home-range size differences. Survival and abundance were higher in plots with high shrub cover vs. plots with low shrub cover. A low-medium intensity, prescribed understory fire in the fall of 1997 had no negative effect on survival or temporary emigration. The only fire-related negative demographic effect was a reduction in the number of juveniles per adult female on burned plots in the spring immediately after the fire. Pr...

MODELING FOREST UNDERSTORY FIRES IN AN EASTERN AMAZONIAN LANDSCAPE

Forest understory fires are an increasingly important cause of forest impoverishment in Amazonia, but little is known of the landscape characteristics and climatic phenomena that determine their occurrence. We developed empirical functions relating the occurrence of understory forest fires to landscape features near Paragominas, a 35‐yr‐old ranching and logging center in eastern Amazonia. An historical sequence of maps of forest understory fire was created based on field interviews with local farmers and Landsat TM images. Several landscape features that might explain spatial variation in the occurrence of understory fires were also mapped and co‐registered for each of the sample dates, including: forest fragment size and shape, forest impoverishment through logging and understory fire, sources of ignition (settlements and charcoal pits), roads, forest edges, and others. The spatial relationship between forest understory fire and each landscape characteristic was tested by regression analyses. Fire probability models were then developed for various combinations of landscape characteristics. The analyses were conducted separately for years of the El Niño Southern Oscillation (ENSO), which are associated with severe drought in eastern Amazonia, and non‐ENSO years.Most (91%) of the forest area that burned during the 10‐yr sequence caught fire during ENSO years, when severe drought may have increased both forest flammability and the escape of agricultural management fires. Forest understory fires were associated with forest edges, as reported in previous studies from Amazonia. But the strongest predictor of forest fire was the percentage of the forest fragment that had been previously logged or burned. Forest fragment size, distance to charcoal pits, distance to agricultural settlements, proximity to forest edge, and distance to roads were also correlated with forest understory fire. Logistic regression models using information on fragment degradation and distance to ignition sources accurately predicted the location of >80% of the forest fires observed during the ENSO event of 1997–1998. In this Amazon landscape, forest understory fire is a complex function of several variables that influence both the flammability and ignition exposure of the forest.

FIRE AND VEGETATION HISTORY IN THE EASTERN CASCADE MOUNTAINS, WASHINGTON

Dendrochronological techniques were used to reconstruct a 433‐yr fire history, and to characterize the historical fire regime (frequency, size, season, severity) of the Teanaway River drainage in the eastern Cascade Mountains of Washington, USA. General Land Office section corner data were used to reconstruct aspects of the late‐19th‐century structure and composition of the forests in the study area. Systematic fire‐scar surveys (∼30 000 ha; 92 sites; 257 fire‐scarred cross‐sections; 1569 individual fire scars) revealed that fire frequency was quite variable spatially; Weibull median probability intervals ranged from seven to 43 years. Fire extent also varied widely; most fires were relatively small (<1000 ha), although several large fires (>4000 ha) were detected. Mean and median fire sizes were 1795 ha and 988 ha, respectively. Large fires occurred every 27 years, on average (every 11 years, on average, between 1708 and 1889), and coincided with periods of annual and seasonal drought (Palmer Drought Severity Index and winter Southern Oscillation Index). Intervals from 1 to 37 years occurred between fires of >4000 ha. Over 80% of fires occurred late in the growing season, or after the onset of cambial dormancy. Sampling locations in dry forest types (dominated by ponderosa pine) yielded fire‐scarred cross‐sections with numerous fire scars leading us to infer that most historical fires were of low intensity, leaving the overstory structure intact. This inference is corroborated by the composition and structure of the historical forest, which was characterized by a preponderance of very large (>100‐cm diameter) ponderosa pines. Mesic forest types (dominated by grand fir and Douglas‐fir) likely exhibited a wider range of fire severities. Moderate and occasional high‐severity understory fires or crown fires did occur within the study area, as indicated by the scarcity or lack of fire‐scar evidence and the presence of relatively even‐aged forests at several mesic forest sites. Historical section corner data indicate that small amounts of these forest types occurred in the study area. Fire frequency and size declined dramatically circa 1900, coincident with the advent of commercial timber harvesting, although most fires, despite their reduced number and size, continued to burn in the late summer or fall.

Road paving, fire regime feedbacks, and the future of Amazon forests

Fire poses the greatest threat to the forests of Amazônia. The magnitude of this threat is amplified by three positive feedback loops that drive the expansion of forest fire in the region: (1) Fire promotes drought, and therefore more fire, by releasing smoke into the atmosphere, thus reducing rainfall. Fire-assisted conversion of forests to pastures may also promote drought by increasing albedo and decreasing water vapor flux to the atmosphere, further inhibiting rainfall. (2) Fire increases the susceptibility of forests to recurrent burning by killing trees, thereby allowing sunlight to penetrate the forest interior, and increasing the fuel load on the forest floor. (3) Finally, fires also self-perpetuate by burning agricultural and forestry systems, discouraging landholders from making those fire-sensitive investments in their land that would allow them to move beyond their dependence upon fire as a management tool. The long-term reduction of Amazon fire, and its substantial costs to society, is most likely to emerge through investments and policy change that stimulate permanent agricultural and forestry production systems within existing frontiers while slowing the rate of frontier expansion. But the Brazilian government’s plan to pave, recuperate or construct 6245 km of roads in the Amazon may have the opposite effect. We present research findings that the government plan would nearly double the area of forestland that is accessible by paved highways, including 192,000 km 2 of fire-prone forest. Our analysis finds that these roads will stimulate 120,000–270,000 km 2 of additional deforestation, and forest impoverishment through logging and understory fire, if the historical relationship between road paving and forest alteration by humans continues. Infrastructural investments are urgently needed in Amazônia to help integrate isolated urban centers into the market economy, to improve the quality of life for millions of rural Amazonians, and to improve the profitability of agribusiness in Brazil’s agricultural belt. But as currently planned, these investments will have the ancillary effects of accelerating deforestation, logging, forest fire, smoke-related illness, and the displacement of small-scale farmers.

Californian mixed‐conifer forests under unmanaged fire regimes in the Sierra San Pedro Mártir, Baja California, Mexico

Abstract Aim This study appraises historical fire regimes for Californian mixed‐conifer forests of the Sierra San Pedro Mártir (SSPM). The SSPM represents the last remaining mixed‐conifer forest along the Pacific coast still subject to uncontrolled, periodic ground fire. Location The SSPM is a north–south trending fault bound range, centred on 31°N latitude, 100 km SE of Ensenada, Baja California. Methods We surveyed forests for composition, population structure, and historical dynamics both spatially and temporally over the past 65 years using repeat aerial photographs and ground sampling. Fire perimeter history was reconstructed based on time‐series aerial photographs dating from 1942 to 1991 and interpretable back to 1925. A total of 256 1‐ha sites randomly selected from aerial photographs were examined along a chronosequence for density and cover of canopy trees, density of snags and downed logs, and cover of non‐conifer trees and shrubs. Twenty‐four stands were sampled on‐the‐ground by a point‐centred quarter method which yielded data on tree density, basal area, frequency, importance value, and shrub and herb cover. Results Forests experience moderately intense understory fires that range in size to 6400 ha, as well as numerous smaller, low intensity burns with low cumulative spatial extent. SSPM forests average 25–45% cover and 65–145 trees per ha. Sapling densities were two to three times that of overstory trees. Size‐age distributions of trees ≥ 4 cm dbh indicate multi‐age stands with steady‐state dynamics. Stands are similar to Californian mixed conifer forests prior to the imposition of fire suppression policy. Livestock grazing does not appear to be suppressing conifer regeneration. Main conclusions Our spatially‐based reconstruction shows the open forest structure in SSPM to be a product of infrequent, intense surface fires with fire rotation periods of 52 years, rather than frequent, low intensity fires at intervals of 4–20 years proposed from California fire‐scar dendrochronology (FSD) studies. Ground fires in SSPM were intense enough to kill pole‐size trees and a significant number of overstory trees. We attribute long fire intervals to the gradual build‐up of subcontinuous shrub cover, conifer recruitment and litter accumulation. Differences from photo interpretation and FSD estimates are due to assumptions made with respect to site‐based (point) sampling of fire, and nonfractal fire intensities along fire size frequency distributions. Fire return intervals determined by FSD give undue importance to local burns which collectively use up little fuel, cover little area, and have little demographic impact on forests.

The late holocene paleohydrology of Pine Lake, Alberta: a multiproxy investigation

This paper reports on a high-resolution, multi-proxy, late-Holocene study from a lake in the Aspen Parkland of southern Alberta, Canada. A sediment core spanning the last 4000+ yrs from Pine Lake was analyzed for charcoal, granulometry, grain roundness, tephra content, geochemistry, mineralogy and pollen. This multi-proxy record indicates: (1) increasing anoxia causing a shift in S deposition from gypsum to pyrite due to increasing moisture availability in the late Holocene; (2) a decrease in Mg flux into the lake due to the development of the aspen forest, which reduced water flow through the Mg-rich shallow sand aquifer; the aspen forest expansion was in turn induced by the extirpation of plains bison prior to settlement; and (3) a change in the upland fire regime from frequent low-biomass grass fires to less frequent but higher biomass under-story fires, also as a result of the expansion of the aspen forest. Not only are the different proxies sensitive to different rates and magnitudes of change, they also show different sensitivities to different types of hydrological change: the mineralogy and geochemistry are sensitive to changes in water level and redox potential, and to changes in the relative strengths of the aquifers feeding the lake, while the granulometry is sensitive to total hydrological balance. Thus, apparently contradictory proxy results should be viewed as complementary.

Effects of Fire Intensity on Competitive Dynamics Between Red and Black Oaks and Mountain Laurel

Abstract We investigated the competitive response of oak and mountain laurel to prescribed fire on two sites in northeastern Connecticut. After nearly a decade, the competitive position of oak on moderately burned portions of both sites was little better, and in some respects worse, than on adjacent unburned controls. However, on portions of both sites where fire had killed much of the overstory, oak regrowth was vigorous and had escaped the dense mountain laurel layer. The results suggest that light understory fires alone are not sufficient for oak regeneration, and that prescribed fire should be used only as part of an integrated strategy involving harvesting of the overstory. Ecologically, the results point to the importance of severe disturbances in maintaining the structure and function of oak ecosystems. North. J. Appl. For. 13(3):119-123.

Comparing the Prescribed Natural Fire Program With Presettlement Fires in the Selway-Bitterroot Wilderness

The severity and extent of recent fires (1979-1990) were compared with that of presettlement fires (pre-1935) by eight major forest types in the Selway-Bitterroot Wilderness (SBW) in Idaho and Montana. Presettlement fire intervals were determined for estimating area burned. Presettlement annual area burned for the entire SBW was 4,290 ha by stand replacement fire and 4,310 ha by nonlethal understory fire. All recent period fires>4 ha were mapped using a GIS system. Area burned was determined by the fire severity classes: crowning, lethal surface, nonlethal surface, and no burning. Annual area burned averaged 2,780 ha by stand replacement and 2,270 by understory fire. Area burned during presettlement was 1.7 times that during the recent period. During the presettlement period, area burned by stand replacement and nonlethal understory fire was 1.5 and 1.9 times greater respectively than during the recent period.

Effects of Goats on Understory Vegetation and Fire Hazard Reduction in a Coastal Forest in California

Abstract This paper examines the effects of goat grazing on understory forage utilization and fire hazard reduction in a 1 ha area of a Monterey pine (Pinus radiata) and red gum (Eucalyptus camaldulensis) forest as well as in a 1 ha area of an adjacent fuelbreak. Stocking rates of 600 Spanish goats/ha for 1 day reduced cover of the brush understory in the forest by 41% and 48% respectively at heights of < 0.5m and 0.5-1.5m. In the fuelbreak stocked at 280 Spanish goats/ha for 3 days the reduction was 46% and 82% at < 0.5m and 0.5-1.5m, respectively. Forage biomass utilization reached 75% in the brush understory and 84% in the fuelbreak. In the brush understory goats grazed more at the upper height (84%) than at the lower (64%). Grazing reduced 1- and 10-hour dead fuels 33% and 58% respectively, while the litter depth was reduced as much as 27%. For. Sci. 35(4):1125-1131.

Fire behavior, fuel consumption, and forest-floor changes following prescribed understory fires in Sierra Nevada mixed conifer forests

We quantified fire behavior, fuel consumption, and forest-floor responses to understory fires prescribed to cover a wide range and variable combination of fuel loadings, fuel moisture levels, and weather conditions in Sierra Nevada mixed conifer forests. At each of three locations, five burns were conducted under each of four different prescriptions (n = 60). Fuel loads ranged from 75–102 Mg/ha on an open site of low productivity to 125–177 Mg/ha on more productive sites. Total fuel consumption ranged from 15% in early spring burns to 92% in early fall burns. Organic horizons (litter and duff) composed 62–84% of the total fuel load. Consumption of the organic horizons ranged from 11 to 94%. Lower duff (Oa horizon) moisture content was 135% in the former and 15% in the latter. Consumption of the organic horizon was significantly correlated with lower duff moisture content (r2 = 0.51). More than 72% of all fuel was consumed after the flaming front had passed over an area. Postburn mineral soil exposure was strongly influenced by fuel consumption, season of burn, and characteristics of the site. In the late spring burns at one site, bare ground exposure was 3% before fire, 60% two months after fire, and 8% thirteen months after fire. There were slight to nonexistent increases in bare ground exposure from burns prescribed during early spring or late autumn (i.e., burns of low to moderate consumption).

Community Structure, Dynamics and Nutrient Cycling in the Okefenokee Cypress Swamp‐Forest

Aspects of community structure and nutrient circulation are described for the cypress (Taxodium distichum) forest in Okefenokee Swamp, Georgia, USA. This bog environment is characterized by low nutrient availability and large peat accumulations. The tree stratum of the forest is dominated by cypress, which is probably due to recurrent understory fires which eliminate other swamp species. In stands throughout the cypress forest, the density and total basal area of living stems >4 cm diameter vary greatly, but mean values (1,465 stems/ha; 52 m2/ha) are high compared to upland forests. Natural thinning appears to be unimportant. Abundant standing dead trees suggest that differences in density among stands are due to past differences in the frequency and intensity of forest fires during periodic droughts. At an intensive study site, biomass and net primary production were measured using techniques of dimension analysis. Cypress trees compose 98% of the total forest above—water biomass of 307 tonnes/ha. Most of the cypress biomass is in tree boles (96%), little is in foliage (0.8%). Presumably due to the acid, nutrient—poor conditions in the swamp, total above—water net primary productivity is low (692 g°m—2°yr—1); the high biomass is the result of the old age (150 yr) of the stand and the high density of trees. Cypress net production is largely channeled into bole wood (41%) and current twigs with needles (41%). It appears that in recent years, above—water biomass has been maintained at a constant level by fire—induced thinning. The nutrient pools in the above—water community are large (666 kg/ha Ca, 111 kg/ha Mg, 230 kg/ha K, 996 kg/ha N and 46 kg/ha P). These nutrients are largely contained in cypress boles and do not freely circulate. The annual nutrient uptake in the forest is rather small compared to that in upland forests. The nutrient uptake (54 kg°ha—1°yr—1 Ca, 11 each for Mg and K and 2.3 for P) is largely channeled to the small amount (231 g/m2) of very efficient cypress foliage. Foliage abscission and foliar leaching by rainfall return 73 to 91% of the annual nutrient uptakes each year; therefore, there are only small permanent additions of nutrients to the above—water pools each year. Except for K which is apparently reabsorbed from foliage before abscission, the cypress trees do not appear to conserve foliar nutrients. However, nutrient conservation may be effected by minimizing the amount of foliage. Litterfall from the forest has resulted in a large accumulation of peat. Although some nutrient regeneration apparently occurs, these peat deposits contain large, permanent nutrient losses from the community. Nutrients cycling is dissolved form (e.g., foliar leachates) and nutrients received from atmospheric precipitation are likely to be of special importance to the nutrition of this closed—basin swamp forest community.