Linking Fuel Heterogeneity to Fire Behavior and Effects chaired by Joseph O'Brien

Soil Temperatures During Fires in Florida Sand Pine Scrub

M. Carrington — Thu, 25 Jun 2009 09:00:00 PDT

Soil temperatures recorded with thermocouples and temperature-sensitive paints on glass petri dishes were quantified during two Florida sand pine scrub prescribed fires during May 1993. Thermocouples and petri dishes were placed either at the soil surface or at 2 cm depth, and either in vegetated or open microsites. Maximum soil surface temperatures were 621.7o C and 628.8o C during fires at Archbold Biological Station and Ocala National Forest, respectively; maximum temperatures at 2 cm depth were less than half of those at the surface. Peak temperature durations also were shorter at the soil surface than at 2 cm depth. Temperatures recorded with thermocouples did not differ between microsites during the Ocala fire, but were higher in open microsites during the Archbold Biological Station fire, probably due to combustion of well-aerated litter. Maximum temperatures of petri dishes generally were lower than those of adjacent thermocouples, due to a greater lag time in melting of temperature-sensitive paints on petri dishes.

Fuel Types and Fire Behavior of Eastern Oak Forests

P. Brose — Thu, 25 Jun 2009 08:40:00 PDT

Forest managers using prescribed fire to sustain and restore eastern oak forests often confront two or more dissimilar fuel types within the same stand or landscape. This fuel bed heterogeneity creates planning and operational problems because of drastic changes in fire behavior and vegetative response. This talk presents preliminary information from an ongoing fire study examining common fuel types of oak forests, their fire behavior, and the post fire vegetative response. Cluster analysis identified six dissimilar fuel types (2 leaf litter, 2 shrub, 2 slash) common to eastern oak forests and measured fire behavior revealed distinct fire behavior for each type. Between the two litter types, oak stands transitioning to mixed mesophytic or northern hardwood species displayed less intense fire behavior than those lacking these species. Both shrub types burned hotter and longer than either of the litter types with mountain laurel exhibiting more intense fire behavior than blueberry. Slash fuels displayed the most intensity with oak clearcuts burning hotter than oak shelterwoods. Forest managers burning oak stands needs to match the fuel type with the appropriate weather conditions to ensure safe, successful prescribed burns.

Forest Soil Response to Wildfire: Relation to Woody Surface Fuels

P. Homann et al. — Thu, 25 Jun 2009 09:20:00 PDT

The 2002 Biscuit wildfire burned 200,000 ha of conifer forest in southwest Oregon, including previously established experimental plots of the Long-term Ecosystem Productivity Study. Prefire manipulations had resulted in substantial heterogeneity among plots with respect to vegetation composition, structure, and woody surface fuel loads. The Biscuit fire resulted in nine plots burned by the wildfire, one plot previously burned by prescribed fire then by the wildfire, and three plots back-burned in the establishment of a fire line. Pre- and postfire sampling allowed direct measurement of soil change, and unburned plots exhibited minimal or no soil change. The soil C loss due to wildfire varied among plots from 3 to 21 Mg C per ha, and the N loss from 0.03 to 0.60 Mg N per ha. On the most severely burned plots, soil C and N losses occurred not only from the O horizon, but also from the mineral soil. Soil C and N losses were positively and curvilinearly related to woody fuels consumed, which varied among the plots from 14 to 80 Mg per ha. This suggests woody fuel loads and their subsequent combustion influenced soil C and N reduction via volatilization, particulate loss, or post-fire erosion. The combination of prescribed fire followed by wildfire resulted in 30% less soil C and N loss than wildfire alone. The three back-burn plots exhibited soil C and N losses comparable to high-severity wildfire plots, in spite of widely varying fuel losses. Overall, the rare occurrence of previously sampled plots being burned by wildfire allowed direct evaluation of wildfire-induced soil changes. More importantly, the prefire heterogeneity of woody fuels resulted in an unprecedented quantification of the relation between soil changes and the magnitude of wildfire fuel consumption, which will be useful in evaluating potential impacts of wildfire on forest soils.

Fuel Loads, Fire Severity, and Tree Mortality in Florida Keys Pine Forests

Jay Sah et al. — Thu, 25 Jun 2009 10:30:00 PDT

In fire dependent forested ecosystems, fire managers are greatly interested in predicting the consequences of their management-oriented prescribed burnings on post-fire tree mortality. While fire intensity is believed to be a strong predictor of tree mortality, fire behavior itself largely depends on fuel characteristics, including both their structure and spatial distribution. We examined the type and distribution of fuels, their effects on fire behavior, and the effects of fire on tree mortality in slash pine forests in the Florida Keys. We conducted a burning experiment in six blocks, and burned eleven plots, three in winter and eight in summer, over a four-year period from 1998 to 2001. Post-fire slash pine mortality was investigated annually for one, two or three years in seven burn plots, three winter burn and four summer burn plots. We used linear regression to model the effects of fuel types on fire severity, and logistic regression to model the effects of burn season, fire severity and tree dimensions on tree mortality. Fire severity increased with surface fuel loads, but was negatively related to the quantity of hardwood shrub fuels. Tree mortality was significantly higher in summer burn than in winter burn plots, and was strongly related to tree size and crown scorch percent. This study suggests that pine tree mortality can be minimized by burning in winter. However, in pine forests where the burning objective is to suppress the growth of hardwoods, winter burning involves a trade-off, in that hardwood shrub fuel consumption is reduced.

Linking Fine Scale Fuel Heterogeneity with Fire Behavior in a Frequently Burned Pinus palustris Ecosystem

J. O'Brien et al. — Thu, 25 Jun 2009 09:40:00 PDT

In ecosystems with frequent surface fire regimes, fire and fuel heterogeneity has been largely overlooked. This could be a result of the relatively complete burns that give an impression of homogeneity in fire behavior and fire effects, or due to the difficulty in capturing fine scale variation in fuel characteristics and fire behavior. While there are often few unburned patches in these systems, there is variation in fire intensity and duration that occurs at fine scales (<10 >m). The diverse vegetation in these ecosystems also varies at a similar fine scale. This diversity could be driven by the influences of local interactions among patches of fuels, both derived from understory vegetation and canopy supplied fine fuels. We will present the results of a study linking fine-scale fuel heterogeneity and in-situ measures of fire intensity within longleaf pine forests of the southeastern USA. Initially, we developed a novel method to capture fine scale fuel spatial variation and structure and found that fuels occur in discrete patches that occur at a scale of approximately 0.5 m. We referred to these patches as “wildland fuel cells” and present here the results of how they influenced fire behavior. We used infrared thermography to couple fire intensity and residence time with fuel cells in a spatially explicit manner. We found that fire temperatures and residence times varied at similar scales to those observed for wildland fuel cells. The results also showed that fuel cells directly affected maximum temperatures, residence time, and an index of intensity, though there were also complex interactions. We plan to couple these measurements to effects on understory vegetation dynamics. The wildland fuels cell concept shows promise as a means to connect empirical studies with models of fire behavior and fire effects, allowing better predictions of the impact of within burn heterogeneity.