Renewed interest in short-rotation woody crops for bioenergy and bioproducts has prompted a reevaluation of the
The southern United States is one of the most productive forested regions in the country, with 81 million ha or 40 percent of the nation’s forests in an area occupying only 24 percent of its land area [
Although new SRWC plantations for bioenergy will likely be established on idle agricultural land because of lower establishment costs and legislative prohibitions [
The southern United States is comprised of the 13 states roughly south of the Ohio River and extending from Texas to the Atlantic Coast. The region primarily has a humid subtropical climate except for a tropical climate in southern Florida and a semiarid climate in western Texas and Oklahoma. Annual daily temperature averages range from >21°C in southern Florida and Texas to 13–16°C in northern areas. Annual precipitation is 1270–1780 mm in the Mid-South including Louisiana, Mississippi, Alabama, and Tennessee, areas of Georgia and Florida, and areas along the Atlantic coast. Precipitation reduces to 1015 and 1270 mm towards Atlantic coastal areas and northern areas of the region, and to 300 and 500 mm towards western Texas and Oklahoma (
The complex role that wildfire plays in shaping forests has been described in terms of vegetation responses, which are characterized as dependent on, sensitive to, independent of, or influenced by fire [
Today, southern Coastal Plain forests are dynamic ecosystems characterized by rapid growth—and hence rapid accumulation of fuels within a favorable climate—and a short fire-return interval of 3–5 years is considered desirable for managed forests [
Area burned in the southern United States by fires of various ignition sources, 2002 to 2010 (Data from National Interagency Fire Center;
The southern Coastal Plain is also a region of rapid land use change [
Wildland fuelbed characteristics are complex both temporally and spatially, and the Fuel Characteristic Classification System (FCCS) provides a means of classifying these fuelbeds in terms of their capacity to support fire and consume fuels [
The initial fuelbed used to begin developing our
Fuel load (Mg ha−1) for initial FCCS fuelbed 165 and starting point for plantation fuelbeds; fuelbed 165 is typical for a longleaf pine savanna with a herbaceous layer dominated by grasses and forbs, burned periodically.
Fuelbed strata | Fuelbed | |
---|---|---|
FCCS 165 | Plantation, preplanting | |
Canopy | 0.9 | 0 |
Shrubs | 2.69 | 0 |
Nonwoody | 0.9 | 0.9 |
Woody | 1.57 | 0 |
Litter | 2.69 | 0 |
Ground | 2.91 | 0 |
Development of the
Once the plantations are described as FCCS fuelbeds, the FCCS software provides estimates of fire potential. Fire potentials in FCCS are summarized by a three -digit number with each digit representing an index describing an aspect of fire potential [
The second digit in FCCS is the crown fire potential (CFP) that utilizes a conceptual model for crown fire developed by Schaaf et al. [
The third digit of a fuelbed’s fire potential in FCCS is the available fuel potential (AFP) which reflects the ovendry, combustible biomass. The total AFP is decomposed by combustion phase (flaming, smoldering, residual). Detailed descriptions of how each component is calculated can be found in the FCCS Users Guide [
In order to consistently determine fire potential requires defining a benchmark set of environmental conditions that includes fuel moisture, topographic slope, and surface wind speed. The default environmental scenario in FCCS is for dry fuel conditions defined as BehavePlus fuel moisture (FM) scenario D2L2 [
Fire potentials for the modeled
Starting from an open grass-dominated system as described in the previous section, after 3 growing seasons, the trees are assumed to reach a height of 5 m with live branches and leaves retained from the ground up to the crown. As the trees develop and the canopy begins to close, the fine fuels begin to shift from being grass dominated to litter dominated as the canopy begins to shade out the grass. By the age of 6, the trees are assumed to have grown to a height of 16 m with a height to live crown of 4 m. Litter and woody debris are accumulating and adding to the fuel load as is bark shedding. By the age of 9, the trees are assumed to reach a height of 20 m with a height to live crown of 15 m. Litter has continued to accumulate at a rate of 2 Mg ha−1, while woody debris has accumulated at a rate of 0.5 Mg ha−1. The fuel loadings from FCCS based on these stand descriptions are shown in Table
Fuel load (Mg ha−1) for
Fuelbed strata | 3 yr old | 6 yr old | 9 yr old |
---|---|---|---|
Canopy | 5.83 | 30.48 | 37.44 |
Shrubs | 0 | 0 | 0 |
Nonwoody | 0.9 | 0.45 | 0.22 |
Woody | 0 | 0.5 | 2 |
Litter | 2 | 8 | 14 |
Ground | 0 | 0 | 0 |
Fuel load (Mg ha−1) for typical pine fuelbeds in the southeastern United States.
Fuelbed strata | Fuelbed | ||
---|---|---|---|
Slash pine plantation (FCCS 156) | Longleaf and slash pine forest with prescribed burning (FCCS 191) | Longleaf and slash pine forest with fire exclusion (FCCS 182) | |
Canopy | 7.17 | 6.5 | 11.21 |
Shrubs | 4.26 | 10.98 | 17.71 |
Nonwoody | 0.22 | 0.45 | 0.45 |
Woody | 13.45 | 4.04 | 2.91 |
Litter | 6.5 | 6.73 | 11.66 |
Ground | 21.07 | 9.42 | 81.6 |
The fire potential estimates from FCCS increase for the
FCCS fire behavior potentials for eucalyptus plantations at 3, 6, and 9 years after planting. FCCS potentials are index values.
3 yr old | 6 yr old | 9 yr old | |
---|---|---|---|
Surface fire behaviour potential | 4 | 7 | 6 |
Reaction potential | 2.4 | 1.8 | 2.3 |
Spread potential | 3.7 | 7.3 | 6.4 |
Flame length potential | 2.1 | 2.5 | 2.8 |
Crown fire potential | 1 | 6 | 6 |
Initiation potential | 0 | 2.3 | 1.7 |
Transmissivity potential | 0 | 7.7 | 8.9 |
Spread potential | 3.2 | 9 | 8.6 |
Available fuel potential | 1 | 3 | 4 |
Flame available fuel potential | 0.6 | 2.8 | 3.6 |
Smoldering available fuel potential | 0 | 0 | 0 |
Residual available fuel potential | 0 | 0 | 0 |
FCCS fire potential code | 411 | 763 | 664 |
Flame length (m) | 0.34 | 0.49 | 0.61 |
Spread rate (m hr−1) | 62.18 | 245.06 | 184.71 |
Another major difference in fire potential between age 6 and 9 is reflected in the decline in the initiation potential for a crown fire. By age 6, crown closure has increased the transmissivity and spread potentials for the stand, but the absence of a shrub layer and the negligible amount of bark shedding keeps the initiation potential relatively low. By the age of 9, despite increased bark shedding, the absence of a shrub layer that would capture the bark and create a significant layer of ladder fuels, along with an increased height to live crown, further reduces the initiation potential. Note, however that in the event of initiation (ignition of the crown) the potentials for transmissivity and spread for a crown fire are very high.
FCCS surface fire behavior potentials do not show any major differences between the
FCCS fire behavior potentials for fuelbeds typical of the southeastern United States. FCCS potentials are index values.
Slash pine plantation (FCCS 156) | Longleaf and slash pine forest with prescribed fire (FCCS 191) | Longleaf and slash pine forest with fire exclusion (FCCS 182) | |
---|---|---|---|
Surface fire behaviour potential | 4 | 7 | 8 |
Reaction potential | 5.1 | 7.9 | 9 |
Spread potential | 4.2 | 6.9 | 7.8 |
Flame length potential | 3.8 | 5.7 | 6.4 |
Crown fire potential | 3 | 2 | 4 |
Initiation potential | 2.7 | 3.1 | 3.4 |
Transmissivity potential | 7.2 | 0 | 7.2 |
Spread potential | 2.1 | 2.7 | 3.5 |
Available fuel potential | 3 | 2 | 6 |
Flame available fuel potential | 1.6 | 1.4 | 1.9 |
Smoldering available fuel potential | 1 | 0.4 | 3.7 |
Residual available fuel potential | 0.1 | 0.1 | 0.3 |
FCCS fire potential code | 433 | 722 | 846 |
Flame length (m) | 0.91 | 2.44 | 3.14 |
Spread rate (m hr−1) | 80.47 | 217.63 | 279.81 |
Even though the
Effect of introducing a shrub layer on FCCS fire behavior potentials for 9 yr old eucalyptus plantation, as compared to a longleaf/slash pine stand with the shrub layer managed by periodic prescribed burning.
9 yr old eucalyptus plantation | 9 yr old eucalyptus plantation with shrub layer | Longleaf and slash pine forest with prescribed burning (FCCS 191) | |
---|---|---|---|
Surface fire behaviour potential | 6 | 7 | 7 |
Reaction potential | 2.3 | 4 | 7.9 |
Spread potential | 6.4 | 7.9 | 6.9 |
Flame length potential | 2.8 | 4.2 | 5.7 |
Crown fire potential | 6 | 6 | 2 |
Initiation potential | 1.7 | 3.1 | 3.1 |
Transmissivity potential | 8.9 | 8.9 | 0 |
Spread potential | 8.6 | 9 | 2.7 |
Available fuel potential | 4 | 4 | 2 |
Flame available fuel potential | 3.6 | 3.9 | 1.4 |
Smoldering available fuel potential | 0 | 0 | 0.4 |
Residual available fuel potential | 0 | 0 | 0.1 |
FCCS fire potential code | 664 | 764 | 722 |
Flame length (m) | 0.61 | 1.31 | 2.44 |
Spread rate (m hr−1) | 184.71 | 248.7 | 217.63 |
Surface fire behavior in young
It is difficult to compare the fuel loads in these hypothetical fuelbeds with those of Project Vesta [
All fire behavior potentials produced using FCCS assumed a default set of environmental conditions. Comparison of these conditions to the historical weather conditions found in the Southern Wildfire Risk Assessment [
Under more extreme burning conditions, many fire behavior predictions in
Introducing select
Our modeling provides a preliminary answer to the question we posed: how fire behavior at the stand level would be affected by deployment of
The work this paper is based upon was funded in part by a special grant from the US Forest Service Research and Development Washington Office for bioenergy research, and The authors acknowledge the support of Marilyn Buford for this work. They also express their thanks to two anonymous reviewers for providing valuable feedback that improved this paper.