Tree mortality due to SOD has the potential to affect fire hazard and behavior by changing the types and distribution of fuels in the forest. Many other factors unrelated to SOD can also influence fuel distribution and fire hazard in forests.
In many stands, the change in fuel distribution due to a few scattered SOD-killed trees will not significantly alter fire behavior in the stand. At the other end of the spectrum, high levels of SOD-related mortality seen in some tanoak stands can greatly alter fire hazard for a number of years. Impacts of SOD need to be considered within the context of the entire landscape to determine how much they change fire hazard and which, if any, management actions may be required.
The presence of SOD-killed trees in a stand can affect fuel amount and distribution as follows:
Because live trees with late SOD symptoms and standing dead trees have an increased likelihood of failure, these trees can pose a hazard to firefighting personnel during fire suppression and mop-up operations.
Fuels generated by SOD-killed trees differ depending on tree species, size, and the amount of time over which the tree declines before it dies (fig. 2-2A, B). In addition, the fuels from killed trees are not static but degrade over time. Once woody materials and leaves decay to the point that they have lost their structural integrity, they typically do not have an effect on fire potential.
Trees killed by SOD begin to degrade before they are dead due to attack of the trunk by beetles and wood decay fungi. Decay and degradation of dead materials proceed faster in wetter sites (high rainfall, frequent fog, deep shade). Degradation patterns and timelines differ somewhat between standing dead trees and material that has fallen to the ground because materials on the ground remain moist longer (fig. 2-2A, B). A photo series showing typical fuels associated with decaying SOD-killed coast live oaks is shown in Table 3-7 in 3.4. Assessing Fire Hazard Related to SOD-Killed Trees. The series is based on data collected primarily in Marin County from 2000-2010.
Rates of fire spread and fire intensity depend on the total amount of fuels present, typically expressed as tons of fuel per acre. Higher fuel loading will increase the amount of heat that can be produced during a fire. In addition, both the horizontal and vertical distribution of fuels affect fire behavior. Fire will spread more uniformly and reliably if a continuous fuel layer is present. Spread will be slower and more unpredictable if fuels are patchy in distribution. Flame length is affected by the height of the fuel bed. Tall piles of debris and dead material that lean up against standing trees can serve as ladder fuels, allowing fire to move from surface fuels on the ground into the canopy.
Tree mortality due to SOD affects forests in other ways that can alter fire behavior. Canopy gaps created by killed overstory trees can open the forest floor to more wind and light. This creates hotter and drier conditions that can increase the rate at which fuels dry out as the season progresses. In addition, higher wind speeds at the forest floor can increase the rate of fire spread.
L ight penetration into canopy gaps can promote the growth of understory vegetation. Gaps may be invaded by annual grasses and forbs that dry out to form fine flashy fuels that ignite easily. The growth of native or invasive shrubs may also be greater in gaps, which can further change fuel types and fuel height distribution. Tree regeneration, especially Douglas-fir seedlings and saplings, may also become dense in canopy gaps and can form dense ladder fuels. Eventually, overly dense Douglas-fir clumps may develop extensive lower canopy dieback and mortality due to competition, forming a more hazardous fuel distribution than existed in the stand prior to SOD.
Within defensible space fuel management areas, SOD affected trees should be removed as soon as the foliage starts to turn brown. Defensible space extends out 100 feet around buildings and requires arranging the tree, shrubs and other fuels sources in a way that makes it difficult for fire to transfer from one fuel source to another (State Board of Forestry and Fire Protection, California Departent of Forestry and Fire Protection, 2006). Data from several studies indicates that the water content of foliage of SOD-affected trees does not change appreciably until the tree changes color.
In more extensive forest areas beyond defensible space envelopes, scattered tree mortality occurring over a period of many years will probably not significantly alter fuel distributions at the landscape level. In such sites, leaving trees to degrade and decay in place is often the best option. Where large patches of trees have died within a relatively short time period (1-3 years), problematic levels of readily ignitable fuels will be present on the ground and in the canopy for a number of years. Canopy fire hazard can be reduced by felling standing dead trees that still have many dead leaves and fine twigs. Once standing dead trees have lost their leaves and fine twigs, they do not act as significant fuels. Cutting dead trees at this stage will reduce the potential for tree failure and reduce the fuel ladders that develop when failed trees hang up on adjacent standing trees.
If dead trees are cut to manage fuels, it will be necessary to treat the biomass from these trees to avoid creating excessive amounts of surface fuel. Treatment options include:
Wood from SOD-killed trees may be used as firewood on the same property (see 3.1.3 Firewood), but should not be moved beyond the local area. State and federal regulations generally prohibit the movement of wood from susceptible oaks and tanoak from SOD-infested counties to noninfested counties.