Fuel Succession

In a forested environment, vegetative biomass tends to accumulate over time. This biomass may accumulate at a rate that is faster than it decays, in which case the forest may be depleted of fuels, or it may accumulate at a rate faster than which it decays. Fuel succession describes the change in a fuel complex over time in terms of distribution of size class, loading, distribution, and available fuel vs biomass. The net gain or loss of available fuels result in either accumulation of fuels or depletion (Brown 1987).

Fuels can accumulate in the litter layer as the result of yearly foliage production. Dead woody fuels are produced as a result of insects, disease or wind/ice events. After a fire event, whatever material is not consumed will add to the surface layer of dead woody fuels. As the lower part of the litter layer begins to decay, it becomes incorporated into the duff layer, which forms the interface between the surface fuels and mineral soil. Finally, Fuel is added to a stand as branches fall from live trees and shrubs.

Fuels can become depleted from a stand as dead plant material oxidizes and decays. Dead woody fuels physically deteriorate and settle over time, and compactness increases as supporting branches decay (Brown 1975). The more compact a fuel bed, the more likely it is to hold moisture and therefore, may be more favorable for decomposition to occur. Also, as crown closure occurs, understory shrubs can decrease significantly. This lack of understory can reduce the amount of available fuel.

Now that we have a general understanding of how fuels can accumulate or be depleted over time, we can understand fuel succession. The generality that downed woody fuels accumulate over time is, in many cases, not true (Brown and See 1981). The amount of forest fuel depends on stand history, whether the stand was visited by insects, disease, wind, and fire, and at what intervals. The size and pattern of disturbance, and amount of fuel that results, can vary with the event, and tree and branch mortality can be compounded by drought (Agee 1993). Changes in the amount of fine and coarse woody fuels over time relate to the amount of biomass present before a disturbance, the severity of the disturbance, and successional patterns after the stand is disturbed.

Fuel succession is more complicated if live and dead fuels are involved (Brown and See 1981). There may be an increase in one class of fuel while another is decreasing or becoming unavailable. Dead woody fuel may decompose while an understory of trees establishes. The loading of dead woody fuel may increase while some trees become tall enough to be much less available to surface fire (Brown and See 1981). In many portions of Rhode Island, pines are beginning to take over oak trees ad the dominant species, and this could result in more complicated prediction of fuel distribution (Sparks 2003).

In their 1981 study, Brown and See found no significant correlation (Brown and See 1981) between stand age and amount of dead woody material. They conclude that the amount of fuel in young and mature forests cannot be related to age because too many other factors are involved. In general, the amount of downed woody desbris does become more predictable as stands acquire old growth charactersistics, but loading is unpredictable from age alone in young, immature, and mature stands.