Ecology - Wildland Fuel Fundamentals and Applications - page 87

example, appear to deposit at least around 0.4 kg m −2 yr −1 (~0.2 kg m −2 yr −1 for
foliage and 0.2 kg m −2 yr −1 for logs), which is about 3 % of the total aboveground
biomass (Table 4.1). Keane ( 2008b ) found that foliage deposits on productive sites
were often 10-60 % of the total litter loading. Deposition rates for logs (Table 6.1 )
are usually measured from historical tree mortality and snag fall rates over time,
which assumes tree fall is the only input to log accumulation. Large branches and
treetops, however, also contribute to log inputs to the forest floor in some ecosys-
tems (Harmon et al. 1986 ). While values in Table 6.1 provide estimates of fuel
deposition rates, actual deposition rates are highly dependent on local vegetation
conditions, such as species composition, stand structure, disturbance history, and
biophysical environment (Keane 2008a ).
Each fuel component has a different spatial and temporal pattern of deposition
on the ground. The finest fuels, such as foliage and small twigs, tend to be more
evenly distributed over time and space (Keane 2008b ). This is because the foliage
and small supporting branches are constantly shed by plants and their small size
facilitates long distance and homogeneous dispersal by wind and gravity. Coarse fu-
els, however, have higher variability in deposition rates from year to year and from
place to place; coarser fuels tend to fall infrequently, usually a result of extreme
events, such as a fires, high wind, or heavy snow load. Because of their size, coarse
fuels fall close to parent plants creating patchy patterns that often reflect the patterns
of the live vegetation. Many years may pass before a large branch or tree bole falls
to the ground and these fallen larger particles tend to be more scattered across the
landscape in heterogeneous patterns.
6.1.3 Decomposition
Decomposition is the process whereby dead organic biomass on the soil surface is
broken down into smaller particles and simpler forms (Millar 1974 ; Swift et al. 1979 ;
Fig. 6.1 ). There are three main sequential mechanisms of decomposition (Marra and
Edmonds 1996 ). Decomposition usually begins with leaching, where soluble carbon
compounds are dissolved in water (precipitation) and this solution eventually seeps
into the soil. Fragmentation then physically splits and breaks fallen plant material
into smaller pieces, creating greater surface areas for microbial colonization and
consumption. Fragmentation is typically accomplished by invertebrate fauna in
the soil such as nematodes, insects, mites, and earthworms, and also by abiotic
weathering (freezing and thawing, drying and wetting). Insects, especially termites,
ants, bark beetles, and wood borers, play important roles in the fragmentation of
large wood particles and they also introduce fungal decomposers to fragmented
material (Harmon et al. 1986 ). Microbes, primarily bacteria and fungi, then invade
the disintegrated plant matter, termed detritus, composed mainly of lignin and
microbial byproducts. Microbial respiration further alters the chemical structure
of the detritus and continues decomposition. Respiration is an oxidation process;
much like fire (see Chap. 2), in that it transforms carbon compounds and oxygen