Ecology - Wildland Fuel Fundamentals and Applications - page 88

into carbon dioxide, water, and energy to sustain metabolic microbial processes (see
Eq. 2.2).
Fuels are composed of a wide variety of organic compounds that decompose
at different rates. Microbes decompose simple sugars to CO 2 and water relatively
quickly and completely, but the decomposition of complicated organic compounds
in forest ecosystems is rarely complete (Meentemeyer 1978 ). Lignin, for example,
has a complex chemical structure that slows the rate of microbial breakdown be-
cause it can only be decomposed by certain fungi. This recalcitrant organic mat-
ter accumulates on and in the soil as humus to become the duff fuel component
(Chap. 3). Every dead fuel component is in various stages of decay, but once ma-
terial from a fuel particle has been broken down by leaching, fragmentation, and
respiration to contain mostly lignin, it is mostly unrecognizable as a fuel particle
and usually lands in the duff for the last stages of advanced decay, mainly from
microbial respiration.
Decomposition alters fuel particle properties in a number of ways. Decay of
woody fuel usually decreases particle densities (specific gravities), decreases heat
content, and increases surface area to volume ratios (SAVR) as cracks develop
(Harmon et al. 2008 ). As decay advances and decomposed organic material moves
to the duff, the mineral content and moisture of extinction also tend to increase.
Duff layers often have high mineral contents because of two factors: (1) the mixing
of mineral soil into the duff by soil macrofauna and (2) the release of minerals into
the duff via decomposition (Keane 2008b ).
Decomposition rates are usually calculated using two approaches. In the first ap-
proach, a parameter k is used to describe rates of decomposition using the following
formula from Olson ( 1963 ):

L
L

(6.1)

t

= e

− kt

,

o

where L t is the loading at time t, L o is loading at the start or when t = 0, and t is time
(yr). The exponential function represents the effect of the remaining material ( L t )
having a greater content of recalcitrant lignin, making decomposition difficult and
longer. The second approach uses a mass loss rate ( MLR, % yr −1 ), calculated as the
difference in loading over a unit time period ( MLR = 100*(L o − L t )/L o ). MLR must
be calculated over long time periods to ensure the estimates include both the rapid
decay of solubles, hemicellulose, and cellulose, and the prolonged decay of lignin.
As with deposition, studies of decomposition rates are mostly for foliage and large
log material, especially in the western USA (Table 6.1 ; MLR can be approximated
in Table 6.1 for the first 10 years of decomposition from the k value by solving
Eq. 6.1 using t = 10 and L o = 10).