Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 70

called hydroscopy . In general, wood is mostly composed of cellulose (~ 50 %),
hemicellulose (~ 20 %), and lignin (~ 20 %); needles have less lignin and more cellu-
lose (Chap. 2). First, cellulose has a greater ability to hold water than lignin because
of its chemical structure (i.e., more hydroscopic). The ability of the fuel particle to
hold water dictates the rate of moisture loss. Second, the internal cell structure of
the fuel strongly influences moisture dynamics and it differs for live and dead fuels.
Cell walls of most dead fuels are hygroscopic. Water molecules that are attracted to
and eventually adhere to cell walls become bound water and have low vapor pres-
sure (Schroeder and Buck 1970 ). Free water consists of those water molecules that
are not bound to the chemical structure of the fuel. Third, the physical properties of
the fuel ultimately control moisture retention. These include all of those properties
described in Chap. 2, but especially particle density, size, and shape. Dense, large,
round coarse fuels dry slower than less dense, small fuels that have high surface
areas.
Live fuels have completely different moisture dynamics than dead fuels. In short,
live fuel moistures are dictated by the ecophysiological processes of transpiration
and soil water dynamics, while dead fuel moistures are driven by the physical pro-
cess of evaporation. Both live and dead fuel moisture dynamics are driven by the
gradient of vapor pressure (humidity) from the particle to the atmosphere; water
molecules tend to migrate to drier conditions. Dead and live FMCs also have a
complex spatial distribution because the biophysical factors that control FMC dy-
namics vary widely across space as they interact with plants, necromass, weather,
and topography.
Although this chapter discusses fuel moisture in the context of how it is used to
predict fire behavior and effects, live and dead fuel moisture dynamics interact with
many other ecological and physical processes. Moist fuels, for example, decompose
more rapidly and are more susceptible to fragmentation than dry fuels. Dry fuels
intercept more precipitation and reduce water availability for plant growth. The
primary objective of this chapter is to familiarize the reader with the biophysical
processes that control fuel moisture, how they relate to wildland fuel dynamics
(Chap. 6), and how to measure or estimate fuel moistures. A more comprehensive
discussion of live and dead fuel moisture dynamics for fire managers is provided in
Schroeder and Buck ( 1970 ) and a more scientific treatment is provided by Nelson
( 2001 ).

5.2 Dead Fuel Moisture Dynamics

Water moves through dead wood fuel particles via three mechanisms—capillarity
forces, infiltration, and diffusion. Capillary forces draw water through fuel particles
via fine capillaries in cell walls and cell structure. Infiltration involves the flow of
free water through a fuel particle via gravitational forces. The primary mechanism
is diffusion in which water in gaseous form (vapor) diffuses into and through a
fuel particle driven by a moisture gradient; water vapor is drawn from areas of