FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

1-hour timelag fuels (Anderson 1990). The surface layer of lichens and
mosses that carries fire in Alaska responds as a 1-hour fuel to
temperature/relative humidity changes (Mutch and Gastineau 1970).
However, conifer needle litter of some species belongs in the 10-hour
timelag category (Anderson 1990), despite its high surface area to
volume ratio. Other factors such as surface covering influence the rate
at which fuel moisture changes in response to environmental
conditions.
d. Fuel properties that affect dead fuel moisture content.
(1) Surface covering. The presence of a surface coating of organic
material can limit movement of water, whether liquid or vapor (Simard
and Main 1982). Dead woody fuel with bark gained and lost moisture at
two-thirds the rate of the same diameter fuels without bark (Simard et
al. 1984).
Moisture exchange in recently cast conifer needle litter is inhibited by a
coating of fat, waxes, and cutin deposits (Anderson 1990). Anderson
(ibid.) noted timelags of 5 to 34 hours for recently cast conifer needle
litter, rather than the expected timelag of 10 hours. Weathering causes
the breakdown and removal of needle coatings that slow vapor transfer.
Timelags of 2 to 14 hours were measured in weathered conifer litter,
which is still slower than the timelag of less than two hours expected for
that diameter of particle (ibid.).
(2) Composition. The material of which a fuel is composed affects its
structure, porosity, ability to gain or lose atmospheric moisture, and the
movement of vapor within the particle. Composition and fuel moisture
response properties vary significantly among dead woody fuels,
deciduous leaf litter, grass litter, and coniferous needle litter (Simard
and Main 1982).
(3) Amount of decomposition. Woody fuels that have been affected by
weathering and decomposition often develop deep cracks that increase
their surface area to volume ratio. Both liquid water and vapor can enter
or leave the fuel through these splits in the wood, increasing the rate of
moisture exchange. There may be few naturally occurring forest fuels
that are actually 1000 hour timelag fuels, because almost all large
pieces of wood have cracks that effectively increase their wetting and
drying rates (Miller 1988, personal observation).