Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 76

Fig. 5.4 The live fuel moisture content (FMC) of new and old pine needle foliage at different
times of the year. Old foliage moisture content does not change throughout the year. (Schroeder
and Buck 1970 )
can be quite heterogeneous because most forest and range communities consist
of a wide diversity of species in different stages of development and phenology
(curing), each having different FMC dynamics.
Plant ecology is just as important as the environment when understanding live
fuel moisture dynamics. Phenology is important because seasonal changes in plant
development influence live FMC throughout the year. Live plant FMCs in the grow-
ing season, for example, are always much higher than FMCs of plants in the dor-
mant season. Foliae age in conifers is also important; older needles have thicker,
hardened cell walls (Fig. 5.4 ). There are also great differences among species in
their ability to sustain foliage under low moisture conditions. Conifers, for example,
have higher minimum foliar FMC values (> 90 % FMC) in the fire season than xeric
shrub species (> 30 % FMC; Kauffman and Martin 1985 ). And FMCs can be quite
different in a single plant; FMCs are typically higher in the upper third of a conifer's
crown than the lower third, which is probably because of the abundance of new
needles in the crown top (Brown 1978 ). Many perennial plants, such as grasses and
forbs, allow their foliage to die when FMCs get too low through curing, while other
species simply close their stomata to reduce transpiration and limiting water loss
thereby maintaining FMC values.
The influence of live fuel FMC on combustion processes is still not well under-
stood (Jolly 2007 ). Why do dead fuels cease burning when FMCs are above 30 %,
while tree crowns burn when their moisture content is above 100 %? Several factors
may contribute to this dilemma. First, water in live fuels is under tension thereby
having a higher pressure and lower boiling point. Second, the chemical composi-
tion of live fuels is different than dead fuels and the many dissolved compounds