Fuel Moisture - Wildland Fuel Fundamentals and Applications - page 75

the soil water becomes depleted, the diffusional gradient must become greater to
exert more tensional pressure to overcome the tensional forces that the soil exerts on
the bound water. The moisture content of live fuels comes from the free and bound
water and water vapor in plant's cellular void spaces, cell walls, and conducting
tissue. This water is under tension due to transpirational pull and other forces such
as capillary and surface tension. If soil water is abundant, plants open their stomata
thereby initiating the vapor pressure gradient to pull soil water through cells for use
in photosynthesis and respiration. However, there always needs to be some water
within the plant to maintain respiration and keep the plant living, so live FMCs
rarely get as low as dead FMCs. Most plants in fire environments have the ability
to shut their stomata to control water loss that ensures the plant stays alive thereby
keeping live FMCs high (Waring and Running 1998 ).
Many exogenous environmental factors control water transport in plants. As
mentioned, humidity of the atmosphere is the primary engine that draws water from
the soil and dictates the rates of transpiration (Campbell 1977 ). Soil properties,
such as depth and percent rock, sand, silt, and clay will dictate soil water-holding
capacity, permeability, and flow rates that then control the amount of free and bound
water available for the plant throughout the year (Eagleson 1978 ). Precipitation is
of critical importance in keeping the soil plenum full. Enough water has to fall so
that it is not evaporated, intercepted by the live foliage, or absorbed by the ground
fuels so that it can eventually seep into the soil and be available for plants. Increases
in solar radiation may increase photosynthesis resulting in greater water usage and
earlier water deficits contributing to lower FMCs. Topography is also important in
that it will influence subsurface water flows to and away from the soil plenum, and
it controls radiation, snowfall, and drainage dynamics.
While live fuel moisture dynamics are more complex than dead fuel mois-
ture, live fuel moistures are also more stable over longer time periods. In general,
live fuels, especially new plant growth, have the highest fuel moistures during
the growing season when water is abundant and the new cells consist mostly
of water with little structural material (Fig. 5.4 ). As cell walls harden and cell
growth consists mostly of structural tissue, the mass of organic matter increases
and the mass of water remains stable. As a result, the ratio of dry matter to wa-
ter increases thereby lowering relative moisture content but not plant moisture.
This is why moisture contents of older foliage are rarely as high as new foliage
(Fig. 5.4 ). As soil water becomes scarce, water in the cells and cell voids becomes
depleted causing major to minor decreases in live FMCs depending on the plant
species. However, live foliar moisture rarely goes below the level at which plant
cells would die from lack of moisture. Chrosciewicz ( 1986 ) found that moisture
contents of jack pine and black spruce were approximately 120-130% in the
spring and fall, but dropped to 90 % in the summer. Live FMCs of actively grow-
ing foliage and branchwood can be as high as 300 % during the growing season
when water is plentiful (Pyne et al. 1996 ). Conifers and many temperate shrubs
may have live FMCs that reach a minimum of 90-100 % during the driest parts of
the year. Some xeric shrubs, such as sagebrush, can have FMCs that are as low as
30-50 % during the dry season. The spatial pattern of live FMCs across an area