Agriculture Reference
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Fig. 5.3 The timelags of
different diameter woody
fuel particles. (Schroeder and
Buck 1970 )
5.3
Live Fuel Moisture Dynamics
Unlike dead fuel moisture, live fuel moistures are dictated by the responses of living
plants to their surrounding environment (Pyne et al. 1996 ). Plants grow by trans-
forming water, carbon dioxide, and the sun's energy into biomass via photosynthesis
(Eq. 2.1) and they maintain this living biomass through the process of respiration,
which also uses water. Plants get the water needed for respiration and photosynthesis
from the soil though a series of complex processes known as water transport. When
the ambient air is dry, there is a difference in vapor pressure (amount of water in the
air) from the atmosphere to the stomatal cavities in the plants, photosynthetically
active biomass (foliage). This difference creates a moisture gradient that pulls water
from the foliar cells into the cell void and then through the stomata and into the at-
mosphere via a process known as transpiration (Campbell 1977 ). The movement of
water out of the cell walls and into the intercellular spaces sets up a diffusion gradient
and provides the tensional pressure to pull water through living cells from the roots,
through the xylem, to the foliage, and out to the atmosphere.
Three factors control water transport in plants and therefore live fuel moisture
dynamics: (1) osmotic forces caused by diffusion of water across the plant's cellular
membranes, (2) capillary tension forces, and (3) diffusion across cell voids. Water
enters the roots through diffusion when the transpirational pull, often called plant
water potential, is greater than the soil water potential (forces that bind water to soil
particles). Water moves through cells via osmosis and through cell voids by diffu-
sion and capillary forces. The diffused water is then pulled up to the foliage through
xylem conductive tissue in the wood by tensional pressure caused by plant water
potential that is driven by the vapor pressure gradient from the air to the plant. As
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