FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

thus increases with increasing fuel diameter.
(a) Dead woody fuel timelag classes. Downed dead woody fuels have
been grouped into size classes that reflect the rate at which they can
respond to changes in atmospheric conditions (Lancaster 1970). The
classes relate to an idealized surface area to volume ratio and an
average timelag that represents each fuel class. Classes relate to the
theoretical length of time required to reach 63 percent of EMC.
i. 1-hour timelag fuels - less than 1/4-inch diameter (less than 0.6 cm).
ii. 10-hour timelag fuels - between 1/4-inch and 1-inch diameter (0.6 to
2.5 cm).
iii. 100-hour timelag fuels - between 1-inch and 3 inches diameter (2.5
to 7.6 cm).
iv. 1000-hour timelag fuels - between 3 and 8 inches (7.6 to 20.3 cm)
diameter.
(b) Forest floor timelag classes. There is a loose correspondence
between these timelag classes and forest floor litter and duff, although
the deeper the duff layer, the more approximate is the relationship. The
corresponding classes assigned for fire danger rating purposes were
(Deeming et al. 1977):
i. 1-hour timelag fuels - dead herbaceous plants and uppermost layer of
forest floor litter.
ii. 10-hour timelag fuels - layer of litter extending from just below the
surface to 3/4 of an inch below the surface.
iii. 100-hour timelag fuels - forest floor from 3/4 inch to 4 inches below
the surface.
iv. 1000-hour timelag fuels - forest floor layer deeper than 4 inches
below the surface.
(4) Timelag of other fine fuels. Weathered aspen leaves, tree lichen
(Alectoria jubata) and some cheatgrass fuel beds were shown to act as