Ecology - Wildland Fuel Fundamentals and Applications - page 85

CBH will decrease with advancing succession or time since disturbance because
of increasing plant density, continued growth, and species compositional shifts
(Reinhardt et al. 2006 ). It is more difficult to describe surface fuel layer changes
because of the interaction of deposition with decomposition and climate, but in gen-
eral, loading for all dead fuel components will tend to increase with succession and
plant development until decomposition rates approximately match deposition rates
with the greatest accumulations occurring on sites with the lowest decomposition
rates (Keane 2008b ). In general, the larger the fuel component (e.g., CWD), the lon-
ger it takes for that component to reach this deposition-decomposition equilibrium
and also the greater the variability in fuel properties, especially particle density,
because large fuels are often in a wider range of decay stages than smaller fuels
as a result of their slow decomposition rates. Surface fuel depth and bulk density
also increase with succession as understories become dense with shrubs, herbs, and
trees and dead fuels accumulate. Conversely, the low light conditions created by
some dense shade-tolerant overstories in late successional stages may also deter
shrub, herbaceous, tree regeneration resulting in lower surface fuel depth in some
forested ecosystems. However, fuelbeds are rarely created solely from the processes
of growth and succession, but instead are formed by the complex interactions of
vegetation productivity with deposition, decomposition, disturbance and the physi-
cal environment (Collins and Roller 2013 ).
6.1.2 Deposition
Deposition is defined as the release of live and dead aerial biomass to fall to the
ground to become dead surface fuels (Fig. 6.1 ). Many ecological studies refer to
this process as litterfall, which is confusing because, in this topic, litter is a term for
a specific fuel component. Fuel deposition can result from normal plant ecophysi-
ological and phenological processes, such as leaf shed and turnover, and also from
the effects of endemic and exogenous disturbances (see Sect. 6.1.4). Light winds,
for example, may dislodge senescing biomass and cause it to fall to the ground
close to the plant, while strong winds may detach both dead and live biomass and
transport it great distances. The interactions of fuel particle characteristics (e.g.,
size, shape, density) with wind distributions (e.g., speed, direction, duration) create
unique fuel component patterns across the landscape (Keane et al. 2012a ).
Rates of deposition (kg m −2 yr −1 ) differ greatly by fuel component and ecosystem
type. Most studies estimated only foliage or log deposition rates because they are
the easiest to measure and comprise the majority of deposited necromass (Harmon
et al. 1986 ; Vogt et al. 1986 ; Table 6.1 ). FWD additions to the forest floor are
rarely reported even though they may be the most important to fuels management
and fire behavior prediction because they influence fire spread. Example deposi-
tion rates are presented in Table 6.1 for US Rocky Mountain ecosystems and other
ecosystems of the world. What is striking about these numbers is the great amount
of organic material that falls to the forest floor each year. Douglas-fir stands, for