Ecology - Wildland Fuel Fundamentals and Applications - page 84

autotrophic respiration losses (Waring and Running 1998 ) and that balance point
is dictated by a multitude of factors in the biophysical environment, primarily the
interactions of plant characteristics (e.g., species, size, density) with environmental
conditions (e.g., available water, nutrients, temperature, and sunlight; Fig. 6.2a ).
The fuelbed becomes taller as plants grow in height and expand into the canopy
layer. And, as more plants become established, the canopy layer becomes denser
(higher CBD). Thus, fuelbeds become heavier, higher, and denser with increasing
time since disturbance (Keane et al. 2002 ). Dead fuels accumulate as live plants
shed various parts, such as needles, branches, and fruits, or die from various mortal-
ity agents. As a result, dead fuel deposition rates will tend to increase with increas-
ing live fuels, and these deposited dead fuels will tend to be more diverse and larger
with increasing time since disturbance (Habeck 1985 ; Muller 2003 ). Dead fuels de-
compose over time so the accumulation will eventually level off when depositional
gains are balanced by decompositional losses (Fig. 6.2a ).
The interaction of life-cycle processes across plant species, often referred to as
vegetation succession, also influences fuelbed characteristics. Succession is often a
contentious term in vegetation ecology because it is generally used in the Clemen-
tsian context where vegetation communities develop along pathways of facilitation
(i.e., one community paves the way for the next; Connell and Slayter 1977 ; Clements
1916 ). This topic, however, uses the term to describe any form of vegetation or fuel
development in any direction (progressive or retrogressive). In succession, species
that have evolved disturbance adaptations, such as thick bark, sprouting, and deep
roots, will tend to persist or populate disturbed sites by surviving the disturbance
and/or dispersing propagules into the disturbed area from great distances (Noble
and Slatyer 1980 ). These disturbance-adapted species are usually unable to grow
in heavy shade (shade-intolerant) because they have developed adaptations to grow
quickly and dominate disturbed areas and take advantage of the ample sunlight
available in postburn environments. As time since disturbance increases, shade-
tolerant species often become established and grow in shaded areas because they
have higher growth rates in low sunlight than shade-intolerant species. These shade-
tolerant species first dominate the understory and then eventually outcompete the
shade-intolerant individuals in the overstory. There is a general increase in shade-
tolerant living biomass (ASB) with succession and this living biomass becomes
more homogeneously distributed across all canopy layers as competition-adapted,
shade-tolerant species replace disturbance-adapted shade-intolerant species (Keane
et al. 2002 ). Some shade-tolerant species have unique morphological characteris-
tics that also influence fuelbed characteristics. Shade-tolerant species usually have
greater leaf biomass to harvest more light under low light conditions (Bazzaz 1979 )
and this leaf biomass tends to be distributed across the entire length of the plant.
Shade-tolerant conifers, for example, tend to have crowns that are denser and closer
to the ground (Brown 1978 ). Branches on some shade-tolerant conifers tend to be
shorter in length and smaller in diameter than those of shade-intolerant trees, espe-
cially those adapted to disturbance (Brown 1978 ).
Plant growth and succession cause great changes in fuelbed characteristics over
time (Fig. 6.2a ). For canopy fuels, CH, CBD and CFL will generally increase, and