Ecology - Wildland Fuel Fundamentals and Applications - page 83

Fig. 6.2 The accumulation of live ( solid line ) and dead ( dashed line ) fuels in the absence of
disturbance and after various disturbances. a only succession and vegetation development and no
disturbance (note development starts with bare ground), b after a major wind event, c after a severe
fire event, and d after a mountain pine beetle outbreak
Below- and aboveground necromass is eventually decomposed by microbes and
soil macrofauna (see Sect. 6.1.3) to create duff and the expended energy during de-
composition is called heterotrophic respiration (HR). The difference between total
productivity (GPP) and the sum of both autotrophic and HR is called net ecosystem
production (NEP = GPP − HR − AR or NEP = NPP − AR). The annual NEP for a
fuelbed generally hovers around zero over long time periods, but in periods when
NEP is positive, fuels are generally accumulating. When NEP becomes negative, it
is likely the result of a disturbance or mortality event has probably reduced GPP be-
cause of the decrease in photosynthetic potential when plants are killed or damaged,
and increased HR through the subsequent decomposition of the recently killed plant
material. Negative NEP can persist for months or years depending on the severity
of the disturbance (Diffenbaugh and Field 2013 ).
Wildland fuels accumulate in different ways depending if they are alive or dead
(Fig. 6.2a ). Live fuels accumulate through the completion of life-cycle processes
of living organisms, primarily plants. Plants first become established on a site via
reproduction and regeneration; they then grow as a result of photosynthesis and
respiration, and they then die from a number of causes but most often disturbances,
especially in fire-prone ecosystems (see Sect. 6.1.4). Live fuel biomass accrues
as plants become established and grow bigger and taller, thereby increasing live
fuel loading (increasing ASB) in both surface and canopy fuel layers. Increases in
ASB from annual NPP eventually level off as photosynthetic gains are balanced by