Agriculture Reference
In-Depth Information
2
Fire and the Fire Regime Framework
A global view of potential vs. actual vegetation distributions points to fire as a
major driver of biome distribution and determinant of community structure
(Bond
et al.
2005
). In ecological terms, fire acts much like an herbivore, consuming
biomass and competing with biotic consumers for resources, and in this sense is an
important part of trophic ecology (Bond & Keeley
2005
). As in other competitive
interactions, not only can fire competitively exclude herbivores by temporarily
eliminating resources, but intensive grazing is known to exclude fire by consuming
herbaceous ground fuels (Savage & Swetnam
1990
). Coexistence is often enhanced
by temporal separation of trophic niches, with herbivores grazing early in the
spring on green herbaceous material that is unavailable for burning, whereas later
in the season the remaining dry thatch is readily consumed by fire. In many
respects fire is a more potent competitor because it is not limited by either toxins
or protein deficiency and readily consumes dead woody biomass, but by contrast
it is often limited by ignition sources and continuity of fuels.
Fire scientists have long symbolized the critical elements of fire in a triangle of
fuel, oxygen and heat (Pyne
et al.
1996
). These are indeed necessary for fire
ignition and propagation but are insufficient for predicting the global distribution
of fire-prone ecosystems. The conditions both necessary and sufficient to explain
the ecological distribution of fire activity can be summarized by four parameters:
biomass, seasonality, ignitions and fuel structure (
Fig. 2.1
). In addition to biomass
fuels to spread a fire there must be a dry season that converts potential fuels to
available fuels. In mediterranean-type climate (MTC) ecosystems summer drought
results in high fire hazard on an annual basis, in contrast to many temperate
forests that are only periodically vulnerable to fire in response to decadal or longer
oscillations in climate. Vegetation only burns when ignitions are present to initiate
the combustion process and landscapes vary markedly in the potential for natural
ignitions from lightning, and in the extent of anthropogenic ignition sources.
However, understanding the ecosystem distribution of fire requires consideration
of a fourth parameter, fuel structure, which is fundamental to recognizing how
different fire regimes develop.
Many landscapes are dominated by ecosystems where fire is a natural and
necessary process for long-term sustainability of those systems. Despite the obvi-
ous resilience of many communities to periodic fire, it is misleading to think of
species as being fire adapted; rather, they are adapted to a particular temporal and
