Agriculture Reference
In-Depth Information
frequency, intensity and seasonality. It has since been recognized as a critical
attribute for describing fire-prone ecosystems (e.g. Heinselman
1981
), although
it is an ecosystem attribute with both temporal and spatial variation (Morgan
et al.
2001
). Building on Gill's original concept, a more complete picture of a fire regime
includes five parameters (Keeley
et al.
2009a
): (1) fuel consumption and fire spread
patterns, (2) fire intensity and severity, (3) frequency, (4) burn patch size and
distribution, and (5) fire seasonality. The key concept is that these factors act in
concert to produce the fire regime and it is the entire fire regime that constrains
functional types, community assembly patterns and biome distribution. Dissecting
out one factor and evaluating responses to that factor alone will lead to erroneous
conclusions unless one is cognizant of how other attributes of the fire regime
covary. For example, as discussed in
Chapter 9
, attempts to define global patterns
of resprouting response to fire frequency or fire severity have proven elusive and
that is because changes in other parameters such as types of fuels consumed
produce thresholds of response that may reverse the relationship.
Patterns of Fuel Consumption and Fire Spread
Ecosystems differ greatly in both the horizontal and vertical pattern of biomass
distribution, and thus in fuel structure (
Fig. 2.1
), and this has a profound impact
on fire spread characteristics. On the basis of patterns of fuel consumption, fires
are often categorized into classes of fire regimes, which include
surface fires
,
crown
fires
and
ground fires
. Within these classes, regimes may vary markedly; for
example, both South African fynbos and California chaparral are crown fire
regimes but differ greatly in fire frequency and intensity and seasonal distribution.
Surface fires, which are sometimes referred to as
understory fires
, spread by fuels
on the ground and apply to many forest types (
Fig. 2.2a
) where there is a
discontinuity between surface fuels and tree canopy fuels (
Fig. 2.2b
). Closed-
canopy forests typically have dead leaf and stem surface fuels whereas more
open-canopy forests have standing herbaceous surface fuels. The type of surface
fuels can result in significant differences in fire regime. For example, herbaceous
fuels respond to high rainfall years by increasing fire activity in subsequent years,
but the same is not observed in systems where surface fuel structure is dominated
by downed leaves and branches (see discussion of antecedent climate effects
below).
Crown fires burn in the canopies of the dominant growth forms (
Fig. 2.2d
), and
the term is most usefully applied to shrub and tree dominated vegetation
(
Fig. 2.2c
). This is the predominant fire behavior in MTC regions. In closed-
canopy shrublands the general lack of surface fuels, coupled with the shrub fuel
structure, results in fires spreading as
independent crown fires
or
running crown
fires.
This type of fire behavior is dependent on either strong winds, steep terrain
or a high proportion of dead to live canopy biomass. Other vegetation types such
as North American lodgepole pine, Australian
Eucalyptus
or Mediterranean oak
