Agriculture Reference
In-Depth Information
include changes in fire frequency, fire season and fire intensity. In this sense fire
per se
should not be thought of as a disturbance, but rather ecosystems are
disturbed by deviations from the natural fire regime.
Plants and animals differ markedly in how they respond to fires and to devi-
ations in fire regime. In many fire-prone MTV communities plants have endogen-
ous means of reproduction from stored seedbanks or vegetative buds, and
colonizing species that come in from outside the burned area are relatively limited
(see
Chapter 3
). Many animals in contrast are often extirpated from burned sites
and must recolonize, and thus the metapopulation dynamics are critical to sus-
taining these ecosystem components. As a consequence, the fire regime character-
istics that are critical for sustainability may differ between plants and animals; for
example, fire size or patchiness often has little impact on plant recovery but huge
effects on animal recovery (Bradstock
et al.
2005
; Parr & Andersen
2006
).
Humans have disturbed fire regimes on many landscapes by altering fire fre-
quency. In many instances they have increased ignition sources and thus decreased
fire intervals. In other cases they have suppressed natural ignitions and increased
fire intervals. The impact of altering fire frequencies (as well as other aspects of
the fire regime) may be best predicted by examining thresholds of tolerance
exhibited by life histories (Keeley
et al.
1989
; Gill & McCarthy
1998
; Bradstock
& Kenny
2003
; Pausas
et al.
2004b
). In general, species face two threats from
alterations in fire regime: immaturity risk, where fire intervals are shorter than the
tolerable level, and senescence risk, where fire intervals are longer than tolerable
(Zedler
1995b
). These risks lead resource managers to be watchful for “thresholds
of potential concern” when the fire regime is altered by too frequent or too
infrequent fires (van Wilgen & Scott
2001
).
Immaturity risk is a major concern in all MTC landscapes due to the high
numbers of unplanned anthropogenic ignitions (Syphard
et al.
2009
). It also is a
potential threat on landscapes where planned ignitions for fuel treatments create
short fire intervals that inhibit recovery of some plant and animal populations (see
Chapter 8
). Risk increases when the interval between fires drops below a critical
threshold of tolerance for a species and on moderately fertile soils can lead to type
conversion of shrublands and woodlands to grasslands (Keeley
1995b
; Jacobson
et al.
2004
; Syphard
et al.
2006
). This risk is greatest for those species dependent
entirely on dormant seedbanks for postfire recovery (obligate seeders) and is
measured by the time after fire required to replenish the seedbank. With animal
populations it is measured by time to recolonize burned sites and interacts
strongly with landscape patterns of burning and metapopulation dynamics.
Senescence risk is a potential threat to some populations that are short lived and
have short-lived seedbanks. Documentation of such risks is largely lacking and
one attempt to look for such risks in California chaparral shrublands unburned
for more than a century failed to detect any evidence for senescence impacts on
postfire recovery (Keeley
et al.
2005b
). In crown fire shrublands it is primarily a
risk for serotinous species with aerial seedbanks (Bond
1980
; Lamont
et al.
2004a
,
2007
). In forests with historical fire regimes of frequent low-intensity surface fire,
