Agriculture Reference
In-Depth Information
was based on the premise that evolution could be understood in terms of climate
and geology and thus there was little attempt to consider fire as a factor. However,
demographic patterns in the most widespread chaparral species illustrate not only
fire resilience but fire dependence. Delaying reproduction to a single risky postfire
pulse of recruitment cannot be accounted for by any other reasonable hypothesis.
Likewise, serotinous pines found throughout California and the Mediterranean
Basin represent as clear an example of fire adaptation (Schwilk & Ackerly
2001
)as
to be found anywhere. Yet this perspective evaded Axelrod (
1980
) who apparently
could not escape his early training that plant distributions are dictated by climate
and geology and these attitudes have been widespread in many disciplines
attempting to explain origins and distributions. Some biogeographical problems
are highly dependent on understanding fire regimes and their interactions with
climate and geology. For example the California endemic tree
Pinus sabiniana
has
long perplexed botanists due to its marked absence from certain watersheds. Life
history analysis and response to fire points to an interesting interaction between
geological uplift, drainage topology and fire intensity as factors that in combin-
ation have acted to eliminate this species from watersheds in the middle of its
range (Schwilk & Keeley
2006
).
One means of affecting community assembly on fire-prone landscapes is by the
acquisition of traits that increase resilience to fire but alter fire regimes in ways
that promote burning. Bond and Midgely (
1995
) explored the concept of
flammability as a competitive force. They linked flammability to variations in
mortality during fires and postfire recovery and demonstrated that selection for
more flammable plant types could occur, provided that less flammable competi-
tors suffered high levels of fire mortality and relatively low levels of recovery.
A drawback of their approach is that processes of fire spread used in the model are
simplistic and variations in flammability among functional types due to exogenous
influences of weather are not considered. The model also assumes that space is
equally available to all functional types: that is, the possibility of variations in
habitat availability among species is not considered. Fire is a particularly interest-
ing phenomenon from an evolutionary perspective because the probability of an
individual burning is a function not just of its own flammability but the flamma-
bility of the community and the extent of fire-prone landscape to collect lightning
ignitions and spread them. Where natural sources of ignition are limiting, the size
of fire-prone patches will determine fire probability for individuals.
There have been recent attempts to link observable patterns of variation in
diversity and resilience (i.e. resprouting capability) to resource and disturbance
syndromes (Bellingham & Sparrow
2000
; Clarke & Knox
2002
). Attribution of
causality is difficult because observed variations in diversity and resprouting often
correspond to confounded gradients of resources (i.e. water and soils, which relate
to productivity) and fire regimes (Clarke
et al.
2005
; Pausas & Bradstock
2007
).
Huston (
2003
) posed a model (Dynamic Equilibrium Model) that attempted to
explain how patterns of diversity may emerge from covarying syndromes of
productivity and disturbance frequency. A strength of Huston's model is that it
