Agriculture Reference
In-Depth Information
yrs of Australian Aboriginal occupation on fire regimes
is well described (Jones
1969
), although the impact of this burning on vegetation
changes is a matter of some debate. Charcoal deposition records show an increase
in fire activity and a shift to more sclerophyllous vegetation during the early
Aboriginal period for a number of sites (Singh
et al.
1981
; Luly
2001
). However,
vegetation shifts were not universally associated with occupied sites, shifts were
not always sustained, and climate has not been ruled out as a factor in both
increased fires and vegetation change (Lynch
et al.
2007
).
Humans also potentially influenced fire regimes indirectly through activities
that affected landscape fuel patterns. For example, in both Australia and North
America the Pleistocene colonization by early people contributed to the mass
extinction of giant herbivores (Martin
1984
; Owen-Smith
1987
; Roberts
et al.
2001
; Ripple & van Valkenburgh
2010
). As is illustrated by elephants today,
these large mammals had the potential for greatly altering vegetation patterns
and subsequent fire activity and thus it is to be expected that as landscape fuel
patterns changed, fire regimes followed suit. Loss of much of the large mammal
fauna in the Cape region of South Africa was accompanied by introduction of
livestock and thus the impacts on fuel structure may not have been as marked
(Hendey
1983
).
Global studies of Holocene fire activity, as reflected in charcoal deposition
records, find a strong link between climate and fire activity (Power
et al.
2008
;
Marlon
et al.
2009
; Daniau
et al.
2010
). Some of these reports have used this
correlation as evidence that humans played a minimal role in Holocene fires. In
some regions, though, this fire record is biased against detecting human impacts;
for example in California, charcoal deposition records come from high-elevation
lakes in landscapes more or less saturated with natural lightning-ignited fires,
whereas Holocene populations were distributed in the foothills and coastal plains
where natural ignition sources were highly limited (Keeley
2002b
).
Additionally, although many late Quaternary fire records are consistent with
climate change, in most records there are inconsistencies that are best interpreted
as evidence of early human use within a framework of changing climates evident
on several continents (Gavin
et al.
2007
; Markgraf
et al.
2007
; Black
et al.
2008
;
Gil-Romera
et al.
2009
). In Australia it is thought that the arrival of humans may
have partially decoupled fire activity from climate (Kershaw
et al.
2002
; Lynch
et al.
2007
), and this effect intensified with increased population growth in the late
Holocene (Hassell & Dodson
2003
; Enright & Thomas
2008
). Early Holocene
(8000 yrs BP) mediterranean people increased fire incidence to levels typical of
contemporary landscapes and this was associated with opening of closed-canopy
woodlands, although there is evidence that changes were exacerbated during
wetter periods (Colombaroli
et al.
2008
). In short, there is no reason to think of
climate and humans as representing mutually exclusive factors determining fire
activity and there is no clearer demonstration of this than recognition that
although contemporary burning patterns are markedly influenced by humans,
climatic signals are still evident in annual changes in fire activity.
The impact of 45 000
รพ
