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frequently, and forests were dominated by Douglas fir, larch, and pine (
Pseudotsuga
,
Larix
,
and
Pinus
). Fires became less frequent and forest more closed from about 4,000 years ago,
when conditions became cooler, until a period of more frequent fire and open forest in the
MWP. From the onset of the LIA, about 700 years ago, wet, closed forest dominated, but cur-
rent trends of increasing temperature might increase the frequency and severity of fire, creat-
ing more open forest canopies and heterogeneous landscapes (Hallett and Walker 2000).
Not all ecosystems burned more frequently in the MWP, however. Fire regimes that are
limited by fire conditions, rather than biomass (see Figure 4.3), respond to changes in rainfall
rather than temperature (Murphy et al. 2011). For example, fossil pollen records from nor-
thern Alaska show that boreal forests, dominated by black spruce (
Picea mariana
) were
prevalent between 1280 and 1490 ce (the MWP), but forest cover declined during the LIA,
when temperature decreased by more than 1.7 °C. Though there may have been some lethal
frost damage, charcoal records suggest that vegetation change was most likely related to
changes in moisture balance and fire; in this ecosystem, fuel would have accumulated in the
cold, dry conditions of the LIA, resulting in more frequent fires in colder temperatures. As a
result, cold- and disturbance-adapted species like fireweed (
Epilobium
) and alder (
Alnus
)
expanded at the expense of spruce forest, leading to a much more open landscape, character-
ized by frequent fires. The boreal forests of today re-established about 150 years ago, and
expanded into tundra communities after the LIA (Tinner et al. 2008).
Regional climate systems, topography, and hydrology affect rainfall patterns and vegeta-
tion distribution, both responding to, and driving patterns of, fire regimes. Although we can
draw broad lessons from looking at the relationship between climate and fire at global scales,
it is essential to look at regional, landscape, and local scales when understanding and pre-
dicting changes in fire regimes in a particular ecosystem (Gavin et al. 2007, Whitlock et al.
2010). Disturbance and landscape fragmentation at local scales affect the frequency of igni-
tion and the amount, continuity and type of fuel available. For example, hurricanes may fell
swathes of trees, creating dead fuel wood that is more likely to burn. Logging and grazing also
affect fuel loads and connectivity, and human ignition and suppression is likely to play out at
local-landscape scales (Long 2009, Marlon et al. 2009, Archibald et al. 2012). There are differ-
ences in resilience and sensitivity to climate change and fire, influenced by substrate, water
balance, and topography (Oswald et al. 2003, Lloyd 2005, Lloyd and Bunn 2007, Williams and
Jackson 2007, Girardin et al. 2011). The understanding of fire regimes therefore requires a
multiscale perspective, whereby the effects of fire management at local-landscape scales,
and the feedbacks between ecology and fire, can be set in the context of regional and global
drivers (Gavin et al. 2007, Whitlock et al. 2010).
Plants are not passengers in the fire-climate-vegetation nexus—they drive as well as respond
to fire regimes. Modelling experiments show that if fire could be switched off, then forests
would occupy a much larger proportion of the Earth's surface (Bond et al. 2005). This is
because many subtropical areas are wet enough to support forests, but are fire-prone, which
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