1 Introduction
There is growing consensus that anthropogenic climate change is causing
longer and more intense fire seasons (Balch et al., 2022; Ellis et al.,
2022; Jain et al., 2022), with a corresponding increase in the number of
wildfires exhibiting extreme behaviour with adverse socio-ecological and
-economic impacts (Bowman et al., 2017; Duane et al., 2021). Climate
projections suggest this trend is likely to continue throughout the
21st century (Flannigan et al., 2013; Wotton et al.,
2017; Abatzoglou et al., 2019; Abatzoglou et al., 2021). Western North
American forests, for example, have shown a rapid increase in annual
burnt area in the last half century (Westerling, 2016; Williams et al.,
2019) – albeit this trend is partially driven by a long history of
deleterious land-use practices such as fire suppression (Abatzoglou et
al., 2018). Current climatological and ecological modelling suggests the
occurrence of catastrophic megafire events in the USA, such as those
seen in the 2020 fire season (Higuera and Abatzoglou, 2020), will
continue to increase across affected forests due to anthropogenic
climate change (Bowman et al., 2020). The potential severity of these
events, however, may be reduced by fuel management intervention that
restores pre-colonial fire regimes (Abatzoglou et al., 2021; Hessburg et
al., 2021). A similar increase in extreme fires is also evident in
southeastern Australia (Sharples et al., 2016; Abram et al., 2021),
where trends in both drought and extreme fire weather combined with the
loss of Indigenous fire management are believed to have contributed to
the catastrophic 2019 – 2020 fire season dubbed the Australian Black
Summer (Boer et al., 2020; Nolan et al., 2020; Canadell et al., 2021;
Collins et al., 2021; van Oldenborgh et al., 2021; Mariani et al.,
2022).
The fuel moisture content of live and dead fine fuels is inherently
linked to the behavioural and ignition determinants of wildfire driving
these trends across multiple spatial and temporal scales (Bradstock,
2010; Murphy et al., 2013). Dead fine fuel moisture content (DFFMC) has
specifically been identified as a key switch determining the spread of
vegetation fire. Dead surface fuels such as leaves, bark, twigs, and
grass are capable of rapidly equilibrating with atmospheric humidity in
under 10 hours, and consequently represent the most ignitable component
of vegetation. As the ignition of these fuels can then provide the
energy to ignite both larger dead fuel components and live fuels, DFFMC
is therefore a key determinant of overall wildfire occurrence,
behaviour, and greater pyrogeographic patterns. To track DFFMC and
associated potential fire danger, DFFMC can be measured directly in the
field (e.g, Bowman et al., 2020) or estimated using meteorological
indices such as vapour pressure deficit (VPD) or the Canadian Fire
Weather Index system (FWI: Van Wagner, 1987). Accordingly, derived or
measured DFFMC functions as a prominent proxy for an ecoregion’s
potential to ignite and sustain wildfire, accurately reflecting the
surface litter most prone to ignition and fire spread (Murphy et al.,
2013; Flannigan et al., 2016; Kelley et al., 2019). DFFMC is also of
fundamental importance in shaping spatiotemporal patterns of landscape
fires from local to global scales. For example, moisture differences are
understood to control the spread of fires across different vegetation
types such as savannas and rainforests (Little et al., 2012).
Prior research has identified specific values of DFFMC as marking the
upper and lower bounds of wildfire potential (Fernandes et al., 2008;
Wotton, 2008; Slijepcevic et al., 2015; Flannigan et al., 2016; Nolan et
al., 2016; Boer et al., 2017; Filkov et al., 2019; Clarke et al., 2022).
For instance, FWI- or VPD-derived DFFMC values of between 8 and 12%
have been linked to uncontrollable wildfire (Slijepcevic et al., 2015;
Flannigan et al., 2016; Nolan et al., 2016; Boer et al., 2017; Filkov et
al., 2019), while values of up to 30% have been found to represent the
upper limits of vegetation fire (Fernandes et al., 2008; Nolan et al.,
2016) under current atmospheric oxygen concentrations due to carbon
fibre saturation (Luke and McArthur, 1978; Scott and Glasspool, 2006).
Global and regional studies frequently estimate DFFMC within the
Canadian FWI system due to its ease of calculation and interpretation
(Field, 2020); these studies have often applied single value thresholds
to assess patterns and likely trends in global fire potential. For
instance, an established 10% threshold (Wotton, 2008; Flannigan et al.,
2016) was used to infer that the proportion of fire seasons falling
below this critical threshold had significantly increased between 1979
and 2019 for most ecoregions worldwide (Ellis et al., 2022). Such a
shift is likely to affect established fire regimes, particularly for
more productive or wet ecoregions.
There are, however, inherent problems with applying single-value
thresholds like this to compare different vegetation types and climate
domains. Forest ecosystems have evolved with fire differently in
response to naturally occurring environmental and biological constraints
like soil fertility, climate, and the local biota. It is thus unlikely
that a single DFFMC value could possibly reflect the critical point of
flammability across grossly different ecoregions. Furthermore, fire
danger indices are developed for specific, regional forest types, with
the FWI having been developed based on a mature Pinus banksianaLamb. and Pinus contorta landscape in southern Canada (Van
Wagner, 1987; Wotton, 2008). Despite the FWI’s established global
applicability, there is no reason to assume that the vegetation in these
forests is globally representative, and its use may consequently be
inappropriate for assessing fire potential or true fuel moisture in
different vegetation types (e.g., Aguado et al., 2007; Wotton and
Beverly, 2007; Schunk et al., 2017). This raises a question about how
fire weather indices can best be used to retain local relevance by
identifying ecoregion-specific thresholds controlling the switch from a
non-flammable to a flammable state (e.g, Clarke et al., 2022).
Building on these studies, we seek to answer the question of whether a
universal fuel moisture threshold exists as a control on fire, while
advancing our understanding of how fuel moisture acts as a switch for
landscape fire both regionally and globally. Using a comprehensive
dataset of over 700 hierarchically defined ecoregions, we identify the
ecoregion flammability thresholds (EFTs) as the FWI-derived DFFMC value
most associated with rapid step changes in remotely sensed fire activity
records. Our methods (Figure 1) bypass the issues inherent with applying
fire weather indices globally by localising the relationship between
DFFMC and fire to each individual ecoregion with associated satellite
fire records. We then use a combination of nonmetric multidimensional
scaling and inferential statistical modelling to verify the veracity of
EFTs as a biological, localised mechanism and examine both the
biogeographic and climate characteristics constraining our identified
EFTs as critical fuel-fire switches.