Climatic anomalies drive wildfire occurrence and extent in semi-arid shrublands and woodlands of southwest Australia

Abstract

Variation in the frequency, extent and intensity of wildfires can drive changes in the composition, structure, diversity and functioning of ecosystems in fire‐prone regions worldwide. However, relationships between climatic variation and wildfire occurrence remain poorly understood in many fire‐prone regions. We investigated fire occurrence and extent across 15,500 km2 of semi‐arid southwest Australia in relation to inter‐annual and/or seasonal variation in regional climate and broad‐scale circulation patterns. Superposed epoch analysis (SEA) was used to determine whether wildfire occurrence was related to anomalously high or low regional rainfall or temperature. In particular, we tested if years of minor fire extent (i.e., <250 km2 burnt) and major fire extent (i.e., >1,000 km2 burnt) occurred under different climatic conditions. We also used SEA to determine if wildfires occurred during or following periods of extremes of drivers of regional climate, including the El Niño southern oscillation (ENSO), the Indian Ocean dipole, atmospheric blocking in the adjacent Southern Ocean, and the southern annular mode (SAM). Fire occurrence was linked to dry and hot conditions typically associated with the El Niño phase of ENSO, with few or no fires in years with cool and wet summers. However, major fire years tended to occur during drought conditions that followed wet and cool conditions in spring and summer of the preceding year. These wet and cool periods were typically associated with the presence of blocking highs in the Southern Ocean to the south of Western Australia. We hypothesise that high rainfall in spring and summer favours the growth of ephemeral plants while subsequent drought conditions promote fuel drying, resulting in more continuous and highly flammable fuel beds capable of sustaining larger fires. Regional climatic patterns are likely driven by interactions among the SAM, atmospheric blocking, and decaying tropical cyclones. As climatic extremes are expected to increase in intensity and frequency in the future, it is likely that the occurrence of extensive wildfires in semi‐arid southwest Australia will also increase, potentially driving changes in the distribution and composition of fire‐sensitive plant communities.

Ecological and climatic controls of modern wildfire activity patterns across southwestern South America

“…We found this same lagged relationship in the arid northern Chilean districts (1-AR, 2 CQ) where ENSO-related rainfall pulses produce large increases in grass cover and productivity (Gutiérrez et al 2000). A lagged influence of increased moisture influences on fire activity is a common pattern in fuel-limited semiarid regions (O'Donnell et al 2011). The last mode of variability (PC4) encompasses fire activity restricted to the Temperate/Sub-Antarctic region characterized by relatively evenly distributed precipitation throughout the year.…”

Scale‐dependent thresholds in the dominant controls of wildfire size in semi‐arid southwest Australia

“…Fire sizes are likely limited to less than approximately 41,000 ha by the spatial distribution of past fires and sparsely-vegetated woodlands. However, fires larger than this are generally driven by inter-annual variation from extremely wet periods to drought periods (O'Donnell et al 2011b), which are both forecast to increase in frequency and intensity in southwest Australia (Hughes 2003, Watterson et al 2007). An increase in the frequency and intensity of extreme wet periods and drought periods is likely to favor an increase in the frequency of large fires in the future.…”

“…v www.esajournals.org particular, fuels ,20 years of age tended to limit fire spread (x x ,, 1), while older fuels (.20 years) did not influence fire spread (x x ' 1; Table 5). Inter-annual variation in climate is important for driving large fires in the LJR (O'Donnell et al 2011b) by promoting fuel conditions capable of sustaining large fires. However, the finding that even large fires (i.e., .…”

“…If the influence of these factors on fire spread and size were constant in time, we might expect a single power law to describe the FSD of fires from semiarid southwest Australia relatively well across the entire range of fire sizes. However, given that extreme climatic events (i.e., unusually wet seasons and drought periods) are also strong drivers of fire occurrence in semi-arid southwest Australia (O'Donnell et al 2011b) and their occurrence frequency is highly variable on interannual time scales, we hypothesized that the FSD would consist of different domains for fires influenced by extreme climate events and those that occurred under milder climatic conditions. We consequently expected that a single power law would not describe the FSD well over the entire range of fire sizes, but instead expected that a segmented model with at least two scaling regions would describe the FSD better.…”

Interactions between Fine Wood Decomposition and Flammability

“…In semi-arid shrublands and woodlands, during rainless seasons, photodegradation instead of microbial decomposition could be a dominant control on above-ground litter decomposition due to the open canopy and intense solar radiation [21,[49][50][51]. Ultraviolet radiation can increase litter lignin loss [50] and this change in photodegraded litter may also greatly increase litter flammability and influence fire behaviors in those highly flammable systems.…”