Wildfire refugia in forests: Severe fire weather and drought mute the influence of topography and fuel age

Abstract

Wildfire refugia (unburnt patches within large wildfires) are important for the persistence of fire‐sensitive species across forested landscapes globally. A key challenge is to identify the factors that determine the distribution of fire refugia across space and time. In particular, determining the relative influence of climatic and landscape factors is important in order to understand likely changes in the distribution of wildfire refugia under future climates. Here, we examine the relative effect of weather (i.e. fire weather, drought severity) and landscape features (i.e. topography, fuel age, vegetation type) on the occurrence of fire refugia across 26 large wildfires in south‐eastern Australia. Fire weather and drought severity were the primary drivers of the occurrence of fire refugia, moderating the effect of landscape attributes. Unburnt patches rarely occurred under ‘severe’ fire weather, irrespective of drought severity, topography, fuels or vegetation community. The influence of drought severity and landscape factors played out most strongly under ‘moderate’ fire weather. In mesic forests, fire refugia were linked to variables that affect fuel moisture, whereby the occurrence of unburnt patches decreased with increasing drought conditions and were associated with more mesic topographic locations (i.e. gullies, pole‐facing aspects) and vegetation communities (i.e. closed‐forest). In dry forest, the occurrence of refugia was responsive to fuel age, being associated with recently burnt areas (<5 years since fire). Overall, these results show that increased severity of fire weather and increased drought conditions, both predicted under future climate scenarios, are likely to lead to a reduction of wildfire refugia across forests of southern Australia. Protection of topographic areas able to provide long‐term fire refugia will be an important step towards maintaining the ecological integrity of forests under future climate change.

Still standing: Recent patterns of post-fire conifer refugia in ponderosa pine-dominated forests of the Colorado Front Range

“…Locations of fire refugia are influenced by both predictable and stochastic factors, creating both persistent and ephemeral refugia [19]. Local topography, such as cold air drainages, rugged terrain, and mesic topographies have been shown to increase the likelihood of fire refugia [29][30][31]. Edaphic factors, such as clay and sand content of soils, are known to support variable historic tree densities in the southwestern ponderosa pine forests and, therefore, we hypothesized may also promote sites of fire refugia [32].…”

“…weather [30,31]. Identifying sites with an increased chance of forested fire refugia may help the design of fuel treatments aimed at increasing forest resilience to wildfire under moderate and severe fire weather [33].…”

Causes and consequences of eastern Australia's 2019–20 season of mega‐fires

“…Drought also triggers leaf senescence and shedding in eucalypt forests, resulting in an increase in surface fine fuel loads, which can increase the rate of fire spread (Ruthrof et al, 2016). Importantly, during drought and extreme fire weather, normally damp gullies and rainforest patches are unlikely to provide their usual impediment to the spread of fire across the landscape (Collins, Bennett, Leonard, & Penman, 2019). Eastern Australia has been experiencing severe drought in the lead up to and during the current fire season.…”

“…Fire-sensitive forest communities, such as remnant rainforests, have persisted in a fire-prone Eucalyptus matrix over geological timeframes within areas where high fuel moisture content is usually maintained throughout the fire season (Bowman, 2000). Extreme drought can dry fuels below critical thresholds within these fire refugia, allowing the encroachment of wildfire into these less fire-resilient vegetation communities (Collins et al, 2019). The 2019-20 fires have burnt significant areas of the Gondwana Rainforests, Australia World Heritage Area (potentially up to ~50% of its current distribution), posing a major threat to this globally significant vegetation.…”

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

“…Finally, another factor that can increase the vulnerability of forest ecosystems in a scenario of reduced precipitation is the increase in the frequency and intensity of fire events. In fact, due to the increase in litter production and reduction in biomass humidity (Collins, Bennett, Leonard, & Penman, 2019;Duursma et al, 2016), the effect of drought on vegetation can significantly increase the fire spread and intensity, even in those regions that would be unlike to be burned (e.g., mesic sites and poleward-facing slopes; Collins et al, 2019;Krawchuk et al, 2016;Leonard, Bennett, & Clarke, 2014). It is important to note that although plants may have a set of traits and strategies to allow them to survive fire episodes and/or recompose burned areas (Hoffmann et al, 2009;Pellegrini, Franco, & Hoffmann, 2016;Schafer, Breslow, Hohmann, & Hoffmann, 2015), the increase in drought intensity can significantly reduce the resilience of tree species from fire-prone habitats (Pratt, Jacobsen, Ramirez, & Helms, 2014).…”