Background
A landscape trap has developed in forests dominated by Mountain Ash (Eucalyptus regnans) and Alpine Ash (Eucalyptus delegatensis, collectively termed montane ash forests) in Victoria, Australia. These forests are often even-aged, having regenerated after stand-replacing wildfires or clearcutting. Fire is essential for natural regeneration in these forests, with the mean fire interval being 75-150 years (McCarthy et al. 1999). These obligate seeder tree species are often killed in high-severity fires and regenerating trees do not produce viable seed until 20-30 years of age (Von Takach Dukai et al. 2018). If repeated high-severity fires were to occur at intervals <30 years, these forests would be replaced by non-ash forest vegetation like Acacia spp. woodlands and grasslands (Photo 1; Lindenmayer et al. 2011) – the “interval squeeze” problem (Enright et al. 2015). This would have major impacts on carbon storage, water production, and biodiversity conservation (Lindenmayer et al. 2011).
Analyses
Throughout this article we refer to several recent empirical (Lindenmayer et al. 2011) studies that provide evidence that the pre-conditions of a landscape trap have been met in the montane ash forests of Victoria. Specifically, these studies quantified: (1) stand-age-fire severity relationships (Taylor et al. 2014, Taylor et al. 2020), (2) the extent of old-growth forests (Lindenmayer and Taylor 2020a) and, (3) the probability of forests reaching maturation (Enright et al. 2015, Von Takach Dukai et al. 2018). Stand-age-fire severity relationships and spatial dependence in fire severity in montane ash forests were quantified using a statistical analysis of fire damage at 9934 sites, following the 2009 wildfires in Victoria (Taylor et al. 2014, Taylor et al. 2020). Such relationships also were quantified for the 2019-20 wildfires by analyzing 33,850 grid points spaced at 500 m intervals across a 988,854 ha section of the fire footprint (Lindenmayer et al. 2021; Appendix 1). In addition, data layers sourced from the Victorian Government were used to map temporal changes in the extent of old-growth in the Wet and Damp Ecological Vegetation Class [EVC] (which encompass Mountain Ash and Alpine Ash forests) from 1995 to 2020 (Lindenmayer and Taylor 2020a) as well as the frequency of fire in different EVCs (Lindenmayer and Taylor 2020b). Finally, work by (Cary et al. 2021) used fire regime distribution models to compute the probability of forests reaching old-growth (180 years), sawlog age (80 years), and canopy maturation (for seed production, ~20 years; Appendix 2).
Necessary pre-conditions for a landscape trap
Three inter-related pre-conditions drive landscape trap development. These are: (1) Stand age-fire severity relationships in which, relative to old-growth stands, young forests are more flammable and are at significantly greater risk of burning at high-severity (which kills entire stands of overstorey trees). (2) Widespread young flammable forests (and rarity of less fire-prone, old-growth forests), leading to high levels of spatial contagion in elevated, high-severity fire. And, (3) Repeated fire at short intervals which can, in turn, interact with key life history attributes such as seed production to reduce or eliminate natural stand regeneration. Below, we present evidence that montane ash forests meet these necessary pre-conditions for a landscape trap. Importantly, the simultaneous expression of all three pre-conditions can be critical for a landscape trap to be sprung (see Fig. 1).
Pre-condition #1 –Stand age-flammability relationships
Climate and extreme fire weather are key drivers of fire ignition, behavior and frequency (Jones et al. 2020), but forest attributes like stand age and composition also affect fire severity (Zylstra et al. 2016, Zald and Dunn 2017, Tiribelli et al. 2018). A pre-condition for a landscape trap is that young, forest stands exhibit markedly higher levels of flammability relative to older stands – a phenomenon observed in several forest types globally (e.g. Taylor et al. 2014, Zald and Dunn 2017, Tiribelli et al. 2018). This may be explained by several inter-related mechanisms, such as crown-density, plant architecture and specific plant-traits within species or groups of species (Zylstra et al. 2016, Pausas et al. 2017). For instance, some plant life-forms that occur at high densities in young montane-ash forests (Bowd et al. 2021) have been associated with an increase in flammability (e.g. some graminoids, Acacia and shrub species; Zylstra et al. 2016, Tumino et al. 2019, Cadiz et al. 2020).
Analysis of wildfires in montane ash forests in 2009 contained evidence of a left-skewed, non-linear, relationship between stand age and fire severity (as reflected by the probability of a crown burn, Fig. 2a; Taylor et al. 2014). This work showed that (after controlling for fire weather), young montane ash forests aged ~10-40 years were subject to elevated fire severity, with the lowest levels of severity in old-growth stands (exceeding 120+ years; Taylor et al. 2014). There also were high levels of spatial dependence in wildfires burning in young-forest dominated landscapes in the 2009 fire (Taylor et al. 2020). That is, young stands close together were significantly more likely to burn (and burn at similar levels of fire severity) than those located a long way apart. Notably, a study by Cruz et al. (2012) of fire behavior showed the 2009 conflagration burnt as a rapidly-spreading crown fire through young forest until it encountered old montane ash forest, where fire severity decreased.
A second study of stand-age fire severity relationships following the 2019-20 fires in the Wet and Damp EVC (which encompass Mountain Ash and Alpine Ash forests) in north-eastern Victoria (Appendix 1) showed a strong negative polynomial relationship between stand age and fire severity (Fig. 2b; Lindenmayer et al. 2021). Hence, as in work completed after the 2009 fire (Fig. 2a; Taylor et al. 2014), young Wet and Damp forest exhibited higher levels of flammability relative to older stands (Fig. 2b; Lindenmayer et al. 2021). Similar to the 2009 fire, there also was a high level of spatial dependence between burnt areas in 2019-20 wildfires in these forests (Lindenmayer et al. 2021).
Whilst the evidence that young montane ash forests are susceptible to high severity wildfire is compelling (Taylor et al. 2014, Taylor and Lindenmayer 2020, Lindenmayer et al. 2021), other kinds of evidence suggest that older forests are more likely to experience lower severity fire (Lindenmayer et al. 1999). For example, old growth montane ash stands are almost never comprised of a single age cohort of overstory trees, but typically support multiple age classes (Lindenmayer et al. 2000), with many of these trees supporting fire scars (Lindenmayer et al. 1991). This suggests that old growth stands can experience multiple lower severity wildfires (Banks 1993, Lindenmayer et al. 1999) that does not kill all of the large old trees they support (Lindenmayer and McCarthy 1998). Moreover, these forests are typically characterized by a lower abundance of species associated with an increase in flammability (e.g graminoids, shrubs, Acacia), and a higher occurrence of potentially less-flammable plant species including tree-ferns (Blair et al. 2016, Cawson et al. 2018).
Pre-condition #2 – Extensive young fire-prone forest
A second pre-condition for a landscape trap is that an ecosystem must be dominated by young forest (with elevated flammability and high risk of reburning, Fig. 2). Spatial analyses of forest cover and fire frequency data indicate that old-growth in the Wet/Damp EVC is now very rare across Victoria due to recurrent fire and widespread logging (Lindenmayer and Taylor 2020a). Approximately 85% of this EVC that was formerly old-growth in 1995 has been heavily disturbed by either fire or logging in the past 25 years (Lindenmayer and Taylor 2020a). One of the key regions for this EVC is the Central Highlands of Victoria, where only 1.16% of Mountain Ash forest is now old-growth or 1/30th-1/60th of what it was historically. Similarly, 0.47% of Alpine Ash forest in the region is old-growth, although its historical extent is unknown. Importantly, not only is the extent of loss of old-growth pronounced, but remaining old-growth patches are small and fragmented (Fig. 2). For example, the remaining 1886 ha of old-growth Mountain Ash forest (of a total 171 400 ha) is distributed across 147 individual patches.
Therefore, historical patterns of forest cover at the landscape scale, where previously small patches of regrowing forest that were once embedded within a matrix of extensive old-growth forest, have now been reversed (Photo 2). That is, old-growth forests are now small patches in a matrix of extensive young, regrowth forest. Indeed, approximately 99% of montane ash ecosystem in regions such as the Central Highlands of Victoria is young forest. Moreover, the flammability of extensive areas of young forest (see pre-condition #1), coupled with the spatial contagion of such fires, means that high-severity fires can destroy adjacent small patches of old-growth forest (Photo 3). Indeed, this problem is underscored by the fact that in wood production montane ash forests such as the Central Highlands region of Victoria, approximately 70% of remaining old-growth patches are less than 200 m of a recently burned or logged forest (Taylor and Lindenmayer 2020). However, the size of old-growth patches in which fire behavior is altered and fire severity is depressed relative to surrounding young stands remains unknown.
Pre-condition #3 – Repeated fire at short intervals
Consistent with pre-condition #3 for the development of a landscape trap, the incidence and extent of wildfire has been increasing significantly in the past 25 years (Lindenmayer and Taylor 2020b), with Wet and Damp EVC heavily impacted (Bowman et al. 2014). Some areas have burnt up to four times since 1995, with the inter-fire interval as short as six years in some places (Lindenmayer and Taylor 2020b), whereas the natural fire return interval should be 75-150 years (McCarthy et al. 1999). In the case of the 155,055 ha of Alpine Ash mapped in State Forests by the Victorian Government, analyses of disturbance data (Department of Sustainability and Environment 2007, Department of Environment and Land and Water Planning 2021) shows that 84% of this forest type was burned between 1980-2020, with 34% burned by two or more fires (Appendix 3). Some regions dominated by Alpine Ash forest have been particularly heavily affected. For example, three fires burned Alpine Ash forests in the Carey River State Forest (located in Central Victoria) between 1998 and 2019 (Appendix 3). Following these fires, areas of Alpine Ash forest were subsequently clearcut (Appendix 3), resulting in four major disturbance events in these forests over a 20-year period.
Feedbacks and interactions between pre-conditions
Interactions between pre-conditions can be important for reinforcing the development of a landscape trap (Fig. 1 and Fig. 4). For example, repeated fire at short-intervals precludes the recruitment of new stands of old-growth (pre-condition #2), while expanding the amount of flammable young fire-prone forest (pre-condition #1). Another feedback from interactions between pre-conditions is a rapid decline in biological legacies like large old dead trees. These trees are critical nest sites for cavity-dependent biota (Lindenmayer et al. 2017) and are most prevalent in old-growth forest (Lindenmayer et al. 2000). They are created when fires burn old-growth stands; with such trees then persisting for several decades in regenerating stands, facilitating colonization by cavity-dependent wildlife (Lindenmayer et al. 2019). Large dead trees are not produced when young stands are burnt repeatedly by high-severity fire, thereby eliminating suitable trees for occupancy by many taxa, including some of conservation concern (Lindenmayer et al. 2019). Hence, a landscape trap can trigger major declines in biodiversity.
Post-fire (salvage) logging is another important kind of interaction between natural disturbance and human disturbance in montane ash forests (Lindenmayer et al. 2018). It can impair plant and animal species recovery, disrupt plant-soil-microbial feedbacks (Bowd et al. 2021) and result in major losses of key biological legacies (such as large old trees; Bowd et al. 2018, Lindenmayer et al. 2019). Moreover, salvage logging also interacts with pre-condition #1 resulting in high-densities of flammable regrowth vegetation, with little structural diversity (Bowd et al. 2018, Lindenmayer et al. 2019). Given that salvage logging will likely increase concurrently with increases in high-severity wildfire, it may reinforce the development of a landscape trap (see Fig. 1).
A further problem with landscape traps is the interaction between recurrent disturbance and critical life history attributes of dominant trees, like time to sexual maturity. Young montane ash trees (<21 years of age) do not produce viable seed crops to facilitate stand regeneration following fire (Von Takach Dukai et al. 2018). The lowest rates of regeneration success are where young stands have been burnt (Smith et al. 2016). Post-fire natural regeneration failure in montane ash forests is now widespread across Victoria (http://tiny.cc/u490tz). Efforts are underway to gather seed in an attempt to revegetate large areas of young forest that was burnt after recent wildfires but failed to recover (see Fig. 2c). When artificial seeding fails, montane ash stands will collapse (see Fig. 1) and will likely be replaced by Acacia-dominated woodland (Photo 1a and Photo 1b) because Acacia produces large, long-lived stores of viable seed at an earlier age than eucalypts (Passos et al. 2017). Notably, in the past, extensive areas of montane ash forest spanning ~ 10 000 ha that were subject to recurrent wildfires suffered ecosystem collapse and became dominated by Acacia spp. and grasslands, with the area then artificially regenerated, in part, with non-native seed stock (see Photo 1; McKimm and Flinn 1979).
Management implications
Our initial work on landscape traps (Lindenmayer et al. 2011) was a theoretical conceptualization of how they might develop and manifest. Since that time, new insights on stand-age and flammability relationships, fire frequency, and other empirical studies indicate that the landscape trap in Victoria’s montane ash forests has been sprung. The trap has significant resource management and conservation implications. First, the optimal age for trees to become sawlogs in these forests is >80 years, but high frequency of reburning means that stands have ~ 80% probability of being burnt before this age (Cary et al. 2021; Appendix 2). Logging industries therefore have highly uncertain access to millable timber. Second, the rarity of old-growth forest (Lindenmayer and Taylor 2020a) will impair ecosystem service provision (e.g. water yield and carbon storage; Keith et al. 2017), and erode habitat suitability for an array of threatened species (Lindenmayer et al. 2017). Moreover, ongoing climate change will increase future wildfire risk (Cary et al. 2012, Jones et al. 2020, van Oldenborgh et al. 2021) including in extensive areas of already young, highly flammable forest where fire severity is highest (Taylor et al. 2014), natural regeneration is lowest (Smith et al. 2014), and biodiversity and ecosystems service values are impaired (Keith et al. 2017). Finally, the giant old trees (up to 100m in height; Photo 4) characterizing montane ash forests (Lindenmayer et al. 2000) may become a thing of the past as the landscape trap continues to preclude the recruitment of old-growth forest.
“Unspringing” the landscape trap – strategies for ecosystem restoration
Substantial policy and management interventions will be required to reverse the problems that have arisen from the development of a landscape trap in montane ash forests. First, these ecosystems have experienced a large amount of recurrent disturbance at frequent intervals. It is therefore important to reduce the number of stressors in montane ash ecosystems, particularly ones which are relatively straight forward to manage such as the extent and amount of logging. This will reduce the rate at which further areas of young, fire-prone forest is added to already highly fire-prone and extensively fragmented ecosystems (Taylor and Lindenmayer 2020). It also will reduce the risk of depleting seed sources needed to facilitate reforestation in the event of future fires. A critical component of reducing the number of stressors in montane ash ecosystems, will be to recognize that some widely recommended strategies to reduce fire severity such as commercial thinning (e.g. Volkova et al. 2017) may in fact have limited effectiveness, and can sometimes, elevate the severity of subsequent wildfires (Taylor et al. 2020). Second, (and related to the first recommendation), we suggest that new policies must attempt to expand the currently limited old-growth estate. This is an enormous challenge given changes toward a warmer and increasing dry climate in the south-eastern Australia (Cary et al. 2012). However, such an approach may have an increased chance of success if it is focused on more sheltered parts of landscapes where fire severity have generally been lower in the past (Lindenmayer et al. 1999) and ecologically mature forests have the highest probability of developing and persisting (Mackey et al. 2002). Third, given that montane ash ecosystems have been exposed to so many wildfires, often of very high severity (see Collins et al. 2021), new approaches, including the use of detection and suppression technologies like drone detection and unmanned autonomous vehicles, may be important to rapidly extinguish ignitions before they become major conflagrations (Roldán-Gómez et al. 2021).
Concluding comments
Feedbacks between natural and human disturbances can produce various kinds of traps in ecosystems. An important kind of trap is a “landscape trap” in which natural and human disturbances such as fire and logging produce young, flammable wet forests at increased risk of repeated re-burning at high-severity, thereby precluding them from growing to ecological maturity. Since initial theoretical work on landscape traps (Lindenmayer et al. 2011), further empirical, field-based evidence has emerged that reinforces the original conceptual proposition for their development. We have presented new empirical evidence that shows that a landscape trap has been sprung in the tall, wet, montane ash forests in south-eastern Australia forests – that support the tallest flowering trees on earth. The trap has been sprung because three key pre-conditions for its development have been met. These include the prevalence of widespread flammable young forest and repeated high-severity fire, which interact to place the ecosystem at high risk of collapse. Historical patterns of forest cover have now been altered from widespread old-growth with small patches of regrowth embedded within it, to the reverse. Landscape traps such as the one that has been sprung in montane ash forests have significant ecological and resource management implications. Key restoration interventions such as strategic expansion to reduce the number of disturbance stressors in montane ash forests and significantly increase the extent of old growth forest will be required to reverse the problems associated with development of a landscape trap in montane ash forests.
Finally, we argue is critical that an examination be conducted globally of the risk of landscape traps developing in other ecosystems (e.g. Mason et al. 2018) including those at risk of regeneration failure under short fire return intervals (e.g. North-American subalpine lodgepole pine forests and Canadian conifer forests). This examination should include forests subject to multiple stand-replacing disturbances that can interact and influence the extent and frequency of the other disturbances, as well as interact with life-history attributes, thereby compounding ecological impacts (e.g. Thompson et al. 2007, Zald and Dunn 2017, Tiribelli et al. 2018).
