Landscape variation in plant leaf flammability is driven by leaf traits responding to environmental gradients

Abstract

Abstract. Landscape differences in environmental conditions select for divergences among plant species in strategically important leaf traits such as leaf mass per area (LMA) and leaf area (LA). Interspecific variation in some of these same leaf traits has been associated to varying degrees with differences among species in leaf flammability, including the attributes ignitibility, sustainability, and combustibility. Yet, how environmentally selected variation in leaf traits drives variation in leaf flammability at landscape scales remains largely unknown. Here, we compared leaf traits and flammability attributes between species of sheltered forest vegetation (low light, moist habitat) and plant species of exposed woodland vegetation (high light, dry habitat) in a fire-prone landscape of south-eastern Australia. We found that leaves of sheltered forest species were significantly more flammable via both higher ignitibility and combustibility compared with exposed woodland species. These significant differences in leaf ignitibility and combustibility were underpinned by sheltered forest species having leaves with significantly larger LA and lower LMA compared with exposed woodland species. Further, multiple regression analyses revealed that both LA and LMA were significantly and uniquely related to faster time to ignition (TTI; ignitibility) and higher mean mass loss rate (combustibility). Most notably, although significantly higher fuel moisture content (FMC) of leaves of sheltered forest species significantly lengthened TTI, the lower LMA of these species played a more critical role in reducing TTI, with low LMA explaining more unique variation (partial r 2 = 0.78) in high leaf ignitibility than low FMC (partial r 2 = 0.49). Our findings provide the first evidence that landscape-scale variation in leaf flammability is tightly coordinated with the primary strategic response of the leaf traits LMA and LA to an environmental gradient. Furthermore, projections for increasing wildfire frequency and intensity in the region will likely allow wildfires to overcome the once protective nature provided by topography to sheltered forest vegetation, which means that higher leaf flammability in sheltered forest species has the potential to exacerbate the effects of changing weather conditions to place sheltered forest habitat, their plants, and their animals, at even higher risk of catastrophic wildfire.

Shoot flammability is decoupled from leaf flammability, but controlled by leaf functional traits

“…Another possible explanation for the decoupling of the leaf‐ and shoot‐level flammability relates to the measurement methods. Leaf flammability was calculated by burning a single leaf in a muffle furnace at an oven temperature of 400–430°C, without applying any ignition source (Mason et al, ), as has been done in several other studies (Krix & Murray, ; Krix, Phillips, & Murray, ; Montgomery & Cheo, ; Murray et al, ). Exposing the leaf to such high temperatures in the absence of a flame is likely to remove any volatile oils present in the leaf before they ignite, and thus overestimate the ignition time of a leaf.…”

“…Flammability can influence the fitness, survival, reproduction and distribution of plants, as seen by the evolution of flammability strategies in fire‐prone environments (Bond & Midgley, ; Mutch, ; Pausas et al, ). Recently, there have been renewed efforts in plant flammability research to tackle fundamental topics such as the evolution of plant flammability (Archibald et al, ; Battersby, Wilmshurst, Curran, McGlone, & Perry, ; Pausas et al, ), and in an applied context to provide information useful for fire management (Krix & Murray, ; Wyse et al, ; Zylstra, ), building on past work in such areas (Bond & Midgley, ; Gill, ; Kerr, Schwilk, Bergman, & Feldman, ; Mutch, ). However, there is still considerable debate on how best to measure plant flammability and therefore, how to appropriately quantify this important plant trait (Schwilk, ; Varner, Kane, Kreye, & Engber, ).…”

Shoot‐level flammability across the Dracophyllum (Ericaceae) phylogeny: evidence for flammability being an emergent property in a land with little fire

“…Plant flammability is a compound trait emerging from the chemical and physical characteristics of a plant (Schwilk, 2015; Pausas et al ., 2017). Different plant species and individuals of the same species growing in different habitats vary in their flammability (Pausas et al ., 2012; Murray et al ., 2013; Wyse et al ., 2016; Krix & Murray, 2018). Investigating the evolution of plant flammability can help us better understand the interaction between fire and plants, and allow us to better prepare for a warmer world, where fire risk may be higher in many regions (Doerr & Santín, 2016).…”

What drives grassland‐forest boundaries? Assessing fire and frost effects on tree seedling survival and architecture

“…(Whether such frost adaptations have also evolved in areas with short frost events with large diurnal temperature variations, as found in subtropical savannas, has not been investigated). Apart from traits already mentioned such as thick bark (Schafer, Breslow, Hohmann, & Hoffmann, 2015), well‐protected buds (Charles‐Dominique et al., 2015), and belowground storage organs (Maurin et al., 2014), trees with smaller leaf area possess lower time to ignition, burn duration, and mean mass loss rate (Krix & Murray, 2018), which may decrease fire damage. Frost‐hardy plants tend to have smaller, thicker leaves (Palta & Li, 1979) with decreased leaf wettability (through waxy cuticles or pubescence) (Aryal & Neuner, 2010).…”