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

Daniel W Krix,Brad R Murray

doi:10.1002/ecs2.2093

Abstract

AbstractLandscape 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 r2 = 0.78) in high leaf ignitibility than low FMC (partial r2 = 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.

Highlights

Research in trait-based ecology has identified fundamental patterns of variation in plant traits along environmental gradients (Reich et al 1992, 2003, Wright et al 2004, 2010, McGill et al 2006, Poorter et al 2009)
We examine interspecific relationships between the three leaf traits and the three flammability attributes to test the predictions that high ignitibility is related to large leaf area (LA) and low leaf mass per area (LMA) found in gully species, and that overall these two leaf traits counteract the buffering effect of high fuel moisture content (FMC) in gully species; and that high sustainability and high combustibility are related to large LA and high LMA in gully species
Leaf traits in relation to habitat and stratum We found that LA was significantly larger in sheltered forest species compared with exposed woodland species (GLM: F1,88 = 11.6, P = 0.0009; phylogenetic generalized least squares (PGLS) [Brownian motion (BM)]: F1,88 = 46.7, P < 0.0001) and in the overstory compared with the understory stratum in both habitats (GLM: F1,88 = 15.2, P = 0.0002; PGLS [BM]: F1,88 = 65.7, P < 0.0001; Fig. 2a), with no significant habitat 9 stratum interaction (GLM: F1,88 = 2.8, P = 0.1; PGLS [BM]: F1,88 = 3.1, P = 0.08)

Summary

Introduction

Research in trait-based ecology has identified fundamental patterns of variation in plant traits along environmental gradients (Reich et al 1992, 2003, Wright et al 2004, 2010, McGill et al 2006, Poorter et al 2009). What has yet to be considered, is the possibility that if leaf traits are found to vary among species as a function of environmental conditions across a landscape, with these traits in turn driving differences among species in leaf flammability, landscape variation in leaf level flammability might be largely predictable on the basis of variation in a few key leaf traits responding to environmental gradients Uncovering such a predictive relationship, underpinned by fundamental environmental filtering of key leaf traits, is critical as it has the potential to provide an important initial link between flammability research at the leaf scale and research examining flammability at the scale of vegetation communities (Gill and Zylstra 2005). Leaves contribute a large proportion of the living biomass available to a fire and recent modeling has shown that the properties of living fuel, in particular plant leaves, are an important factor in landscape fire (Zylstra et al 2016)

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