Abstract
The flammability of plant leaves influences the spread of fire through vegetation. Exotic plants invading native vegetation may increase the spread of bushfires if their leaves are more flammable than native leaves. We compared fresh-leaf and dry-leaf flammability (time to ignition) between 52 native and 27 exotic plant species inhabiting dry sclerophyll forest. We found that mean time to ignition was significantly faster in dry exotic leaves than in dry native leaves. There was no significant native-exotic difference in mean time to ignition for fresh leaves. The significantly higher fresh-leaf water content that was found in exotics, lost in the conversion from a fresh to dry state, suggests that leaf water provides an important buffering effect that leads to equivalent mean time to ignition in fresh exotic and native leaves. Exotic leaves were also significantly wider, longer and broader in area with significantly higher specific leaf area–but not thicker–than native leaves. We examined scaling relationships between leaf flammability and leaf size (leaf width, length, area, specific leaf area and thickness). While exotics occupied the comparatively larger and more flammable end of the leaf size-flammability spectrum in general, leaf flammability was significantly correlated with all measures of leaf size except leaf thickness in both native and exotic species such that larger leaves were faster to ignite. Our findings for increased flammability linked with larger leaf size in exotics demonstrate that exotic plant species have the potential to increase the spread of bushfires in dry sclerophyll forest.
Highlights
Fires shape the ecology and evolution of many plant communities throughout the world [1,2,3]
Comparisons of Leaf Traits between Natives and Exotics There was a significant interaction between plant status and leaf condition (F1,154 = 17.13, P,0.001) in the two-way analysis of variance (ANOVA) model which showed that exotic leaves had significantly faster ignition times than native leaves but only when the leaves were dry (Fig. 1a)
Consistent with the pattern represented by the significant interaction, phylogenetic logistic regressions showed that while fresh-leaf flammability did not differ significantly between natives and exotics (F1,77 = 3.23, P = 0.08), dry-leaf flammability was significantly different between natives and exotics (F1,77 = 27.22, P,0.001) with faster ignition times in dry exotic leaves
Summary
Fires shape the ecology and evolution of many plant communities throughout the world [1,2,3]. There is the troubling prospect that the ecological impacts of increased bushfire coverage might be compounded by increased bushfire intensity and frequency in systems where introduced exotic plant species have become established [11,12,13,14]. Such situations will only arise if exotic plants have intrinsic fuel properties that differ from those of native plants, properties such as increased flammability that enhance the spread of fire. Identifying how variation among species in functionally important traits such as leaf size relates to variation in leaf flammability is still in an early stage [21]
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