Fuel architecture influences interspecific variation in shoot flammability, but not as much as leaf traits

Abstract

Plant flammability is strongly influenced by functional traits, meaning that the quantitative measurement of trait–flammability relationships is key to understanding why some species burn better than others. While relationships between flammability and leaf traits are well‐studied, the role of architectural traits has rarely been assessed. Shoots preserve some of the architecture of plants; therefore, shoot‐level trait–flammability relationships offer great promise for determining the relative influence of fuel architecture and leaf traits on flammability. We quantified plant flammability by burning 70‐cm‐long shoot samples from 65 species of indigenous and exotic New Zealand trees and shrubs and measured a range of leaf and fuel architectural traits on the same individuals. The influence of species' evolutionary history on flammability variation was also quantified. Most of the variation in flammability and functional traits was explained by between‐species differences. No significant phylogenetic signal was detected for the flammability variables measured in this study. Fuel architecture influenced shoot flammability, and along with leaf traits, explained a high proportion (41%–54%) of flammability variation. Branching patterns (number of ramifications and sub‐branches) was the key architectural trait that was strongly positively correlated with flammability. Other architectural traits, such as foliage and twig fraction mass, and fuel bulk density were also significantly associated with some flammability variables. Leaf dry matter content (LDMC; positive relationship) and leaf thickness (negative relationship) were the leaf traits most strongly correlated with shoot flammability. Synthesis. Our study addresses a key knowledge gap by demonstrating the influence of fuel architecture on shoot flammability and improves our understanding of why species with certain architecture (e.g. highly branched) burn better than others. However, leaf traits such as leaf dry matter content (LDMC) and leaf thickness emerged as having a relatively stronger influence on flammability than architectural traits. Where available, traits such as LDMC, leaf thickness and branching pattern can be effective surrogates of plant flammability and can be used to improve global dynamic vegetation models and fire behaviour models. However, several architectural traits are time‐consuming to measure, so where they are not available, it will be quicker to simply measure shoot flammability.