Burn or rot: leaf traits explain why flammability and decomposability are decoupled across species

Abstract

Summary In fireprone ecosystems, two important alternative fates for leaves are burning in a wildfire (when alive or as litter) or they get consumed (as litter) by decomposers. The influence of leaf traits on litter decomposition rate is reasonably well understood. In contrast, less is known about the influence of leaf traits on leaf and litter flammability. The aim of this study was twofold: (i) to determine which morphological and chemical leaf traits drive flammability and (ii) to determine whether different (combinations of) morphological and chemical leaf traits drive interspecific variation in decomposition and litter flammability and, in turn, help us understand the relationship between decomposability and flammability. To explore the relationships between leaf traits and flammability of individual leaves, we used 32 evergreen perennial plant species from eastern Australia in standardized experimental burns on three types of leaf material (i.e. fresh, dried and senesced). Next, we compared these trait–flammability relationships to trait–decomposability relationships as obtained from a previous decomposition experiment (focusing on senesced leaves only). Within the three parameters of leaf flammability that we measured, interspecific variation in time to ignition was mainly explained by specific leaf area and moisture content. Flame duration and smoulder duration were mostly explained by leaf dry mass and to a lesser degree by leaf chemistry, namely, nitrogen, phosphorus and tannin concentrations. The variation in the decomposition constant across species was unrelated to our measures of flammability. Moreover, different combinations of morphological and chemical leaf properties underpinned the interspecific variation in decomposability and flammability. In contrast to litter flammability, decomposability was driven by lignin and phosphorus concentrations. The decoupling of flammability and decomposability leads to three possible scenarios for species’ influence on litter fates: (i) fast‐decomposing species for which flammability is irrelevant because there will not be enough litter to support a fire; (ii) species with slow‐decomposing leaves and a high flammability; and (iii) species with slow‐decomposing leaves and a low flammability. We see potential for making use of the decoupled trait–decomposition–flammability relationships when modelling carbon and nutrient fluxes. Including information on leaf traits in models can improve the prediction of fire behaviour. We note that herbivory is another key fate for leaves, but this study was focused on fire and decomposition.

Determinants of flammability in savanna grass species

“…) but species variation in flammability has received relatively little attention (but see Scarff & Westoby ; Murray, Hardstaff & Phillips ; Grootemaat et al . ). Flammability is an emergent property of a plant's chemical and physical traits.…”

Assessment of Flammability of Moroccan Forest Fuels: New Approach to Estimate the Flammability Index

“…Few studies have tried to assess flammability using field-scale fire experiments [10] due to authorization, safety and cost constraints [9,11,12], while the most studies performed standardized laboratory-scale flammability measurements [2,5,7,[12][13][14][15][16][17][18][19][20][21], including some recent studies that still use the method of Valette [2,7,12,15]. Some of these methods have the disadvantage of using ground samples (do not take into account the fuel structure) and are criticized for combustion tests under conditions different from the natural conditions [1].…”

“…This FH parameter also gives insight into a fire propagation rating [14]. Sustainability is assessed using the duration of combustion (DC) [1,14,17,34].…”

“…Moisture content (MC) is a key factor influencing flammability (e.g., [5,17,35,36]), acting both physically on the plant by controlling the thermal capacity of tissues and chemically through the adjustment of the combustion process [1,37]. Therefore, flammability variation within species depends on fuel water content [1,35,38].…”

“…FI combines the capability of fine fuels to ignite when given a heat source [12] to spread fire to the adjacent fuels and to sustain the combustion [14]. Ranking these species according to FI as well as integrating FI data into a fire behaviour prediction system in order to improve its accuracy and map the high-risk areas based on vegetation maps would help in forest fire management as these would facilitate the selection of the appropriate afforestation species for mitigating fuel hazards [7,8,17].…”