Seasonal and Local Differences in Leaf Litter Flammability of Six Mediterranean Tree Species

Abstract

One of the suggested management options for reducing fire danger is the selection of less flammable plant species. Nevertheless, vegetation flammability is both complex and dynamic, making identification of such species challenging. While large efforts have been made to connect plant traits to fire behavior, seasonal changes and within species variability of traits are often neglected. Currently, even the most sophisticated fire danger systems presume that intrinsic characteristics of leaf litter stay unchanged, and plant species flammability lists are often transferred from one area to another. In order to assess if these practices can be improved, we performed a study examining the relationship between morphological characteristics and flammability parameters of leaf litter, thereby taking into account seasonal and local variability. Litter from six Mediterranean tree species was sampled throughout the fire season from three different locations along a climate gradient. Samples were subjected to flammability testing involving an epiradiator operated at 400 °C surface temperature with 3 g sample weight. Specific leaf area, fuel moisture content, average area, and average mass of a single particle had significant influences on flammability parameters. Effects of sampling time and location were significant as well. Due to the standardized testing conditions, these effects could be attributed to changes in intrinsic characteristics of the material. As the aforementioned effects were inconsistent and species specific, these results may potentially limit the generalization of species flammability rankings. Further research is necessary in order to evaluate the importance of our findings for fire danger modeling.

Do relationships between leaf traits and fire behaviour of leaf litter beds persist in time?

“…An increase in moisture content tends to decrease the particle rigidity and breakability, thus decreasing the particles’ resistance to compression. In this study and in our previous work [30,42] we used this property to flatten leaf litter particles for surface measurements. Furthermore, as particles absorb water their fresh mass inevitably increases and they exert higher pressure on the particles below them.…”

“…Even though the importance of understanding the relationship between functional traits, decomposition and fire behaviour is recognised [32,69], only few studies have directly investigated the effects of aging on particle characteristics and fire behaviour of litter fuels. In these studies, fuel aging is addressed through repeated sampling of leaf litter [35,42], the time since treatment for masticated fuels [43,44], or the reduction in wood density for twigs [70]. While twigs were tested as individual standardised fragments, leaf litter and masticated fuels were tested as constructed litter beds (i.e.…”

“…So far only few studies directly measured the effects of litter bed dynamics on its fire behaviour [35,41–44]. These studies do account for changes in chemical composition [44] and particle size [42]. Nevertheless, they do not account for in-stand observed alterations of the litter bed structure [35], the mass loss induced by decomposition, or effects of the addition of new material atop of the existing litter bed.…”

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

“…Most flammability experiments have been conducted in the laboratory on small plant components (i.e. leaves, leaf litter, small twigs or needles, bark, woody debris) due to ease of sample collection and manipulation during burning (Ganteaume, Jappiot, Lampin, Guijarro, & Hernando, 2013;Grootemaat, Wright, Bodegom, Cornelissen, & Shaw, 2017;Kauf, Fangmeier, Rosavec, & Španjol, 2015;Mason, Frazao, Buxton, & Richardson, 2016;Zhao, Logtestijn, Werf, Hal, & Cornelissen, 2018). It has been argued that these fine-level laboratory tests do not scale up to predict canopy flammability or crown fire in the field, at least in part because they do not adequately account for plant architecture (Fernandes & Cruz, 2012).…”

How Much Does Fire Behavior of Leaf Litter Beds Change within Two Months?

“…Litter beds are dynamic fuels and their properties are continuously changing. Decomposition [16,17] and particle fragmentation [18,19] are recognized as important processes altering the fire behavior of the leaf litter beds. Another process appears important as well: compaction (i.e., an increase in the bulk density and packing ratio with a decrease in the amount of empty space within the fuel bed) of fuel beds under exposure to local weather condition was previously observed [18], but only recently quantified [20].…”