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

Abstract

Wildland fires are a dominant disturbance on Earth. On the local scale, fire activity is also influenced by species-specific fire behaviour of leaf litter beds. Thus, researchers strive to identify plant functional traits governing fire behaviour. The currently accepted relationships between morphological characteristics of the individual particles, fuel bed structure and resulting fire behaviour have been established on freshly constructed leaf litter beds. To investigate to what degree these relationships are altered upon exposure of constructed leaf litter beds to outside weather conditions, a novel testing system was designed. It enables outdoor exposure of the constructed litter beds, their subsequent retrieval and fire behaviour testing without disturbing the sample structure. Two treatments were applied on seven monospecific leaf litters. “Fresh treatment” corresponded to the common practice of testing fire behaviour directly after fuel bed construction. In the “settled treatment” constructed fuel beds were exposed for 30 days to outside weather conditions before being tested. The “settled treatment” was designed to address physical changes in the fuel bed structure which occur due to repeated wetting of the fuel bed. Thus, to minimise the effects of decomposition and fragmentation, winter exposure was chosen. Within the “fresh treatment” previously established relationships between size, curl, bulk density and fire behaviour characteristics could be confirmed. In the “settled treatment” the majority of these relationships lost their significance. The “settled treatment” had significantly lower bulk density (BD), rate of spread, maximum flame height and maximum sand temperature at 1 cm depth; and significantly higher flaming duration and amount of unburned residues compared to the “fresh treatment”. Species with low initial BD were more affected by the treatment than species with high initial BD. The abrupt change in the fire behaviour of some leaf litter beds and the loss of numerous relationships between morphological characteristics of the individual particles and fire behaviour characteristics upon settled treatment indicate that fast occurring changes in the fuel bed structure should be taken into consideration if we are to understand the relationships between functional traits and local fire activity.

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

“…Samples in treatment "0" were constructed directly in the testing cage, while samples in treatments "10" and "60" were established in the exposure construction and transferred to the testing cage prior to testing. Details on the exposure construction, testing cage and transfer process are given in [20]. In brief, the exposure construction was composed of a 20 cm × 20 cm aluminum mesh bottom with an edge 2.5 cm in height, and a fabric enclosure 13 cm in height.…”

“…To enable fire behavior testing of the samples formed in the exposure construction without disturbing their structure, a special combustion chamber was developed. For details on the combustion chamber refer to [20]. In brief, the stainless steel combustion chamber has an open front side providing air supply and an unobscured view of the burning samples.…”

“…Our initial work [20] revealed species specific and potentially large effects of a 30 days winter exposure to outside weather conditions on fuel bed structure and fire behavior of deciduous tree species. Species with high initial bulk density (BD) were less affected by the exposure than species with low initial BD.…”

“…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]. To quantify the effects of leaf compaction on fire behavior of the leaf litter beds a novel exposure and testing system was developed.…”

“…), carob (Ceratonia siliqua L.), and downy oak (Quercus pubescens Willd.). Whereas our initial work [20] focused solely on deciduous trees, here we included species belonging to different plant groups (evergreen conifers, evergreen broadleaved species and deciduous broadleaved species, respectively) and provide the first information on compaction of evergreen leaf litter beds. Based on the established relationships between leaf traits and fire behavior of litter beds [2,12,13,17,[21][22][23] we expected different fire behaviors and different responses to the treatments for the tested species.…”