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

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.