Scaling from leaf traits to fire behaviour: community composition predicts fire severity in a temperate forest

1. Although species differ in flammability, identifying the traits that influence flammability and linking them to other axes of trait variation has yet to be accomplished. Leaf length may be a key trait influencing the flammability of leaf litter. 2. Differences in species composition across a landscape or changes in composition through time may alter fire behaviour. Forests in the Sierra Nevada of CA, USA, have experienced changes in species composition that have modified the distribution of leaf litter traits. 3. Across three independent data sets, at scales from a single watershed to multiple watersheds and elevations, we tested if mean community leaf length in patterns of fire severity. We used structural equation models to disentangle direct effects of site characteristics from the contribution of species corn position. 4. Fire severity was greater at sites inhabited by species with longer leaves than at sites containing short-leaved species, probably as a result of lower litter density. The effect cannot be explained merely by the joint influence of site characteristics on both fire behaviour and species composition. 5. A significant portion of this pattern is driven by shifts in the abundance of Pinus species. In this system, pines are among the longest-leaved species and this makes it difficult to separate leaf-length effects from other possible flammability-enhancing characteristics of pines. Evidence from one data set, however, suggests that the pattern cannot be entirely explained by proportion of pines alone. 6. Synthesis. We demonstrate that a simple integration of a species trait predicts fire severity at landscape scales. This provides a link between the two scales at which most previous work has occurred: species-specific measurements of traits and landscape-level characterisation of fuel loads. Investigations of trait effects on fire behaviour are important because climate change may lead to novel climates and no-analogue species assemblages. In this ecosystem, shorter-leaved species, which have increased in density during the period of fire exclusion, may act as a positive feedback by reducing fire severity and thereby favouring their own establishment. Conversely, restoration of fire to these forests, by increasing the dominance of long-leaved species, may increase flammable fuels.