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

Abstract

Summary1. 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 composition. 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.

Is fire severity increasing in the Sierra Nevada, California, USA?

“…In fact, in the more extensive fires studied, fire severity tended to decrease with very long fire intervals Hanson 2006, 2008). This is supported by recent research on flammability trends in Sierran forests by Schwilk and Caprio (2011). In addition, in the dry forest types in the interior northwest California, which range from open to closed canopy, found modest amounts of high-severity fire that did not differ in long-unburned forests relative to more recently burned forests and, in moister closed forests of this region, fire severity decreased with time since fire, consistent with earlier research (Odion et al 2004.…”

Short Communication. Comparing flammability traits among fire-stricken (low elevation) and non fire-stricken (high elevation) conifer forest species of Europe: A test of the Mutch hypothesis

“…The evolution of a flammability trait is more likely to occur when the trait also provides additional benefits to the plant. Schwilk and Caprio (2011), working on pine litter fuel beds, state that single flammability traits of species (such as pine needle length) can lead to prediction of the fire behaviour in the whole plant communities. From this study it can be inferred that differences in fire regimes among low and high elevation conifer species in Europe cannot be attributed to differences in single flammability traits of species, but rather to the whole plant community fuel properties or to other factors (climatic or anthropogenic).…”

Characterization of Worm-eating Warbler (Helmitheros vermivorum) breeding habitat at the landscape level and nest scale

“…The structural makeup of leaf litter could differ on NE-and SW-facing slopes in our study system based on differences between mesophytic and xeric tree species composition and the associated leaf-litter characteristics (Crosby and Loomis 1974, Schwilk and Caprio 2011, Kreye et al 2013, potentially providing better nesting material for Worm-eating Warblers on SW-facing slopes. Leaves from xeric tree species (e.g., oaks) provide a drier, more loosely compacted litter layer than mesophytic tree species (e.g., maples; Crosby and Loomis 1974).…”