title: Fuel moisture content enhances nonadditive effects of plant mixtures on flammability and fire behavior.
abstract: Fire behavior of plant mixtures includes a complex set of processes for which the interactive contributions of its drivers, such as plant identity and moisture, have not yet been unraveled fully. Plant flammability parameters of species mixtures can show substantial deviations of fire properties from those expected based on the component species when burnt alone; that is, there are nonadditive mixture effects. Here, we investigated how fuel moisture content affects nonadditive effects in fire behavior. We hypothesized that both the magnitude and variance of nonadditivity in flammability parameters are greater in moist than in dry fuel beds. We conducted a series of experimental burns in monocultures and 2-species mixtures with two ericaceous dwarf shrubs and two bryophyte species from temperate fire-prone heathlands. For a set of fire behavior parameters, we found that magnitude and variability of nonadditive effects are, on average, respectively 5.8 and 1.8 times larger in moist (30% MC) species mixtures compared to dry (10% MC) mixed fuel beds. In general, the moist mixtures caused negative nonadditive effects, but due to the larger variability these mixtures occasionally caused large positive nonadditive effects, while this did not occur in dry mixtures. Thus, at moister conditions, mixtures occasionally pass the moisture threshold for ignition and fire spread, which the monospecific fuel beds are unable to pass. We also show that the magnitude of nonadditivity is highly species dependent. Thus, contrary to common belief, the strong nonadditive effects in mixtures can cause higher fire occurrence at moister conditions. This new integration of surface fuel moisture and species interactions will help us to better understand fire behavior in the complexity of natural ecosystems.
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: Luke G
authors.initials: LG
authors.lastname: Blauw
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: Niki
authors.initials: N
authors.lastname: Wensink
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: Lisette
authors.initials: L
authors.lastname: Bakker
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: Richard S P
authors.initials: RS
authors.lastname: van Logtestijn
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: Rien
authors.initials: R
authors.lastname: Aerts
authors.affiliation: Conservation Biology Department, Institute of Environmental Sciences, Leiden University Leiden, The Netherlands.
authors.forename: Nadejda A
authors.initials: NA
authors.lastname: Soudzilovskaia
authors.affiliation: Systems Ecology, Department of Ecological Sciences, VU University Amsterdam Amsterdam, The Netherlands.
authors.forename: J Hans C
authors.initials: JH
authors.lastname: Cornelissen
keywords: Combustion; heather; moss; plant traits; species composition; surface fuels