Abstract
Invasive species can cause shifts in vegetation composition and fire regimes by initiating positive vegetation-fire feedbacks. To understand the mechanisms underpinning these shifts, we need to determine how invasive species interact with other species when burned in combination and thus how they may influence net flammability in the communities they invade. Previous studies using litter and ground fuels suggest that flammability of a species mixture is nonadditive and is driven largely by the more-flammable species. However, this nonadditivity has not been investigated in the context of plant invasions nor for canopy fuels. Using whole shoots, we measured the flammability of indigenous-invasive species pairs for six New Zealand indigenous and four globally invasive plant species, along with single-species control burns. Our integrated measure of flammability was clearly nonadditive, and the more-flammable species per pairing had the stronger influence on flammability in 83% of combinations. The degree of nonadditivity was significantly positively correlated with the flammability difference between the species in a pairing. The strength of nonadditivity differed among individual flammability components. Ignitability and combustibility were strongly determined by the more-flammable species per pair, yet both species contributed more equally to consumability and sustainability. Our results suggest mechanisms by which invasive species entrain positive vegetation-fire feedbacks that alter ecosystem flammability, enhancing their invasion. Of the species tested, Hakea sericea and Ulex europaeus are those most likely to increase the flammability of New Zealand ecosystems and should be priorities for management.
Highlights
Biological invasions pose an ever-increasing threat to terrestrial ecosystems worldwide (Pysek and Richardson 2010)
One sample was taken per individual, with 36 samples collected per species for the indigenous species and 48 samples collected per species for the invasive species
Bulk density and dry biomass per burn were more strongly associated with the second principal components analysis (PCA) axis and the burn time flammability component (Figure 3); these variables explained little of the variation in the ordination
Summary
Biological invasions pose an ever-increasing threat to terrestrial ecosystems worldwide (Pysek and Richardson 2010). Some invasive species change only the composition of the communities they invade, but those that are ‘ecosystem engineers’ (sensu Jones and others 1994) can affect the structure and functioning of entire ecosystems (Crooks 2002; Gaertner and others 2014; Levine and others 2003). Such high-impact invasive species drive environmental change through feedbacks that enhance their own success (Gaertner and others 2014). The most common examples involve plant invasions that increase fire frequency and severity, but invaders may modify ecosystem processes by suppressing fire activity (Brooks and others 2004; Crooks 2002). Invasive species can modify the fire regime by altering the inherent flammability of the available fuel in a system by, for example, changing the relative proportion of live and dead material, as well as the amount and arrangement of fuel loads (Berry and others 2011; Brooks and others 2004; D’Antonio and Vitousek 1992)
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