Shoot-Level Flammability of Species Mixtures is Driven by the Most Flammable Species: Implications for Vegetation-Fire Feedbacks Favouring Invasive Species

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.

Community‐level flammability declines over 25 years of plant invasion in grasslands

“…However, these same shoot‐level experiments suggest that other flammability components (e.g. combustibility and ignitability) are determined by the most flammable species in a mixture, and that this influence can be disproportionately large compared to their relative biomass (Wyse et al., ). Field‐based experimental burns would help resolve the role of particular species in suppressing or promoting fire and more broadly test the applicability of a community‐weighted mean approach to assessing community flammability in these systems.…”

“…() (Figure ; Table ), with adjustments for larger plants (the vertical cylinder described above). All samples were stored air‐dry at room temperature for 24 hr before burning (Wyse et al., ) to ensure that the moisture content of the samples matched the ignition source (Wyse, Perry, & Curran, ). The burners were turned on until the grill reached c .…”

New Zealand forest dynamics: a review of past and present vegetation responses to disturbance, and development of conceptual forest models

“…are examples of invasive weeds that are highly flammable (Wyse et al 2016) and can rapidly recolonise recently burned areas, replacing native pioneers (Perry et al 2014(Perry et al , 2015. Their combined effects may increase ecosystem flammability (Wyse et al 2017), fire frequency, and lead to arrested successions (Perry et al 2014). …”

“…into early successional communities are of particular concern. These species may also affect disturbance regimes by encouraging increased fire frequencies, thereby facilitating further invasions and novel ecosystems Perry et al 2014;Wyse et al 2017).…”

Managing Fire and Biodiversity in the Wildland-Urban Interface: A Role for Green Firebreaks

“…While field-based experiments should be considered the gold standard for testing the effectiveness of green firebreaks, useful insights can be gleaned using laboratory experiments, especially those that retain plant architecture by burning shoots [12,13] or whole plants [14]. Such tests could burn multiple species together to determine the relative contribution of low or high flammability species to the resultant fire [15], and hence help assess just how much biomass of low flammability plants is required to extinguish a fire that is burning a high flammability species. While green firebreaks hold promise as a fire management and biodiversity conservation tool, they should be used in conjunction with other active and passive fire-fighting approaches, especially in extreme fire conditions.…”