Exotic plant invasions can alter fire regimes in plant communities. Invaders often possess traits that differ from native plants in the community, resulting in increases or declines in community-level flammability, changing fire regimes and potentially causing long-term modifications to plant community composition. Although considering traits of multiple invaders and native species together is useful to better understand how invasions change community-level flammability, few studies have done this. We measured morphological and flammability traits of 51 native and exotic plant species common in tussock grasslands in New Zealand's south-eastern South Island to examine relationships between morphology and whole-plant and shoot-level flammability. Plant community data from 103 permanent transects in this region measured over a 25-year period (c. 1982–2007) were used to determine how flammability changed with increasing levels of plant invasion. Invasion by exotic plants has led to reduced community-level flammability due to shifts from native tussock grasses with high flammability and high fuel loads to mat-forming exotic forbs with low flammability and little fuel. These changes will likely lead to considerable alterations to the fire regime, resulting in lower intensity fires that burn more patchily and for shorter amounts of time, potentially causing further changes in floristic composition. We found considerable differences in flammability across the wide range of species and growth forms that we studied, emphasising the importance of quantifying species-level flammability and the need to avoid treating grasslands as homogenous in terms of their flammability. Total biomass, leaf length and leaf area were the traits most positively correlated with flammability in these tussock grasslands. Synthesis. We show how plant invasions over decadal time-scales have reduced the community-level flammability of tussock grasslands and, for the first time, demonstrate how this can be driven by exotic forbs. The total biomass of constituent species is a useful surrogate for community flammability across a wide range of species and growth forms in both temperate grasslands and savanna ecosystems and should be used in dynamic global vegetation models to assess how flammability varies under various global change scenarios.
Bibliographical noteFunding Information:
This work was funded by grants to H.L.B. from Lincoln University
the New Zealand Department of Conservation, and the Marsden Fund administered by the Royal Society of New Zealand, and grants to N.J.D. and also to T.J.C. and H.L.B. from the Miss E. L. Hellaby Indigenous Grasslands Research Trust. The European Union provided funding through a NESSIE-Erasmus Mundus Postdoctoral scholarship to J.P.C.; LU provided additional funding via an Early Career Researcher award and LU Research Fund to T.J.C., as well as a Summer Scholarship for R.P. Many thanks to M. Mackintosh for field logistics and field assistance, and Z. Lazare for helping with the field work. We thank J. Sullivan for early advice, S. Wyse and G. Perry for comments on and discussion of this manuscript, V. Smith for translating the abstract into Te Reo Māori, and S. Wilson and two anonymous referees for their helpful comments. We are also grateful to the lessees for allowing us access to sample on their properties. The authors have no conflicts of interest to declare.
- community dynamics
- forb invasion
- plant flammability
- temporal change
- tussock grassland