Dr Tim Curran and colleagues have written a blog post about their recently accepted paper ‘Community-level flammability declines over 25 years of plant invasion in grasslands‘…

It is widely recognised that plant invasions result in changes to fire regimes, in some cases even leading to reduced fire frequency and fire intensity.

In our study, we show how the invasion by mat-forming exotic forbs, the hawkweeds; Pilosella spp. and Hieracium spp., have reduced community flammability in grasslands in New Zealand over a 25-year period.

New Zealand grasslands, once restricted to alpine areas above the treeline, expanded substantially after human settlement due to widespread clearing of forests through burning and subsequent introduction of grazing mammals. This, coupled with the relatively recent establishment of invasive plant species, has led to dramatic changes in the distribution and structure of indigenous grasslands.

In our paper we explored how 25 years of invasions by multiple plant species have affected the flammability of grassland communities. To do this, we examined relationships between morphology and whole-plant and shoot-level flammability of 51 dominant species in grasslands.

Flammability measurements of sampled species were carried out on a specially designed ‘plant barbecue’ (Fig. 1) that allows the preservation of original plant architecture (Fig. 2). By using the recorded frequency and abundance of our species in 103 transects over three separate sampling periods between 1982 and 2007, we were able to scale up from single species to community-level flammability.

Figure 1: Our device for measuring plant flammability (the ‘plant barbecue’)

Figure 2: Examples of how we retained plant architecture during our burning experiment (left: Chionochloa macra; right: Dracophyllum acerosum). Ninety percent of our 51 species were burnt at the whole plant or whole ramet scale.

We show that plant invasions have 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 (Fig. 3). We also show that there were considerable differences in flammability across the wide range of species and growth forms 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 (as measured by fresh mass) of a plant was highly correlated with flammability, a pattern that has also been observed in savanna grasses in South Africa. This suggests that total biomass is a useful surrogate for species flammability across a range of growth forms in temperate and tropical grassland ecosystems and should be considered for use in dynamic global vegetation models to examine future changes in grassland flammability.

Figure 3: Invasion-induced shifts in composition from native tussock grasses with high flammability and high fuel loads (1980s) to exotic mat-forming forbs with low flammability and low fuel loads (2000s) have decreased community flammability in these grasslands.

We predict that these changes in community flammability will likely cause considerable alterations to the fire regime; resulting in lower intensity fires that burn more patchily and for shorter periods of time. If exotic species are favoured by such fire regimes, this may establish positive feedbacks, whereby invasions further suppress fire, again enhancing invasion. This could lead to permanent shifts in species composition and substantially different fire regimes.

The next step in this research is to validate these findings using field experiments, so we have a lot more burning ahead of us!

Josep Padullés Cubino, Hannah L Buckley, Nicola J Day and Timothy J Curran

Read the full paper, published open access, online: Community-level flammability declines over 25 years of plant invasion in grasslands including a translated abstract written in Maori, one of New Zealand’s official languages.