Flammability of native and invasive alien plants common to the Cape Floristic Region and beyond: Fire risk in the wildland–urban interface

Abstract

High-impact mega-fires in many temperate forests and shrublands of the world have called for pre-emptive approaches to mitigate fire risk. Comparative appraisals of the characteristics and flammability of plant fuels can inform fire risk predictions and vegetation management efforts in the wildland–urban interface of such fire-prone regions. Prompted by recent extreme fires in the Cape Floristic Region of South Africa, we assessed the flammability and fuel traits of excised terminal branches of 30 woody species (many of which have never been studied before) that commonly occur as native, alien or invasive species in this region and some other temperate regions of the world. We furthermore assessed changes in flammability and fuel moisture in these species after partial drying of excised branches under ambient conditions for 2–4 weeks to simulate extreme drought conditions. Several prominent alien invasive taxa in the study region and elsewhere showed high flammability, namely Pinus radiata (median flammability index score of 2.5), Eucalyptus camaldulensis (2.3), Acacia mearnsii (2.2), Acacia melanoxylon (2.2), Pinus pinaster (2.2), Acacia cyclops (2.0) and Cestrum laevigatum (2.0). The high fire risk posed by these species needs to be considered in potential further plantings and in invasive plant clearing efforts. Conversely, Osteospermum moniliferum (median flammability index score 0.9) and Acacia saligna (1.3) are species with large invasive ranges that showed low flammability, which may suppress fire regimes in fire-prone ecosystems but present suitable options for fire mitigation in their native ranges. Fynbos and thicket species native to the study region mostly displayed high and low flammability, respectively. Species of which the flammability is likely to increase disproportionately under extreme drought conditions include select fynbos and thicket species, and among the alien invasives, all four Acacia species, Leptospermum laevigatum, Callistemon viminalis, Pinus pinaster and Osteospermum moniliferum. Management of these species should be a secondary priority (after high flammability species) in the wildland–urban interface and other high fire risk areas.