Patterns of flammability across the vascular plant phylogeny, with special emphasis on the genus Dracophyllum : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of philosophy at Lincoln University

Abstract

Fire has been part of the environment for the entire history of terrestrial plants and is a common disturbance agent in many ecosystems across the world. Fire has a significant role in influencing the structure, pattern and function of many ecosystems. Plant flammability, which is the ability of a plant to burn and sustain a flame, is an important driver of fire in terrestrial ecosystems and thus has a fundamental role in ecosystem dynamics and species evolution. However, the factors that have influenced the evolution of flammability remain unclear. The aim of this thesis is to explore the evolutionary patterns of plant flammability and examine the potential factors that have influenced its evolution. To do this, I examined evolutionary patterns of shoot-level flammability at a range of taxonomic levels, including across the Tracheophyta (194 vascular plant species), within a single genus (21 Dracophyllum species), and within a single species (eight populations of Dracophyllum rosmarinifolium). I also explored the potential factors that have influenced variation in flammability across different taxonomic groups. Firstly, I examined evolutionary patterns of flammability across the Tracheophyta (vascular plants). I measured shoot-level flammability of 194 vascular plant species and related these to phylogeny, the fire-proneness of the species’ natural habitat and species’ growth form. I found phylogeny, fire-proneness of habitat and growth form were important predictors of the shoot flammability of vascular plant species. Shoot flammability was generally correlated with phylogenetic relatedness, although some closely related species in some families, such as Dracophyllum species (Ericaceae), varied in their flammability. Species in fire-prone ecosystems tend to have higher flammability than species from non-fire-prone ecosystems, suggesting that fire may play an important role in the evolution of plant flammability. Growth form also influenced flammability: forbs were less flammable than grasses, trees and shrubs, while grasses had higher biomass consumption by fire than other groups. The results suggested that shoot flammability of plants was largely correlated with phylogenetic relatedness and high flammability may result in parallel evolution driven by environmental factors, such as fire regime. Secondly, I examined evolutionary patterns of flammability within a single genus. In the first objective, I found flammability varied widely across Dracophyllum genus. So in the second objective, I explored the phylogenetic patterns of variation in shoot-level flammability across 21 Dracophyllum (Ericaceae) species. I found species in the subgenus Oreothamnus had higher flammability and smaller leaves than those in the subgenus Dracophyllum. Shoot flammability (ignitability, combustibility and consumability) and leaf length showed phylogenetic conservatism across genus Dracophyllum, but exhibited lability among some closely related species, such as D. menziesii and D. fiordense, perhaps due to occupying different habitats. Shoot flammability of Dracophyllum species was negatively correlated with leaf length and shoot moisture content, but had no relationship with the geographic distribution of Dracophyllum species. In conclusion, I found that shoot-level flammability varied widely in the genus Dracophyllum, but showed phylogenetic conservatism. The higher flammability of the subgenus Oreothamnus may be an incidental or emergent property due to the evolution of flammability-related traits, such as smaller leaves, which were selected for other functions that may have facilitated drought and frost tolerance during the Pleistocene and incidentally changed flammability. Finally, I examined evolutionary patterns of flammability within a single species D. rosmarinifolium. D. rosmarinifolium is an extremely polymorphic species. There is considerable variation in the size of the lamina across D. rosmarinifolium populations. In this objective, I measured shoot-level flammability of 62 D. rosmarinifolium individuals from eight populations across the South Island of New Zealand. To explore the potential factors that influenced the intraspecific variation in flammability, I examined the relationship between flammability and a suite of climatic and geographic variables, including latitude, mean annual air temperature and mean annual precipitation of the sample locations, and elevation. I found all flammability components varied significantly across populations. Populations at higher elevations had higher combustibility. My results suggest that elevation appear to have influenced the intraspecific variability of flammability within D. rosmarinifolium, suggesting that shoot flammability may be influenced by habitat environment in the largely fire-free environment of New Zealand. In this study, I demonstrated that flammability has a strong phylogenetic component, which is consistent with the idea that flammability having evolved and been selected for. However, I also found evidence for high flammability having arisen in the absence of selective pressure by fire. These results suggest that flammability has likely both emerged and been selected for; it depends on the context and whether a species or population occurs in a non-fire-prone or fire-prone habitat