Fire in the tree: The origin and distribution of fire-adapted traits within conifers and their influence on speciation rates across the conifer phylogeny.

Premise: Considering rapidly changing fire regimes due to anthropogenic disturbances to climate and fuel loads, it is crucial to understand the underpinnings driving fire-adapted trait evolution. Among the oldest lineages affected by fire is Coniferae. This lineage occupies a variety of fire prone and non-fire prone habitats across all hemispheres and has four fire-adapted traits: (1) thick bark; (2) serotiny; (3) seedling grass stage; and (4) resprouting ability. We seek to determine the historic origins of these traits, the degree of convergent evolution among species, how fire adaptations affect diversification rates in conifers, and if there is a link between climate and the evolution of fire adaptations.

Methods: To investigate these questions, we use a combination of ancestral state reconstructions, multiple diversification analyses, and Pagel trait correlations.

Results: Our results point to multiple evolutionary origins of fire adaptations. We find certain climates, particularly Subtropical and Mediterranean, are highly correlated with species possessing fire adaptations. Several lineages evolved fire adaptations after the Mid-Miocene Climactic Optimum, which coincides with the expansion of the then novel Mediterranean Climate type. Generally possessing a fire adaptation does not increase diversification rates, with the possible exceptions of Pinus subsections Australes and Ponderosae.

Conclusions: The appearance of novel climates and associated fire regimes seem to have been the primary drivers of fire adaptation evolution in conifers. However, most increases in diversification rates are within clades that responded favorably to cooler drier climates post Mid-Miocene Climactic Optimum, regardless of whether the clade is fire adapted.