Community phylogenetics of North American conifers through the lens of fire-adapted traits.

Premise: In the face of novel fire regimes driven by anthropogenic changes to climate, ignitions, and fuels, understanding the evolution and present distribution of fire-adapted traits is critical. Four common fire adaptations in conifers are thick bark, serotinous cones, seedling grass stage, and resprouting.

Methods: We focused on these fire-adapted traits and their abundance in North American conifers within a community phylogenetic context. Using ranges derived from numerous sources, we divided the continent into 20 conifer regions. We assessed regions for species richness, number of fire adaptations, number of taxa with each fire adaptation, and the percent composition of species with fire adaptations. We calculated regional phylogenetic dispersion metrics for each fire adaptation as well.

Results: For almost every analysis, a north-south trend was evident and had strong spatial correspondence with fire intensity and frequency metrics of contemporary fire regimes. Species richness and nearly every measure of fire adaptation abundance, increases with decreasing latitude. Northern latitudes also lack resprouting and the grass stage. We identified three fire-prone regions: California, Mexico and Central America, and the Gulf Coast. In terms of community composition, northern and mid-latitude regions trend toward neutral assembly, while southern regions are phylogenetically clustered. In most regions, fire adaptations are phylogenetically clustered, indicating that even in neutrally assembled ecosystems, fire adaptations are restricted to a few clades.

Conclusions: Given this information, we advocate tailoring management strategies toward present fire adaptations (or lack thereof), emphasizing strengthening fire resilience in fire-adapted ecosystems and maintaining the integrity of fire refugia.