Fire‐adapted traits of Pinus arose in the fiery Cretaceous

Abstract

Summary The mapping of functional traits onto chronograms is an emerging approach for the identification of how agents of natural selection have shaped the evolution of organisms. Recent research has reported fire‐dependent traits appearing among flowering plants from 60 million yr ago (Ma). Although there are many records of fossil charcoal in the Cretaceous (65–145 Ma), evidence of fire‐dependent traits evolving in that period is lacking. We link the evolutionary trajectories for five fire‐adapted traits in Pinaceae with paleoatmospheric conditions over the last 250 million yr to determine the time at which fire originated as a selective force in trait evolution among seed plants. Fire‐protective thick bark originated in Pinus c. 126 Ma in association with low‐intensity surface fires. More intense crown fires emerged c. 89 Ma coincident with thicker bark and branch shedding, or serotiny with branch retention as an alternative strategy. These innovations appeared at the same time as the Earth’s paleoatmosphere experienced elevated oxygen levels that led to high burn probabilities during the mid‐Cretaceous. The fiery environments of the Cretaceous strongly influenced trait evolution in Pinus. Our evidence for a strong correlation between the evolution of fire‐response strategies and changes in fire regime 90–125 Ma greatly backdates the key role that fire has played in the evolution of seed plants.

Ecology and evolution of pine life histories

“…Functional trait analysis in pines shows that thick bark is always associated with fireadapted pines with fire-tolerater strategies and lacking in species adapted to stresses of extreme cold or aridity or to crown fire regimes (Keeley and Zedler 1998). Ancestral trait reconstruction supports a Cretaceous origin for this trait, strongly implying that fire adaptation was already an important driver of pine evolution at this time (He et al 2012). Angiosperms at this time were rapidly diversifying into mostly fast growing weedy species (Wing and Boucher 1998), and it is possible this contributed to fuel continuity that enhanced fire spread (Bond and Scott 2010).…”

“…By the mid-Cretaceous, both the fossil record (Millar 1998) and molecular phylogenies He et al 2012) show Pinus split into subgenera Strobus and Pinus. These have largely followed very different adaptive trajectories: the former subgenus to abiotically stressful conditions and the latter into fire-prone landscapes.…”

An experimental assessment of the ignition of forest fuels by the thermal pulse generated by the Cretaceous–Palaeogene impact at Chicxulub

“…Wildfires were a common feature of the Cretaceous landscape based on the high abundances of fossil charcoals found in rocks from this period (Belcher et al 2005;Glasspool & Scott 2010;Brown et al 2012). Moreover, high fire frequencies in the Cretaceous appear to have driven plant adaptations to fire at this time; for example, Pinus has been shown to evolve the fire-adaptive traits of thick bark and serotinous cones between 129 and 89 Ma (He et al 2012), implying that some plants were becoming increasingly adapted to fire throughout this period. Our experimental results imply that surface fires could have been ignited in areas where litter was seasonally dry, but that at proximal and intermediate distances from the impact direct ignition of canopy fuels was unlikely.…”

Fire intensity impacts on post-fire temperate coniferous forest net primary productivity

“…Numerous studies have linked morphological traits to post-fire survival; thicker bark, deep rooting depth, and a high, open tree crown have all been identified as characteristics that increase relative fire resistance of a tree (Fischer and Bradley, 1987;Harrington, 2013;He et al, 2012;Keeley, 2012;Midgley et al, 2011;Ryan and Reinhardt, 1988;Starker, 20 1934;VanderWeide and Hartnett, 2011). However, many studies assume a binary response regarding fire impacts on vegetation: either mortality (immediate or delayed) or no physiological effect .…”