Using carbon economics of tree height to estimate evolutionary timing of cold tolerance in conifers

• Little is known of the timing of cold adaptations in conifer species.
• Tree height and cold tolerance are related in modern-day conifers.
• We calculated tree height from plant fossil metrics in conifers.
• Tree height in Mesozoic conifers was significantly different than in Cenozoic.
• Differences in conifer tree height throughout the Cenozoic were not significant.

Modern conifer species express many cold tolerant traits that allow for their persistence and dominance in ecosystems at high latitudes. The evolution of these traits is believed to have occurred at some point after the Cretaceous, but there exist no studies that support this hypothesis. Understanding when frost tolerance evolved within the conifer lineage is imperative as the presence of this flora is often used as a palaeoclimatic indicator under the nearest living relative (NLR) method. A trade-off relationship between tree height and cold stress tolerance is indicated for modern-day conifer species, providing clues as to what would have occurred physiologically and structurally to conifers upon the evolution of these traits. To approximate the evolutionary timing of frost tolerance, conifer basal stem diameters were obtained, from the Triassic to present day, and used to predict tree height measures using allometric equations. The plotting of tree height through time and statistical analyses suggest a significant decline of 11.31 m between the Cenozoic and Mesozoic, and height similarities within the Cenozoic and Mesozoic. This decline suggests carbon resource reallocation from tree height to frost tolerance which is further supported using genetic data, during the Cenozoic Mesozoic boundary. However, since there is only minimal research on the phylogeny of frost tolerance in conifers, we are unable to refine our estimate any further. Our overall goal was to incite collaborative research to address this critical evolutionary step in modern-day conifers.