Spatial and evolutionary parallelism between shade and drought tolerance explains the distributions of conifers in the conterminous United States

Aim Gymnosperms do not follow a latitudinal diversity gradient across the Northern Hemisphere but are influenced by geography at continental scales. Tolerance to physiological aridity is thought to be the main driver of this distribution, yet through evolutionary time conifers have also faced conditions of frost, shade and fire. We tested four predictions to evaluate how environmental stressors and geographical and evolutionary patterns of traits influence conifer distributions: (1) environmental variables related to aridity are most important in explaining geographical patterns of traits; (2) traits responsible for survival in stressful conditions have evolved under a niche conservatism constraint; (3) phylogenetic correlations among traits as the result of complex evolutionary responses to multiple abiotic stressors are widespread; (4) there are parallelisms between spatial trait associations and correlated trait evolution. Location The conterminous United States. Methods We combined conifer occurrences with 10 traits related to drought, freezing, shade and fire. The spatial distribution of traits was mapped and the relationship between environment and the geographical patterns of traits was explored. Niche conservatism was assessed comparing patterns of trait evolution against Brownian motion. We computed geographical and phylogenetic correlations among traits to determine the correspondence between spatial and evolutionary trade-offs. Results (1) Maximum temperature followed by precipitation were the environmental variables that best described the geographical distributions of traits. (2) Most traits contain a phylogenetic signal consistent with niche conservatism: major exceptions being fire-related traits and frost tolerance. (3) Drought and shade tolerances show one of the strongest negative phylogenetic correlations. (4) The drought-shade tolerance trade-off is mirrored at the biogeographical scale. Main conclusions Unlike in angiosperms, cold does not seem to have been a major driver in the evolutionary history of temperate conifers. A strong tradeoff between drought and shade tolerance is the simplest explanation for understanding the current distribution of conifers in North America.