Evidence for a freezing tolerance-growth rate trade-off in the live oaks (Quercus series Virentes) across the tropical-temperate divide

It has long been hypothesized that species are limited to the north by minimum temperature and to the south by competition, resulting in a trade-off between freezing tolerance and growth rate. We investigated the extent to which the climatic origins of populations from four live oak species (Quercus series Virentes) were associated with freezing tolerance and growth rate, and whether species fitted a model of locally adapted populations, each with narrow climatic tolerances, or of broadly adapted populations with wide climatic tolerances. Acorns from populations of four species across a tropical-temperate gradient were grown under common tropical and temperate conditions. Growth rate, seed mass, and leaf and stem freezing traits were compared with source minimum temperatures. Maximum growth rates under tropical conditions were negatively correlated with freezing tolerance under temperate conditions. The minimum source temperature predicted the freezing tolerance of populations under temperate conditions. The tropical species Q. oleoides was differentiated from the three temperate species, and variation among species was greater than among populations. The trade-off between freezing tolerance and growth rate supports the range limit hypothesis. Limited variation within species indicates that the distributions of species may be driven more strongly by broad climatic factors than by highly local conditions.