Within and between population comparisons suggest independently acting selection maintaining parallel clines in Scots pine (Pinus sylvestris)

Parallel clines in traits related to adaptation in a species can be due to independent selection on a pair of traits, or due to selection in one trait resulting in a parallel cline in a correlated trait. To distinguish between the mechanisms giving rise to parallel adaptive population divergence of multiple traits along an environmental gradient we need to study variation, correlations, and selective forces within individual populations along the gradient. In many tree species, budset timing forms a latitudinal cline, and parallel clinal variation is also found in other seedling traits, such as first year height and fall frost injury. In this study, we set up a common garden experiment with open pollinated progeny from natural populations of Scots pine (Pinus sylvestris), with one large sample from single population (500 families) and smaller samples from across a latitudinal gradient. Budset timing, first year height and induced fall frost injury were first measured in a greenhouse. Th...