Distinct male reproductive strategies in two closely related oak species.

Reproductive strategies of closely related species distributed along successional gradients should differ as a consequence of the trade-off between competition and colonization abilities. We compared male reproductive strategies of Quercus robur and Q. petraea, two partly interfertile European oak species with different successional status. In the studied even-aged stand, trees of the late-successional species (Q. petraea) grew faster and suffered less from intertree competition than trees of the early-successional species (Q. robur). A large-scale paternity study and a spatially explicit individual-based mating model were used to estimate parameters of pollen production and dispersal as well as sexual barriers between species. Male fecundity was found to be dependent both on a tree's circumference and on its environment, particularly so for Q. petraea. Pollen dispersal was greater and more isotropic in Q. robur than in Q. petraea. Premating barriers to hybridization were strong in both species, but more so in Q. petraea than in Q. robur. Hence, predictions based on the competition-colonization trade-off are well supported, whereas the sexual barriers themselves seem to be shaped by colonization dynamics.