1. The genetic and phenotypic variability of life-history traits determines the demographic attributes of tree populations and, thus, their responses to anthropogenic climate change. Growth-and survival-related traits have been widely studied in forest ecology, but little is known about the determinism of reproductive traits.2. Using an elevation gradient experiment in the Pyrenees, we assessed the degree to which variations in reproductive effort along climatic gradients are environmentally or genetically driven, by comparing oak populations (Quercus petraea) growing under field and common garden conditions.3. In situ monitoring revealed a decline in reproductive effort with increasing elevation and decreasing temperature. In common garden conditions, significant genetic differentiation was observed between provenances for reproduction and growth: trees from cold environments (high elevations) grew more slowly, and produced larger acorns in larger numbers. Our observations show that genetic and phenotypic clines for reproductive traits have opposite signs (counter-gradient) along the environmental gradient as opposed to growth, for which genetic variation parallels phenotypic variation (co-gradient).4. The counter-gradient found here for reproductive effort reveals that genetic variation partly counteracts the phenotypic effect of temperature, moderating the change in reproductive effort according to temperature. We consider the possible contribution to this counter-gradient in reproductive effort as an evolutionary trade-off between reproduction and growth.