Ontogeny refers to the genetically regulated shifts in phenotypic traits across developmental phases. While ontogenetic variation is well documented for some plant traits, there is a lack of research investigating the potential of these developmental trajectories to evolve. I investigated genetic variation in ontogenetic trait trajectories by studying leaf stomatal traits in Betula pendula, from a long-term field experiment in Finland. Stomata are major regulators of photosynthetic rate, which in turn drives patterns of growth and ecosystem productivity. Epidermal peels were made from leaves collected for 8 birch genotypes at two ontogenetic stages, and replication within genotypes included a total of 263 trees sampled at both stages. The stomatal traits were digitized and analyzed using ImageJ, and density and stomatal size estimates were collected from 3 sub-samples per peel. Genetic variation in ontogenetic trait trajectories is detected as the interaction between genotype and ontogeny in stomatal traits. Results indicate considerable variation in stomatal traits both within and among genotypes, with complex patterns across ontogeny. This study provides the first test for genetic variation in ontogenetic trait trajectories in long-lived woody plants, providing key insights into the evolvability of these trajectories.