How could two genetically identical oak trees have different heights when tree height is a heritable trait?

The heritable characters are the traits that are determined by the heredity material (genetic material) of the organism.

The heritable trait ensures the reproductive success of the species but the trait is also influenced by the environment.

In the given question, the height of the oak trees is determined by the genetic material but the variation on height is the result of the environmental characteristics. The height of the tree could have varied due to the exposure to the sunlight (more sunlight more height) or the available nutrients.